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Crustacean burrows in the Seychelles, Indian Ocean
Palaeogeography, Palaeoelimatology, Palaeoecology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
CRUSTACEAN BURROWS IN T H E SEYCHELLES, I N D I A N OCEAN C. J. R. BRA1THWA1TE AND M. R. TALBOT
Department of Geology, The University, Dundee, Scotland (Great Britain (Received April 24, 1971~ (Resubmitted February 29, 1972)
ABSTRACT Braithwaite, C. J. R. and Talbot, M. R., 1972. Crustacean burrows in the Seychelles, Indian Ocean. Palaeogeogr., Palaeoelimatol., Palaeoecol., 11: 265-285. Crustacean burrows are common in Recent sediments associated with fringing reefs around Mah6, Seychelles. Burrow morphology is related 1o function. Unbranched burrows produced by the crabs Cardisoma, Sesarma, Oeypode spp., Uca and Macropthalma are primarily dwellings, whereas multiramous systems constructed by the shrimps Callianassa spp. serve in addition as a means of sediment exploitation. The distribution of morphotypes shows only general correspondence to recognised biofacies zones, and well-preserved burrows from Pleistocene limestones on Aldabra demonstrate that, while providing useful corroborative evidence, burrows are of limited value as primary zone indicators.
INTRODUCTION
Growing interest in trace fossils has stimulated the study of modern burrows and the organisms producing them. Among these, crustacean burrows are particularly distinctive (Hayasaka, 1935; Weimer and Hoyt, 1964; Shinn, 1968; and Frey, 1970) and are of potential geological value in reconstructing ancient environments. The present study has centred on Mah6, principal island of the Seychelles group, where burrows are common in Recent carbonate sediments associated with fringing reefs. The burrow systems and surface traces resulting from the activities of six species of crab, four shrimps and a stomatopod have been investigated: two of them are described here for the first time. The structure of burrows reflects function and both dwelling and sediment-exploiting systems are present. Distribution of morphotypes only broadly corresponds to biofacies zones defined by other criteria. The problems and circumstances of preservation of these structures have been examined and their value as palaeoecological indicators tested on a limited scale in Pleistocene limestones on Aldabra. This work is part of a larger study of the sediments and reef environments Palaeogeogr., Palaeoclimatol., Palaeoecol., 11 (1972)265-285
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of the Seychelles. Earlier results of this and details of the geology, ecology and physiography of Mah6 are to be found in papers by Taylor (1968), Lewis (1969), Braithwaite (1971) and Rosen (1971).
METHODS
The technique used in saturated sediment resembled that described by Shinn (1968) and involved the production of epoxy resin replicas of burrows. This proved impractical on dry material where high permeabilities resulted in resin loss through burrow walls. Under these conditions portland cement was used as a casting medium. The method was developed in discussion with W. J. Kennedy and gave good results, preserving additional information on laminae around burrows. Resin impregnated cores (McMullen and Allen, 1964) occasionally produced useful replicas of burrows, but were chiefly of value in revealing other structures. Identification of crustaceans is based primarily on the work of Taylor (1968, and personal communications, 1965 et seq.). The authors, however, accept responsibility for any errors. DESCRIPTIONS OF ENVIRONMENTS
The main features of reef environments in the Seychelles have been described in a number of previous papers (noted above). Some new data are given here but in general this kind of information will be limited to providing reference to the sedimentological and ecological framework in which these organisms and their biogenic structures are found. 1
2
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cordimanus; 5 = Uca annulipes; 6 ~ Macropthalma ~ A l p h e u s sp.; 9 = P s e u d o s q u i l l a ciliata; 10 ~ C a l l i a n a s s a
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sp. 2. Palaeogeogr., Palaeoelimatol., Palaeoeeol.,
11 (1972) 265-285
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Fig.2. The general spatia! relationships o f biofacies on the reefs o f Mah6 and distribution of the crustacean burrows described.
P l a t f o r m ranging in w i d t h f r o m 0 - 1 , 0 0 0 metres.
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Taylor (1968) has described the ecological zonation in the area which we have adopted. This can be paraphrased as: (1) supra-tidal (dry) areas; (2) mangrove environments; (3) beaches; (4) rippled sand zone; (5) grass beds; (6) open sand area; (7) sargassum zone with algal cobbles; (8) bioconstructed reef edge with coral-defined sub-zone; (9) reef front or reef slope; (10) pro-reef floor. The distribution of burrowing crustaceans in relation to these zones is shown in Fig.l, while Fig.2 illustrates their joint control by sea level. It can be seen that only zones 1-6 and 10 are significant as burrowed areas.
Supra-tidal (dry) areas Although crab burrows are occasionally associated with low-level laterites, their main occurrence is in the so-called "plateaux". These are essentially flat areas of uncertain origin which occur within a few metres of high-water mark behind almost all bays on the island. They are particularly wide on Anse aux Pins on the east coast and Grande Anse and Anse & la Mouche on the west. The greater part of their sediments are bioclastic sands but these become quartzose and include lateritic clays on landward margins and close to streams (Baker, 1963; Piggot, 1968; and Lewis, 1969). On Mah6 the plateaux form the main cultivated areas and the natural vegetation (Sauer, 1967), soil structure, and probably crab distribution have been modified. The seaward fringes of these areas are occupied by the crabs Cardisoma carnifex and Sesarma longipes. On adjacent islands Birgus latro is also present but on Mah6 this is now probably extinct.
Mangrove environments These merge laterally with the plateaux and are associated particularly with high deltaic salients constructed by freshwater streams. The largest area in which mangroves (Rhizophora, Bruguiera, Ceriops and others) occur now is north of Port Glaud on the west coast, but they appear in small groups at most river mouths, and were once extensive on the east coast around Port Victoria. Here the swamp sediments remained until recently as high muddy areas behind the harbour reef edges. The sediments are generally poorly sorted and characterized by high percentages of quartz and other minerals derived from the lateritized granite. Three crabs are associated with this environment: Sesarma longipes, Uca annulipes and Macropthalma parvimanus.
Beaches In areas where a beach berm is developed, these range from gently-sloping, fine-grained, structures in low energy environments to coarse rudaceous aggregates with steep slopes formed in higher energy situations. Berms are absent from shores adjacent to stream mouths, and beaches are not usually present around rocky granite headlands. Two species of Ocypode are common in berms and foreslopes, Palaeogeogr., Palaeoelimatol., Palaeoecol.,
1 l (1972) 265-285
CRUSTACEAN BURROWS IN SEYCHELLES
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except where sediment instability or coarse grain sizes prevent effective burrowing. The lower margins of beaches are also occupied by rapid-burrowing bivalves, but only crab burrows have been identified in cores. Rippled sand zone The contact between the beach and the rippled sand zone below varies on reef flats from an abrupt step to a gradational slope, although sediments are usually distinct. The more abrupt changes normally occur in higher energy situations. The surface of this zone is typically marked by ripples with crests orientated parallel or slightly oblique to the shore, Ripple size varies; in high-energy areas such as Anse Forbans amplitude may be 10-15 cm and wavelength up to 1 m. Such mobile sands contain few burrows, and a certain level of stability is clearly necessary before they can be occupied. In more sheltered positions on the wider reefs (e.g., Anse aux Pins: up to 750 m) infaunal activity in the more stable substrates augments ripples with a wide variety of surface traces (Fig.3). Two species of Ca/lianassa and the stomatopod Pseudosquilla are active in this zone. With their traces are paired siphonal openings of bivalves together with simple cylindrical holes of millimetre dimensions, stellate feeding traces and " w o r m casts" produced by a number of organisms which include polychaetes and (probably) balanoglossids. Mucous-lined annelid tubes 1-2 mm in diameter are locally important in binding the sediment surface.
Fig.3. Surface of rippled sand zone on Anse aux Pins. Note the stellate annelid feeding trace and paired entrances to Callianassa sp.1 burrows. The coin is approximately 2.5 cm diameter. Palaeogeo~r., Palaeoclimatol., Palaeoecol., 11 (1972) 265-285
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In exposed sites these sands may merge into those adjacent to the reef edge. Elsewhere they give way to extensive marine grass beds.
Grass beds These are characterized by more or less dense growths of Thalassia hemprichi and similar marine angiosperms. The area colonized depends, among other factors, upon reef width and exposure, and in some cases beds may be 200-300 m wide. A few corals are present (see Rosen, 1971) and detailed descriptions of the grass beds are given by Taylor and Lewis (1970). Within this zone a large proportion of the sediment volume close to the surface consists of burrow space (estimated 10-15~o). The principal burrowing crustaceans are two species of Callianassa, one of which constructs the mounds so characteristic of this environment on Anse aux Pins, Anse ~ la Mouche and similar areas (Fig.4), and Pseudosquilla. Infaunal molluscs (Taylor, 1968) and other burrowing organisms are common but appear to leave few recognizable traces.
Fig.4. Callianassa sp.2 m o u n d s on Anse ~. la Mouche. The h a m m e r handle is approximately 40 cm long. N o t e that m a n y m o u n d s in this area are large composite structures.
Open sand area Sediment in this outer sand zone is commonly thin, interdigitating with coarser cobble debris derived from the reef edge. Although rapid-burrowing bivalves and gastropods are present, no burrows have been identified on reef-flats such as Anse aux Pins. North of Port Victoria, however, extensive sand patches with scattered clumps of Enhalus and colonies of Favia and Galaxaea extend to the Palaeogeogr., Palaeoclimatol., Palaeoecol., 11 (1972) 265-285
CRUSTACEAN BURROWS 1N SEYCHELLES
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line of brown algae marking the reef edge and occur on top of more isolated pinnacles in front of the edge. These sediments characteristically contain burrows of a shrimp which has been tentatively identified as Alpheus sp.
Pro-reef.floor Since little loose sediment accumulates within any normal edge zone, only borings of organisms such as Lithophaga and Cliona can occur. To seaward, infaunal activity has produced structures which include mound systems and burrow entrances similar in all superficial respects to those ascribed to Callianassa species on the reef flats. These are common in certain areas, occupying level sandy floors in front of the main reef slope, particularly within channels. It is emphasized, however, that the sediments are relatively stable since the channels themselves do not appear to be contemporary features (Braithwaite, 1971). Adjacent floors which bear coral banks, loose coralgal cobbles, or more mobile sands, are all devoid of normal burrowing organisms. BURROW MORPHOLOGY, ORGANISM ACTIVITY AND DISTRIBUTION
This section describes the morphology of the burrow systems and surface traces examined. Observations on behaviour are given where these bear on construction, while distribution is summarized in Fig. 1 and 2.
Cardisoma ¢arnifex and Sesarma Iongipes These genera produce burrows of similar form: Plate I, l shows a typical example. The aperture is sub-circular in plan and funnel-shaped, with a generally short taper. The entry shaft varies from sub-vertical to nearly horizontal. It is not differentiated from the burrow proper, which is always a single tube, gently curving, or rather irregular with segments of spiral arcs reversing direction. Less regular geometries are particularly associated with stony or otherwise difficult ground. Burrow dimensions are related to the size of the occupant, and those of Sesarma are generally smaller (3-6 cm in diameter) than those of Cardisoma which may be up to 10 cm in diameter and over 1.5 m deep. Both burrows are normally roughly cylindrical but in one Cardisoma burrow-cast the lowest 35-40 cm was flattened, with angular margins which suggested that it represented the residual of a floored cavity comparable with those described by Shinn (1968). Excavation appears to be by use of the chelae, but these leave no obvious traces on the burrow walls. The sediment removed may be randomly strewn on the surrounding surface or carefully placed, in the form of discrete irregular pellets, to form a ridge several centimetres high around the burrow entrance. The typical habitat for Sesarma longipes is high among mangrove roots or in equivalent positions elsewhere in the lower supra-tidal zone. The entrance is Palaeogeogr., Palaeoelimatol., Palaeoecol., 11
(1972) 265 285
272 PLATE
C. J. R. BRA1THWAITE AND M. R. TALBOT I
1. Cardisoma burrow cast in concrete, note the crab in position adjacent to the scale. 2a, b, c. Variation in burrows produced by Ocypode sp. J-shape in 2a and spiral in 2c. 3a, c. Uca burrows; b. d. Macropthalma. There is considerable variation within this group. Palaeogeogr., Palaeoclimat ol., Palaeoecol., 11 (1972) 265-285
CRUSTACEAN BURROWS IN SEYCHELLES
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normally just above the level of high-water spring tides, with the burrow extending below the water table. The ground is appreciably more moist than that usually favoured by Cardisoma although burrows of this genus have also been observed with their entrances flooded at high water. Both genera use the burrow primarily as a refuge. Sesarma at least is willing to enter dwellings other than its own when not forcibly prevented from doing so, and it seems likely that individual burrows may have a succession of occupants. This would favour preservation by the compacting effect of continued use on wall structure and by limiting destruction by re-working.
Uca annulipes and Macropthalma parvimanus These broadly sympatric genera construct similar burrows (Plate I, 3a-d). They are common in muddy, high-intertidal areas, particularly those bordering mangroves. The aperture is typically a sub-circular hole, although two openings are occasionally present. The burrow is a single tube which may be a simple curve, a series of irregular curves or, more rarely, a spiral. The angle of entry is also variable, ranging from sub-vertical to near horizontal. The tube is usually constricted close to the aperture, opening out below to as much as two and a half times this minimum diameter. The end of the tunnel may be further inflated into a chamber, which is sometimes bilobed. The termination lies at a maximum depth of about 30 cm below the sediment surface. Neither of these burrows has any obvious added lining, but since they occur in muddy sediment, compaction of the walls would serve a similar function. The horizontal portions of some burrows have a flattened profile so that sections are oval rather than circular. It is not clear whether this is a constructional feature or the result of addition of sediment to the floor. The entrance is often surrounded by a peripheral ridge of sediment, 1-3 cm high and about three times the hole diameter. This is constructed as the tide falls from material which has been used to plug the opening during high water. Uca may maintain an air bubble during this period, since the first sign of activity, while the entrance is still covered, is a disturbance of the mud and release of a series of bubbles. Irregular friable pellets of sediment are strewn about the aperture during subsequent excavation. The general behaviour of Uca has been well described by Crane (1957). Additional geologically significant activities have been noted on Aldabra. Here much of the interior of the islet of Moustique is a high-intertidal sand flat, only flooded at spring tides. The surface is partly lithified by evaporation and in some areas contains large numbers of Uca sp. burrows. The crabs pick up fragments of cemented crust and carry them as far from their burrow entrances as neighbours will allow. Since the whole population is performing the same action the net result is that each burrow is surrounded by a neat polygonal wall shared with those Palaeogeogr., Palaeoelimatol., Palaeoecol., 11 (1972) 265-285
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c. J. R. BRAITHWAITEAND M. R. TALBOT
about it and completing a striking pattern of polygons of 20-30 cm diameter extending over a wide area (Fig.5). Both Uca and Macropthalma are scavengers, using their burrows as dwellings and retreats, vacating them at low water to forage in their immediate territory.
Fig.5. Regular polygons produced by Uca on intertidal flat on Moustique, Aldabra. Hammer, giving scale, is approximately 40 cm in length.
Ocypode ceratopthalma and O. cordimanus Both are beach dwellers, their distribution in general being limited by the inability of the crab to excavate coarse debris. O. cordimanus inhabits the beach crest and produces permanent burrows up to and above high water spr'ngs, while O. ceratopthalma lives lower on the beach slope. Although spatially separated, tile burrows of these two species show no clearly defined differences (Plate I, 2a-c). The circular, funnel-shaped aperture leads into an unlined, sub-circular or oval-sectioned tunnel 1-6 cm in diameter, Palaeogeogr., Palaeoclimatol.,Palaeoecol., 11 (1972)265-285
CRUSTACEAN BURROWS IN SEYCHELLES
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entering the sediment at a high angle. Typical burrows are unbranched, straight, gently curved, or occasionally spiral tubes, reaching to depths of between 10 and 50 cm. More acutely-curved burrows also occur, basically of a J-shape with a short vertical branch down from the deepest part of the curve (Plate I, 2a), but the function of this branch is not known. The termination of the main shaft has clearly acted as a refuge in several of the burrows investigated, the crab having retreated into it and sealed off the passage. In a few examples the termination is slightly inflated. No multiple branched forms have been recovered (compare Hayasaka, 1935). Farrow (1971) has suggested that the burrows of Ocypode are dimorphic, the male producing a spiral and the female U or J-shapes. The aperture is commonly obliterated at high water and re-excavated as the tide falls. Two kinds of pellet occur around the entrance. One is friable, subspherical, and up to 2 cm diameter, and is formed simply from material removed from the burrow. These may be scattered over an area of about a square metre, either isolated or in small groups linked to the entrance by surface traces (footprints). During periods of more intense activity a small ramp-like pyramid is built by the crab which carries pellets up the ramp and throws them over the slip-off face. Smaller and more coherent pellets are associated with surface "grazing" traces. Both genera appear to take advantage of the filtering action of the beach sediment. Organic particles carried by the swash are deposited as the water drains through the surface. Concentrations are normally low, and O. ceratopthalma feeds
Fig.6. Surface grazing traces and pellets produced by Ocypode. Lower beach slope Anse ~_ la Mouche. Tape marked in centimetres.
Palaeogeogr., Palaeoclimatol., Palaeoecol., I I (1972) 265-285
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by rushing down the beach between swash or by random walks at low tide to retrieve such particles. Where concentrations are higher, as on Anse ~ la Mouche, systematic "grazing" of the sediment by this species has been observed. The chelae are used to scrape the surface, forming pairs of short parallel furrows. The sand removed is manipulated by the mouth parts, presumably to separate food particles, and the surplus extruded as pellets. These are sub-spherical or rod-like bodies up to 1 cm in length, their surfaces are smooth and coherent and probably mucus bound (pellets and parallel grooves are illustrated in Fig.6). Faecal pellets were not noted but are presumably sometimes also present. Callianassa sp. 1 A number of solitary shrimps were trapped within resin casts of burrows of this group. These have been tentatively identified as specimens of a Callianassid, here referred to as Callianassa sp. 1. Two separate genera may be involved since the burrows, although similar in design, fall into two distinct groups clustered about mean diameters of 4.8 and 7 m m (examples of both can be seen in Plate II). In the following account these are treated as a single morphotype, the range of dimensions given covering both groups. Apertures are normally paired (Plate II, 1); the inhalent opening lies in a conical depression 1-2 cm deep and 2-3 cm in diameter, while the exhalent tube terminates in a conical mound, of sediment and faecal pellets, of similar dimensions. These surface features are typical of the rippled sand zone in more stable areas and of the inner grass zone. The burrow is characteristically a series of sub-spherical rooms joined by a tunnel 3-8 m m in diameter sloping downwards at 15-30 o (Plate II, 2a-c). Up to six rooms may be linked in this way, to a total depth of approximately 30 cm. In the most c o m m o n system the burrow consistently changes direction through 60 ° of arc at each r o o m so that the series forms a roughly equilateral triangular spiral (Plate II, 3a-c). In others this angle is not uniform or consistent in direction and the spiral pattern is lost. Up to four short galleries leave each room, but only rarely are all of these open. In a few examples a gallery may swell at its termination to form a subsidiary r o o m from which further galleries arise. Open galleries tend to occur at lower levels and are probably functional while those above, having been abandoned, collapse. This pattern is, however, a response to one particular erosion/deposition balance and may not be repeated where rates of sedimentation or sediment thickness are different. The main shaft to the surface is vertical or steeply inclined, connecting with either the first or second room. In the latter case a short blind-ending tube, which probably represents an abandoned entrance, projects upwards from the first room. In one specimen this had remained open so that the burrow had two entrances. The exhalent tube is much narrower (2-3 mm), vertical, and linked to the Palaeogeogr., Palaeoclimatol., PalaeoecoL, 11
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PLATE II
Resin casts of burrows Callianassa sp.1. 1, 2a, b, c. Lateral views. Note the frequent directional changes and the excurrent shafts visible in I and on the right hand side of 2b. 3a, b, c. Views from below looking up through the lumen of the spiral indicating its triangular character. 3b is from the larger sub-group noted in the text. 4. Resin cast of Pseudosquilla burrow.
Palaeogeogr., Palaeoclimatol., Palaeoecol., 11 (1972) 265-285
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C. J. R. BRA1THWAITE AND M. R. TALBOT
tapered end of a gallery from one of the first three rooms. The shrimp seems too large to produce this exhalent shaft by simple excavation, but the method of construction is not known. One of the larger diameter burrow casts imprisoned within it a small unidentified crab. The exact nature of the relationship with the occupying shrimp is not known but may be commensal.
Callianassa sp.2 The tentative identification of a species of Callianassa is based partly upon glimpses of these elusive animals disappearing down burrows, and partly upon the similarity between burrow casts recovered and those described by Shinn (1968) and Farrow (1971) and ascribed to this genus. These structures are common in grass-zone sediments and in the margins of adjacent rippled sands. The burrow system is similar in construction to that of Callianassa sp. 1 and consists of a series of rooms (cf. Shinn, 1968) linked by a gently sloping, roughly spiral tunnel (Plate lII, a, b). The rooms are usually slightly inflated to about 7 cm diameter, and between one and four sub-horizontal galleries radiate from each. These may also be inflated but are commonly of similar dimensions to the main tunnel, 1.5-2 crrl in diameter and 5-10 cm long. Most horizontal galleries are open, although small amounts of loose sediment may rest on floors, but one of those in the cast illustrated (Plate III) is stuffed with a mixture of coarse sediment and matted Thalassia leaves. Similar packings have been described by Shinn (1968). Two distinct apertures are present. The larger of these, functioning as an entrance, is normally a circular opening on a level sediment surface or occupying a slight depression. It is funnel-shaped tapering from about 8 cm diameter at the surface to 1.5-2 cm, the normal tube width, 2-3 cm below. The shaft beneath is initially vertical but curves before entering the first room at a low angle 10-15 cm below the surface. The exhalent aperture is 3-5 mm diameter and, in low energy environments, is typically surrounded by a volcano-like mound of sediment about 35 cm in diameter and 10-15 cm high. More persistent composite structures 30 cm high and over a metre in length occur locally, and where mounds are emersed at low tide, as the larger ones commonly are, their margins are often marked by miniature strand-lines. Such structures, if preserved, would form valuable sea-level indicators. The mounds are built up of sediment which is periodically discharged from the exhalent aperture. This is linked by a slender (3-5 mm) shaft to the tapered end of an inflated gallery from the first room. In both this burrow and that of Callianassa sp.1 the shape of this gallery is distinctive and corresponds closely to that of the posterior of the occupant. Shrimps generally maintain an anteroposterior current by movements of the swimmerets and by taking up position in the inflation this activity could be used to circulate fresh water in the upper galleries and flush faecal pellets from the system. The latter are cylindrical, 5-8 mm Palaeogeogr., PalaeoclimatoL, Palaeoecol., 11
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PLATE 111
A.
B.
T h e lateral view of a resin cast o f the b u r r o w system of Callianassa sp.2. T h e exhalent tube is missing but the tapered gallery leading to it is visible to the right of the entry shaft. Three successive levels of descent are present. T h e view from above indicates the spiral nature of the system and the radiating arrangemenl of galleries above rooms. Note the association of specimens of Quidnipa
PalaeogeoF, r., Palaeoclimatol., Palaeoecol., 11 (1972) 265 285
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C. J. R. BRAITHWAITE AND M. R. TALBOT
long and 1-3 mm in diameter, and are commonly strewn on the lower slopes of the mound. All tunnels and galleries are lined with a fine-grained sediment consisting of silt-size quartz and carbonate fragments and, probably, organic material. The lining in galleries is usually 1-2 mm thick but may be as much as 6 mm in the main tunnel. When removed from the sediment it is normally coherent and slightly flexible, but is sometimes hard and brittle. The inner surface is smooth, without scratch marks, while the outer is irregular. This is chiefly the result of packing of the lining between coarser grains, but a knobby pelleted structure is sometimes present (though less conspicuous than that described by Weimer and Hoyt, 1964). The shafts and chambers are excavated oversize and then stabilized by plastering sediment along the walls, often producing a concentric banding. In some preserved burrows two or three distinct layers occur within the original perimeter, but it is not clear whether such additions to the initial lining are part of a deliberate policy of accretion or are simply a re-distribution of loose sediment finding its way into the system. Most of the burrow is presumably excavated by digging using the first few pairs of legs (Pohl, 1946). This is unlikely for the exhalent tube since it is smaller than the shrimp, but the method of construction is not known. Little is known either of the life habits of Callianassa or of the functions of its complex burrow system. The general layout suggests that it served both as a secure dwelling and as a means of sediment exploitation. This may, however, be an over-simplification since the lining would prevent access to the surrounding sediment other than during excavation. MacGinitie (1934) and Pohl (1946) demonstrated that the food of some North American callianassids consisted of micro-organisms and organic debris obtained by actively sifting sediment. This seems reason enough for the systematic, mine-like construction of the burrow, and MacGinitie (1934) suggests that the system is extended as particular areas are exhausted. The presence of galleries stuffed with a mixture of plant debris and sediment raises the interesting possibility that Callianassa may actually culture bacteria by permitting organic material to decompose within the burrow. In several resin casts living specimens of the bivalve Quidnipagus were trapped in porous sediment adhering to gallery terminations. The positions of the siphons in life are not known, but there can be little doubt that this location was of some benefit to the molluscs. Callianassa sp.2 and the mounds it produces is particularly abundant in grass-zone sediments, but mound systems are also common in certain areas of the pro-reef floor. These have not been investigated in detail but they closely resemble those ascribed to Callianassa sp.2. Depths of occurrence vary from a few metres in parts of the Port Victoria area to at least 30 m in North-West Bay. The maximum habitable depth is not known. In marginal channels off Cerf Passage in calm waters 10-15 m deep, mounds rise 40-50 cm above the floor. On shallower and
Palaeogeogr., Palaeoelimatol., Palaeoecol., 11 (1972) 265-285
CRUSTACEAN BURROWS IN SEYCHELLES
28l
more open floors exposed to larger waves produced by the southeast monsoon, mounds decrease in size and burrows may occur without them. Beyond a threshold of sediment stability no burrows are present. Farrow (1971) noted morphological variations in casts of burrows of Callianassa which he related to bed mobility and sediment thickness. Present evidence suggests that differences in mound sizes and indications in casts of the formation of new entrances are most likely to be the result of sediment movement. They are normally paralleled by a progressive change in the sorting characteristics of the sediment. Variations in gross morphology are more readily related to the depth of sediment available for exploitation.
Alpheus sp. This shrimp is c o m m o n in lower-energy open sand areas. The burrows have not been investigated with resin techniques. The entrance has a U-shaped rim of small gravel-sized bioclasts, the burrow passing between the open ends and beneath the lower loop of the U. The angle of entry is low (10-15 °) and thus differs from that of other shrimps studied in that an initial tunnel is formed rather than a shaft. The burrow is excavated by use of the chelae, small piles of sand being carried forth on them and dumped away from the entrance. Larger bioclasts are placed with some precision about the aperture, presumably for their value in maintaining stability. No distinctive mound system is formed. Farrow (1971) has described Alpheus burrows which are similar and noted the commensally-associated gobiid fish, one of which occupies almost every burrow entrance.
Pseudosquilla ciliata One cast from a burrow produced by a stomatopod, thought to be Pseudosquilla ciliata, was recovered. This is probably not a true reflection of the density and distribution of this organism, regarded by Taylor (1968) as a common inhabitant of the grass zone and similar low energy intertidal areas. Surface recognition of the burrow is difficult since the aperture has similar dimensions and general appearance to that of Callianassa sp.2 burrows. In the specimen collected (Plate II, 4), the aperture was a well-defined circular opening 2.5 cm in diameter, lying almost flush with the surface. The shaft expands below to 4-5 cm diameter, so that in the cast the entrance appears as a constriction. The burrow is a simple cylindrical tube 74 cm in length. Descending as a vertical shaft for about 15 cm, it curves to lie almost horizontal 20 cm below the surface, rising again slightly in the last 10-15 cm. The termination is smoothly rounded and slightly inflated. No mud lining is present, but sediment adhering to the cast, particularly on the lower surface, is rather nodular. Nodules are roughly ovoid and about 1 cm in length; they are not well defined and are probably not faecal.
Palaeogeogr., Palaeoclimatol., Palaeoecol., 11 (1972) 265 285
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C. J. R. B R A I T H W A I T E A N D M. R. T A L B O T
A few fragments of Thalassia leaves are included within the immediate wall. These may be significant since Taylor (1968) has noted the presence of large numbers of Thalassia leaves and similar organic material on the walls of Pseudosquilla burrows. DISCUSSION A N D C O N C L U S I O N S
Contemporary burrows The burrows described above fall into two broad morphological groups. Crab burrows are in general large-diameter simple curves lacking branches, while shrimp burrows are usually of smaller diameter, multibranched, and with frequent directional changes. This architectural separation reflects a divergence in function, the crab burrows are largely dwellings, while those of the shrimp serve both as shelter and as a means of sediment exploitation. The palaeoecological significance of this grouping is that it corresponds to an imprecise but nevertheless real boundary between terrestrial and marine environments. If the specific identity of the burrow were known further refinement should be possible, for example, the Cardisoma/Sesarma group, the Uca/Macropthalma group or Ocypode spp. could be used to differentiate between supra-tidal environments, upper intertidal low-energy areas or upper beaches. Distribution of burrows is summarized in Fig.2 but it has not been possible to test their value in precise boundary definition. In the sub-tidal zones a more diverse infauna is present including Callianassa spp., Alpheus and Pseudosquilla. These have an overlapping distribution which bears little relationship to the biofacies zonation of the reef. Some environmental information may, nevertheless, be gained from them. The mounds associated with Callianassa sp.2 indicate that the burrow was formed in a low-energy environment, while burrows without mounds suggest a higher energy but still stable condition. Preserved mounds have not been observed during this study but are certainly present in some ancient sediments such as the Oxfordian of southern England. The disposition of occupied and vacant regions within the burrow and their relationship to the sediment surface provides evidence of erosive or depositional variation of that surface. Gross morphology can give some indication of the thickness of workable sediment.
Preservation Adequate preservation is a primary necessity for crustacean burrows or body fossils to be of geological value. In contrast to their high density in many modern environments, neither decapods nor stomatopods are common as fossils. Research on the Pleistocene limestones of Aldabra has so far produced few crabs and no shrimps. The carapace of both is, of course, fragile in comparison to the shells of most common molluscs, but it might be supposed that a burrowing habit would
Palaeogeogr., Palaeoclimatol.,Palaeoecol., 11 (1972) 265-285
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provide a potentially favourable environment for fossilization. The advantages are clearly insufficient, and remains have presumably either been fragmented beyond recognition or destroyed by diagenetic changes. That being so, even greater emphasis must be placed upon their trace fossils. Some burrows, by virtue of their situation or construction, seem more likely to be preserved than others. For example, the great depth of penetration of some Callianassa burrows might place them beyond the limits of normal sediment removal and reworking, but the depth of penetration is itself a function of sediment thickness and a lack of reworking, so the relationship is not a simple one. The linings to Callianassa sp.2 burrows are often quite hard, even in occupied systems, and are thus relatively resistant to early compaction, so that Shinn (1968) was able to demonstrate open burrows at more than twice their normal inhabited depth. Linings, by their differing porosity and (probably) chemistry, may also act as loci for early lithification. This suggestion is based partly on the observations of Taylor and llling (1969), who reported recently-cemented sediment closely associated with burrows in the Persian Gulf, but cemented burrows with multiple linings have been found in the Seychelles. These are unfortunately rare and their origin is uncertain. Crab burrows do not have the advantage of linings, although the walls may be strengthened by surface packing and binding with mucus or similar substances. In addition, the high intertidal and low supra-tidal position which they occupy seems particularly vulnerable to erosion. It seems that to be preserved they must either be filled and buried during a prograding phase in sedimentation, or cemented at a very early stage in order to resist compaction. The instability of the beach environment suggests that preservation of Ocypode burrows should be a particularly unlikely event. Nevertheless, open burrows ascribed to this genus are present in beach-rock on Mab6 (at Bel Ombre) and on Aldabra (Dune D'Messe). It seems probable that these were re-excavated rather than preserved open, but recent fillings are not differentiated from host sediment in ways which might have encouraged selective erosion. Well-preserved burrows, believed to have been formed by crabs, are present at two localities in the Aldabra Pleistocene. Low angle tunnels of 3-5 cm diameter with few branches occur at Passe Horeau. These resemble some Ocypode burrows and the fact that they are overlain by well-sorted sediments containing beachdwelling limpets (identified by J. D. Taylor) lends support to this interpretation. Similar large diameter systems with few branches are present at the eastern end of Picard. These occur in a unit which overlies calcarenites containing large numbers of small diameter burrows showing the same frequent angular directional changes as those formed by the smaller callianassids. The sequence is interpreted as recording the seaward accretion of a beach over shrimp-burrowed sand flats. The picture of an emergent sand cay is completed by the associated sediments which contain terrestrial molluscs, tortoise bones and plant rootlet impressions. Palaeogeogr., Palaeoclimatol., Palaeoecol., I I (1972)265 285
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Other probable crustacean burrows occur east of Passe Gionnet. These consist of a vertically disposed system of tubes about 3 cm in diameter with bulbous turning areas, but because of interference the extent to which they branch is not known. They resemble in some respects the casts of burrows of the mole crab Albunea figured by Farrow (1971) and are unlike those of any living forms described here. Associated with them are smaller burrows of general callianassid form. CONCLUSIONS
The lack of specific identity of individual burrows in this study and the broadcast nature of morphological groups places a limit on their use as environmental indicators. It is, however, clear from our observations on Mah6, tested on a limited scale in the Pleistocene of Aldabra, that crustacean burrows are of value, both individually and conjointly, for general environmental definition and the elaboration of themes suggested by other evidence. ACKNOWLEDGEMENTS
This study forms part of a larger investigation of carbonate sediments in the Seychelles financed by The Carnegie Trust for the Universities of Scotland and the Natural Environment Research Council. The work on Aldabra was sponsored by The Royal Society. The University of Dundee generously granted leave of absence. We are particularly indebted to Mr. G. Lionnet, Director of the Department of Agriculture of the Seychelles, and to his staff for the help given to us in support of our field work. REFERENCES Baker, B. H., 1963. Geology and mineral resources of the Seychelles archipelago. GeoL Surv. Kenya, Mem., 3" 1-140. Braithwaite, C. J. R., 1971. Seychelles reefs: structure and development. In: D. R. Stoddart and C. M. Yonge (Editors), Regional Variation in Indian Ocean Coral Reefs. Symp., Zool. Soc. Lond., 28: 39-63. Crane, J., 1957. Basic patterns of display in fiddler crabs (Ocypodidae: gen. Uca). Zoologica, 42: 69-82. Farrow, G. E., 1971. Back-reef and lagoonal environments of Aldabra Atoll, distinguished by their crustacean burrows. In: D. R. Stoddart and C. M. Yonge (Editors), Regional Variation in Indian Ocean Coral Reefs. Symp., Zool. Soc. Lond., 28: 455-500. Frey, R. W., 1970. Environmental significance of Recent marine lebensspuren near Beaufort, North Carolina. J. Palaeontol., 44: 507-519. Hayasaka, I., 1935. The burrowing activities of certain crabs and their geologic significance. Am. Midland Naturalist, 16: 99-103. Lewis, M. S., 1969. Sedimentary environments and unconsolidated carbonate sediments of the fringing coral reefs of Mah6, Seychelles. Mar. Geol., 7: 95-127. MacGinitie, G. E., 1934. The natural history of Callianassa californiensis Dana. Am. Midland Naturalist, 15: 166-177.
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McMullen, R. M. and Allen, J. R. L., 1964. Preservation of sedimentary structures in wet unconsolidated sands using polyester resins. Mar. Geol., I : 88 97. Piggot, C. J., 1968. A soil survey of the Seychelles. Minist. Overseas Dev. Tech. Bull., 2:1 89. Pohl, M. l., 1946. Ecological observations on Callianassa major Say. at Beaufort, North Carolina. Ecology, 27:71 80. Rosen, B. R., 1971. Principal features of reef coral ecology in shallow water environments of Mah6, Seychelles. In: D. R. Stoddart and C. M. Yonge (Editors), Regional Variation in Indian Ocean Coral Reefs. Symp., Zool. Soe. Lond., 28:163 183. Sauer, J. D., 1967. Plants and Man on the Seychelles Coast. Univ. Wisc. Press, Madison, Wise., pp.l-132. Shinn, E. A., 1968. Burrowing in recent lime sediments of Florida and the Bahamas. J. Palaeontol., 42(4): 879-894. Taylor, J. C. M. and Illing, L. V., 1969. Holocene intertidal calcium carbonate cementation, Qatar, Persian Gulf. Sedimentology, 12: 69-107. Taylor, J. D., 1968. Coral reef and associated invertebrate communities (mainly molluscan) around Mah6, Seychelles. Philos. Trans. R. Soc. Lond., Ser. B., 254(793): 129-206. Taylor, J. D. and Lewis, M. S., 1970. The flora, fauna and sediments of the marine grass beds of Mah6, Seychelles. J. Nat. Hist., 4: 199-220. Weimer, R. J. and Hoyt, J. H., 1964. Burrows of Callianassa major Say. geologic indicators of littoral and shallow marine environments. J. Palaeontol., 38: 761-767.
Palaeogeogr., Palaeoclimatol., Palaeoecol., I 1 (1972) 265-285
CRUSTACEAN BURROWS IN T H E SEYCHELLES, I N D I A N OCEAN C. J. R. BRA1THWA1TE AND M. R. TALBOT
Department of Geology, The University, Dundee, Scotland (Great Britain (Received April 24, 1971~ (Resubmitted February 29, 1972)
ABSTRACT Braithwaite, C. J. R. and Talbot, M. R., 1972. Crustacean burrows in the Seychelles, Indian Ocean. Palaeogeogr., Palaeoelimatol., Palaeoecol., 11: 265-285. Crustacean burrows are common in Recent sediments associated with fringing reefs around Mah6, Seychelles. Burrow morphology is related 1o function. Unbranched burrows produced by the crabs Cardisoma, Sesarma, Oeypode spp., Uca and Macropthalma are primarily dwellings, whereas multiramous systems constructed by the shrimps Callianassa spp. serve in addition as a means of sediment exploitation. The distribution of morphotypes shows only general correspondence to recognised biofacies zones, and well-preserved burrows from Pleistocene limestones on Aldabra demonstrate that, while providing useful corroborative evidence, burrows are of limited value as primary zone indicators.
INTRODUCTION
Growing interest in trace fossils has stimulated the study of modern burrows and the organisms producing them. Among these, crustacean burrows are particularly distinctive (Hayasaka, 1935; Weimer and Hoyt, 1964; Shinn, 1968; and Frey, 1970) and are of potential geological value in reconstructing ancient environments. The present study has centred on Mah6, principal island of the Seychelles group, where burrows are common in Recent carbonate sediments associated with fringing reefs. The burrow systems and surface traces resulting from the activities of six species of crab, four shrimps and a stomatopod have been investigated: two of them are described here for the first time. The structure of burrows reflects function and both dwelling and sediment-exploiting systems are present. Distribution of morphotypes only broadly corresponds to biofacies zones defined by other criteria. The problems and circumstances of preservation of these structures have been examined and their value as palaeoecological indicators tested on a limited scale in Pleistocene limestones on Aldabra. This work is part of a larger study of the sediments and reef environments Palaeogeogr., Palaeoclimatol., Palaeoecol., 11 (1972)265-285
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of the Seychelles. Earlier results of this and details of the geology, ecology and physiography of Mah6 are to be found in papers by Taylor (1968), Lewis (1969), Braithwaite (1971) and Rosen (1971).
METHODS
The technique used in saturated sediment resembled that described by Shinn (1968) and involved the production of epoxy resin replicas of burrows. This proved impractical on dry material where high permeabilities resulted in resin loss through burrow walls. Under these conditions portland cement was used as a casting medium. The method was developed in discussion with W. J. Kennedy and gave good results, preserving additional information on laminae around burrows. Resin impregnated cores (McMullen and Allen, 1964) occasionally produced useful replicas of burrows, but were chiefly of value in revealing other structures. Identification of crustaceans is based primarily on the work of Taylor (1968, and personal communications, 1965 et seq.). The authors, however, accept responsibility for any errors. DESCRIPTIONS OF ENVIRONMENTS
The main features of reef environments in the Seychelles have been described in a number of previous papers (noted above). Some new data are given here but in general this kind of information will be limited to providing reference to the sedimentological and ecological framework in which these organisms and their biogenic structures are found. 1
2
3
4
f
5
[.~ M a n g r o v e
6
7
8
9
10
areas
HWME Supra -'tidal
Rippled sands
Beach
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LWMS
~ro\
A I metre V
?3m
?
reef floor ?30m
Fig.1. The distribution of the crustacean burrows described in relation to tide levels and environments. Note that while the vertical axis represents a real scale, divisions of the horizontal are arbitrary. 1 ~ C a r d i s o m a carn(fex; 2 ~ S e s a r m a longipes; 3 ~ O c y p o d e ceratopthalma; 4 = Ocypode p a r v i m a n u s ; 7 = Callianassa sp.
cordimanus; 5 = Uca annulipes; 6 ~ Macropthalma ~ A l p h e u s sp.; 9 = P s e u d o s q u i l l a ciliata; 10 ~ C a l l i a n a s s a
1; 8
sp. 2. Palaeogeogr., Palaeoelimatol., Palaeoeeol.,
11 (1972) 265-285
O0 ~h
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\ . Algally c r u s t e d bare surfaces.
\
~
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~
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as in channels.
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~
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.
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MUddy f l a t s within and m a r g i n a l to m a n g r o v e delta as w i t h Uca annulipe5 and M a c r o p t h a l m a parvimanus.
M a n g r o v e areas w i t h S e s a r m a Iongi_peS_ a m o n g r o o t s a b o v e H.W.M.
Fig.2. The general spatia! relationships o f biofacies on the reefs o f Mah6 and distribution of the crustacean burrows described.
P l a t f o r m ranging in w i d t h f r o m 0 - 1 , 0 0 0 metres.
Low slooe.
etc..
Sargassum z o n e weea concentrated on calcareous algae-encrusted r i d g e s . Acrop.ora ~ . / ~oclllqpora ~ n - a e - m e a n d r i n a ¢ ~ Calcareous algae. ~ .
Mtgratmg
Grass zone dora nated by \ ~ , , ~_~~ L ~ ~ Thallassla. b u r r o w s of bivalves, ~ ~" Callianassa spp. 1 and 2 and ~ " ~ Pseudosqullla. ~-~.~
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Beach, laminated clean washed sands, Ocypode cordimanus abo and (3. c~atO--Et',ha_ ~ .
Dry supratidal a r e a w i t h Cardisoma carnifex and Ss~arma Iong ipes at l o w e r levels •
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268
C. J. R. BRAITHWAITE AND M. R. TALBOT
Taylor (1968) has described the ecological zonation in the area which we have adopted. This can be paraphrased as: (1) supra-tidal (dry) areas; (2) mangrove environments; (3) beaches; (4) rippled sand zone; (5) grass beds; (6) open sand area; (7) sargassum zone with algal cobbles; (8) bioconstructed reef edge with coral-defined sub-zone; (9) reef front or reef slope; (10) pro-reef floor. The distribution of burrowing crustaceans in relation to these zones is shown in Fig.l, while Fig.2 illustrates their joint control by sea level. It can be seen that only zones 1-6 and 10 are significant as burrowed areas.
Supra-tidal (dry) areas Although crab burrows are occasionally associated with low-level laterites, their main occurrence is in the so-called "plateaux". These are essentially flat areas of uncertain origin which occur within a few metres of high-water mark behind almost all bays on the island. They are particularly wide on Anse aux Pins on the east coast and Grande Anse and Anse & la Mouche on the west. The greater part of their sediments are bioclastic sands but these become quartzose and include lateritic clays on landward margins and close to streams (Baker, 1963; Piggot, 1968; and Lewis, 1969). On Mah6 the plateaux form the main cultivated areas and the natural vegetation (Sauer, 1967), soil structure, and probably crab distribution have been modified. The seaward fringes of these areas are occupied by the crabs Cardisoma carnifex and Sesarma longipes. On adjacent islands Birgus latro is also present but on Mah6 this is now probably extinct.
Mangrove environments These merge laterally with the plateaux and are associated particularly with high deltaic salients constructed by freshwater streams. The largest area in which mangroves (Rhizophora, Bruguiera, Ceriops and others) occur now is north of Port Glaud on the west coast, but they appear in small groups at most river mouths, and were once extensive on the east coast around Port Victoria. Here the swamp sediments remained until recently as high muddy areas behind the harbour reef edges. The sediments are generally poorly sorted and characterized by high percentages of quartz and other minerals derived from the lateritized granite. Three crabs are associated with this environment: Sesarma longipes, Uca annulipes and Macropthalma parvimanus.
Beaches In areas where a beach berm is developed, these range from gently-sloping, fine-grained, structures in low energy environments to coarse rudaceous aggregates with steep slopes formed in higher energy situations. Berms are absent from shores adjacent to stream mouths, and beaches are not usually present around rocky granite headlands. Two species of Ocypode are common in berms and foreslopes, Palaeogeogr., Palaeoelimatol., Palaeoecol.,
1 l (1972) 265-285
CRUSTACEAN BURROWS IN SEYCHELLES
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except where sediment instability or coarse grain sizes prevent effective burrowing. The lower margins of beaches are also occupied by rapid-burrowing bivalves, but only crab burrows have been identified in cores. Rippled sand zone The contact between the beach and the rippled sand zone below varies on reef flats from an abrupt step to a gradational slope, although sediments are usually distinct. The more abrupt changes normally occur in higher energy situations. The surface of this zone is typically marked by ripples with crests orientated parallel or slightly oblique to the shore, Ripple size varies; in high-energy areas such as Anse Forbans amplitude may be 10-15 cm and wavelength up to 1 m. Such mobile sands contain few burrows, and a certain level of stability is clearly necessary before they can be occupied. In more sheltered positions on the wider reefs (e.g., Anse aux Pins: up to 750 m) infaunal activity in the more stable substrates augments ripples with a wide variety of surface traces (Fig.3). Two species of Ca/lianassa and the stomatopod Pseudosquilla are active in this zone. With their traces are paired siphonal openings of bivalves together with simple cylindrical holes of millimetre dimensions, stellate feeding traces and " w o r m casts" produced by a number of organisms which include polychaetes and (probably) balanoglossids. Mucous-lined annelid tubes 1-2 mm in diameter are locally important in binding the sediment surface.
Fig.3. Surface of rippled sand zone on Anse aux Pins. Note the stellate annelid feeding trace and paired entrances to Callianassa sp.1 burrows. The coin is approximately 2.5 cm diameter. Palaeogeo~r., Palaeoclimatol., Palaeoecol., 11 (1972) 265-285
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C. J. R. BRAITHWAITE AND M. R. TALBOT
In exposed sites these sands may merge into those adjacent to the reef edge. Elsewhere they give way to extensive marine grass beds.
Grass beds These are characterized by more or less dense growths of Thalassia hemprichi and similar marine angiosperms. The area colonized depends, among other factors, upon reef width and exposure, and in some cases beds may be 200-300 m wide. A few corals are present (see Rosen, 1971) and detailed descriptions of the grass beds are given by Taylor and Lewis (1970). Within this zone a large proportion of the sediment volume close to the surface consists of burrow space (estimated 10-15~o). The principal burrowing crustaceans are two species of Callianassa, one of which constructs the mounds so characteristic of this environment on Anse aux Pins, Anse ~ la Mouche and similar areas (Fig.4), and Pseudosquilla. Infaunal molluscs (Taylor, 1968) and other burrowing organisms are common but appear to leave few recognizable traces.
Fig.4. Callianassa sp.2 m o u n d s on Anse ~. la Mouche. The h a m m e r handle is approximately 40 cm long. N o t e that m a n y m o u n d s in this area are large composite structures.
Open sand area Sediment in this outer sand zone is commonly thin, interdigitating with coarser cobble debris derived from the reef edge. Although rapid-burrowing bivalves and gastropods are present, no burrows have been identified on reef-flats such as Anse aux Pins. North of Port Victoria, however, extensive sand patches with scattered clumps of Enhalus and colonies of Favia and Galaxaea extend to the Palaeogeogr., Palaeoclimatol., Palaeoecol., 11 (1972) 265-285
CRUSTACEAN BURROWS 1N SEYCHELLES
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line of brown algae marking the reef edge and occur on top of more isolated pinnacles in front of the edge. These sediments characteristically contain burrows of a shrimp which has been tentatively identified as Alpheus sp.
Pro-reef.floor Since little loose sediment accumulates within any normal edge zone, only borings of organisms such as Lithophaga and Cliona can occur. To seaward, infaunal activity has produced structures which include mound systems and burrow entrances similar in all superficial respects to those ascribed to Callianassa species on the reef flats. These are common in certain areas, occupying level sandy floors in front of the main reef slope, particularly within channels. It is emphasized, however, that the sediments are relatively stable since the channels themselves do not appear to be contemporary features (Braithwaite, 1971). Adjacent floors which bear coral banks, loose coralgal cobbles, or more mobile sands, are all devoid of normal burrowing organisms. BURROW MORPHOLOGY, ORGANISM ACTIVITY AND DISTRIBUTION
This section describes the morphology of the burrow systems and surface traces examined. Observations on behaviour are given where these bear on construction, while distribution is summarized in Fig. 1 and 2.
Cardisoma ¢arnifex and Sesarma Iongipes These genera produce burrows of similar form: Plate I, l shows a typical example. The aperture is sub-circular in plan and funnel-shaped, with a generally short taper. The entry shaft varies from sub-vertical to nearly horizontal. It is not differentiated from the burrow proper, which is always a single tube, gently curving, or rather irregular with segments of spiral arcs reversing direction. Less regular geometries are particularly associated with stony or otherwise difficult ground. Burrow dimensions are related to the size of the occupant, and those of Sesarma are generally smaller (3-6 cm in diameter) than those of Cardisoma which may be up to 10 cm in diameter and over 1.5 m deep. Both burrows are normally roughly cylindrical but in one Cardisoma burrow-cast the lowest 35-40 cm was flattened, with angular margins which suggested that it represented the residual of a floored cavity comparable with those described by Shinn (1968). Excavation appears to be by use of the chelae, but these leave no obvious traces on the burrow walls. The sediment removed may be randomly strewn on the surrounding surface or carefully placed, in the form of discrete irregular pellets, to form a ridge several centimetres high around the burrow entrance. The typical habitat for Sesarma longipes is high among mangrove roots or in equivalent positions elsewhere in the lower supra-tidal zone. The entrance is Palaeogeogr., Palaeoelimatol., Palaeoecol., 11
(1972) 265 285
272 PLATE
C. J. R. BRA1THWAITE AND M. R. TALBOT I
1. Cardisoma burrow cast in concrete, note the crab in position adjacent to the scale. 2a, b, c. Variation in burrows produced by Ocypode sp. J-shape in 2a and spiral in 2c. 3a, c. Uca burrows; b. d. Macropthalma. There is considerable variation within this group. Palaeogeogr., Palaeoclimat ol., Palaeoecol., 11 (1972) 265-285
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normally just above the level of high-water spring tides, with the burrow extending below the water table. The ground is appreciably more moist than that usually favoured by Cardisoma although burrows of this genus have also been observed with their entrances flooded at high water. Both genera use the burrow primarily as a refuge. Sesarma at least is willing to enter dwellings other than its own when not forcibly prevented from doing so, and it seems likely that individual burrows may have a succession of occupants. This would favour preservation by the compacting effect of continued use on wall structure and by limiting destruction by re-working.
Uca annulipes and Macropthalma parvimanus These broadly sympatric genera construct similar burrows (Plate I, 3a-d). They are common in muddy, high-intertidal areas, particularly those bordering mangroves. The aperture is typically a sub-circular hole, although two openings are occasionally present. The burrow is a single tube which may be a simple curve, a series of irregular curves or, more rarely, a spiral. The angle of entry is also variable, ranging from sub-vertical to near horizontal. The tube is usually constricted close to the aperture, opening out below to as much as two and a half times this minimum diameter. The end of the tunnel may be further inflated into a chamber, which is sometimes bilobed. The termination lies at a maximum depth of about 30 cm below the sediment surface. Neither of these burrows has any obvious added lining, but since they occur in muddy sediment, compaction of the walls would serve a similar function. The horizontal portions of some burrows have a flattened profile so that sections are oval rather than circular. It is not clear whether this is a constructional feature or the result of addition of sediment to the floor. The entrance is often surrounded by a peripheral ridge of sediment, 1-3 cm high and about three times the hole diameter. This is constructed as the tide falls from material which has been used to plug the opening during high water. Uca may maintain an air bubble during this period, since the first sign of activity, while the entrance is still covered, is a disturbance of the mud and release of a series of bubbles. Irregular friable pellets of sediment are strewn about the aperture during subsequent excavation. The general behaviour of Uca has been well described by Crane (1957). Additional geologically significant activities have been noted on Aldabra. Here much of the interior of the islet of Moustique is a high-intertidal sand flat, only flooded at spring tides. The surface is partly lithified by evaporation and in some areas contains large numbers of Uca sp. burrows. The crabs pick up fragments of cemented crust and carry them as far from their burrow entrances as neighbours will allow. Since the whole population is performing the same action the net result is that each burrow is surrounded by a neat polygonal wall shared with those Palaeogeogr., Palaeoelimatol., Palaeoecol., 11 (1972) 265-285
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about it and completing a striking pattern of polygons of 20-30 cm diameter extending over a wide area (Fig.5). Both Uca and Macropthalma are scavengers, using their burrows as dwellings and retreats, vacating them at low water to forage in their immediate territory.
Fig.5. Regular polygons produced by Uca on intertidal flat on Moustique, Aldabra. Hammer, giving scale, is approximately 40 cm in length.
Ocypode ceratopthalma and O. cordimanus Both are beach dwellers, their distribution in general being limited by the inability of the crab to excavate coarse debris. O. cordimanus inhabits the beach crest and produces permanent burrows up to and above high water spr'ngs, while O. ceratopthalma lives lower on the beach slope. Although spatially separated, tile burrows of these two species show no clearly defined differences (Plate I, 2a-c). The circular, funnel-shaped aperture leads into an unlined, sub-circular or oval-sectioned tunnel 1-6 cm in diameter, Palaeogeogr., Palaeoclimatol.,Palaeoecol., 11 (1972)265-285
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entering the sediment at a high angle. Typical burrows are unbranched, straight, gently curved, or occasionally spiral tubes, reaching to depths of between 10 and 50 cm. More acutely-curved burrows also occur, basically of a J-shape with a short vertical branch down from the deepest part of the curve (Plate I, 2a), but the function of this branch is not known. The termination of the main shaft has clearly acted as a refuge in several of the burrows investigated, the crab having retreated into it and sealed off the passage. In a few examples the termination is slightly inflated. No multiple branched forms have been recovered (compare Hayasaka, 1935). Farrow (1971) has suggested that the burrows of Ocypode are dimorphic, the male producing a spiral and the female U or J-shapes. The aperture is commonly obliterated at high water and re-excavated as the tide falls. Two kinds of pellet occur around the entrance. One is friable, subspherical, and up to 2 cm diameter, and is formed simply from material removed from the burrow. These may be scattered over an area of about a square metre, either isolated or in small groups linked to the entrance by surface traces (footprints). During periods of more intense activity a small ramp-like pyramid is built by the crab which carries pellets up the ramp and throws them over the slip-off face. Smaller and more coherent pellets are associated with surface "grazing" traces. Both genera appear to take advantage of the filtering action of the beach sediment. Organic particles carried by the swash are deposited as the water drains through the surface. Concentrations are normally low, and O. ceratopthalma feeds
Fig.6. Surface grazing traces and pellets produced by Ocypode. Lower beach slope Anse ~_ la Mouche. Tape marked in centimetres.
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by rushing down the beach between swash or by random walks at low tide to retrieve such particles. Where concentrations are higher, as on Anse ~ la Mouche, systematic "grazing" of the sediment by this species has been observed. The chelae are used to scrape the surface, forming pairs of short parallel furrows. The sand removed is manipulated by the mouth parts, presumably to separate food particles, and the surplus extruded as pellets. These are sub-spherical or rod-like bodies up to 1 cm in length, their surfaces are smooth and coherent and probably mucus bound (pellets and parallel grooves are illustrated in Fig.6). Faecal pellets were not noted but are presumably sometimes also present. Callianassa sp. 1 A number of solitary shrimps were trapped within resin casts of burrows of this group. These have been tentatively identified as specimens of a Callianassid, here referred to as Callianassa sp. 1. Two separate genera may be involved since the burrows, although similar in design, fall into two distinct groups clustered about mean diameters of 4.8 and 7 m m (examples of both can be seen in Plate II). In the following account these are treated as a single morphotype, the range of dimensions given covering both groups. Apertures are normally paired (Plate II, 1); the inhalent opening lies in a conical depression 1-2 cm deep and 2-3 cm in diameter, while the exhalent tube terminates in a conical mound, of sediment and faecal pellets, of similar dimensions. These surface features are typical of the rippled sand zone in more stable areas and of the inner grass zone. The burrow is characteristically a series of sub-spherical rooms joined by a tunnel 3-8 m m in diameter sloping downwards at 15-30 o (Plate II, 2a-c). Up to six rooms may be linked in this way, to a total depth of approximately 30 cm. In the most c o m m o n system the burrow consistently changes direction through 60 ° of arc at each r o o m so that the series forms a roughly equilateral triangular spiral (Plate II, 3a-c). In others this angle is not uniform or consistent in direction and the spiral pattern is lost. Up to four short galleries leave each room, but only rarely are all of these open. In a few examples a gallery may swell at its termination to form a subsidiary r o o m from which further galleries arise. Open galleries tend to occur at lower levels and are probably functional while those above, having been abandoned, collapse. This pattern is, however, a response to one particular erosion/deposition balance and may not be repeated where rates of sedimentation or sediment thickness are different. The main shaft to the surface is vertical or steeply inclined, connecting with either the first or second room. In the latter case a short blind-ending tube, which probably represents an abandoned entrance, projects upwards from the first room. In one specimen this had remained open so that the burrow had two entrances. The exhalent tube is much narrower (2-3 mm), vertical, and linked to the Palaeogeogr., Palaeoclimatol., PalaeoecoL, 11
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PLATE II
Resin casts of burrows Callianassa sp.1. 1, 2a, b, c. Lateral views. Note the frequent directional changes and the excurrent shafts visible in I and on the right hand side of 2b. 3a, b, c. Views from below looking up through the lumen of the spiral indicating its triangular character. 3b is from the larger sub-group noted in the text. 4. Resin cast of Pseudosquilla burrow.
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tapered end of a gallery from one of the first three rooms. The shrimp seems too large to produce this exhalent shaft by simple excavation, but the method of construction is not known. One of the larger diameter burrow casts imprisoned within it a small unidentified crab. The exact nature of the relationship with the occupying shrimp is not known but may be commensal.
Callianassa sp.2 The tentative identification of a species of Callianassa is based partly upon glimpses of these elusive animals disappearing down burrows, and partly upon the similarity between burrow casts recovered and those described by Shinn (1968) and Farrow (1971) and ascribed to this genus. These structures are common in grass-zone sediments and in the margins of adjacent rippled sands. The burrow system is similar in construction to that of Callianassa sp. 1 and consists of a series of rooms (cf. Shinn, 1968) linked by a gently sloping, roughly spiral tunnel (Plate lII, a, b). The rooms are usually slightly inflated to about 7 cm diameter, and between one and four sub-horizontal galleries radiate from each. These may also be inflated but are commonly of similar dimensions to the main tunnel, 1.5-2 crrl in diameter and 5-10 cm long. Most horizontal galleries are open, although small amounts of loose sediment may rest on floors, but one of those in the cast illustrated (Plate III) is stuffed with a mixture of coarse sediment and matted Thalassia leaves. Similar packings have been described by Shinn (1968). Two distinct apertures are present. The larger of these, functioning as an entrance, is normally a circular opening on a level sediment surface or occupying a slight depression. It is funnel-shaped tapering from about 8 cm diameter at the surface to 1.5-2 cm, the normal tube width, 2-3 cm below. The shaft beneath is initially vertical but curves before entering the first room at a low angle 10-15 cm below the surface. The exhalent aperture is 3-5 mm diameter and, in low energy environments, is typically surrounded by a volcano-like mound of sediment about 35 cm in diameter and 10-15 cm high. More persistent composite structures 30 cm high and over a metre in length occur locally, and where mounds are emersed at low tide, as the larger ones commonly are, their margins are often marked by miniature strand-lines. Such structures, if preserved, would form valuable sea-level indicators. The mounds are built up of sediment which is periodically discharged from the exhalent aperture. This is linked by a slender (3-5 mm) shaft to the tapered end of an inflated gallery from the first room. In both this burrow and that of Callianassa sp.1 the shape of this gallery is distinctive and corresponds closely to that of the posterior of the occupant. Shrimps generally maintain an anteroposterior current by movements of the swimmerets and by taking up position in the inflation this activity could be used to circulate fresh water in the upper galleries and flush faecal pellets from the system. The latter are cylindrical, 5-8 mm Palaeogeogr., PalaeoclimatoL, Palaeoecol., 11
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PLATE 111
A.
B.
T h e lateral view of a resin cast o f the b u r r o w system of Callianassa sp.2. T h e exhalent tube is missing but the tapered gallery leading to it is visible to the right of the entry shaft. Three successive levels of descent are present. T h e view from above indicates the spiral nature of the system and the radiating arrangemenl of galleries above rooms. Note the association of specimens of Quidnipa
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long and 1-3 mm in diameter, and are commonly strewn on the lower slopes of the mound. All tunnels and galleries are lined with a fine-grained sediment consisting of silt-size quartz and carbonate fragments and, probably, organic material. The lining in galleries is usually 1-2 mm thick but may be as much as 6 mm in the main tunnel. When removed from the sediment it is normally coherent and slightly flexible, but is sometimes hard and brittle. The inner surface is smooth, without scratch marks, while the outer is irregular. This is chiefly the result of packing of the lining between coarser grains, but a knobby pelleted structure is sometimes present (though less conspicuous than that described by Weimer and Hoyt, 1964). The shafts and chambers are excavated oversize and then stabilized by plastering sediment along the walls, often producing a concentric banding. In some preserved burrows two or three distinct layers occur within the original perimeter, but it is not clear whether such additions to the initial lining are part of a deliberate policy of accretion or are simply a re-distribution of loose sediment finding its way into the system. Most of the burrow is presumably excavated by digging using the first few pairs of legs (Pohl, 1946). This is unlikely for the exhalent tube since it is smaller than the shrimp, but the method of construction is not known. Little is known either of the life habits of Callianassa or of the functions of its complex burrow system. The general layout suggests that it served both as a secure dwelling and as a means of sediment exploitation. This may, however, be an over-simplification since the lining would prevent access to the surrounding sediment other than during excavation. MacGinitie (1934) and Pohl (1946) demonstrated that the food of some North American callianassids consisted of micro-organisms and organic debris obtained by actively sifting sediment. This seems reason enough for the systematic, mine-like construction of the burrow, and MacGinitie (1934) suggests that the system is extended as particular areas are exhausted. The presence of galleries stuffed with a mixture of plant debris and sediment raises the interesting possibility that Callianassa may actually culture bacteria by permitting organic material to decompose within the burrow. In several resin casts living specimens of the bivalve Quidnipagus were trapped in porous sediment adhering to gallery terminations. The positions of the siphons in life are not known, but there can be little doubt that this location was of some benefit to the molluscs. Callianassa sp.2 and the mounds it produces is particularly abundant in grass-zone sediments, but mound systems are also common in certain areas of the pro-reef floor. These have not been investigated in detail but they closely resemble those ascribed to Callianassa sp.2. Depths of occurrence vary from a few metres in parts of the Port Victoria area to at least 30 m in North-West Bay. The maximum habitable depth is not known. In marginal channels off Cerf Passage in calm waters 10-15 m deep, mounds rise 40-50 cm above the floor. On shallower and
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more open floors exposed to larger waves produced by the southeast monsoon, mounds decrease in size and burrows may occur without them. Beyond a threshold of sediment stability no burrows are present. Farrow (1971) noted morphological variations in casts of burrows of Callianassa which he related to bed mobility and sediment thickness. Present evidence suggests that differences in mound sizes and indications in casts of the formation of new entrances are most likely to be the result of sediment movement. They are normally paralleled by a progressive change in the sorting characteristics of the sediment. Variations in gross morphology are more readily related to the depth of sediment available for exploitation.
Alpheus sp. This shrimp is c o m m o n in lower-energy open sand areas. The burrows have not been investigated with resin techniques. The entrance has a U-shaped rim of small gravel-sized bioclasts, the burrow passing between the open ends and beneath the lower loop of the U. The angle of entry is low (10-15 °) and thus differs from that of other shrimps studied in that an initial tunnel is formed rather than a shaft. The burrow is excavated by use of the chelae, small piles of sand being carried forth on them and dumped away from the entrance. Larger bioclasts are placed with some precision about the aperture, presumably for their value in maintaining stability. No distinctive mound system is formed. Farrow (1971) has described Alpheus burrows which are similar and noted the commensally-associated gobiid fish, one of which occupies almost every burrow entrance.
Pseudosquilla ciliata One cast from a burrow produced by a stomatopod, thought to be Pseudosquilla ciliata, was recovered. This is probably not a true reflection of the density and distribution of this organism, regarded by Taylor (1968) as a common inhabitant of the grass zone and similar low energy intertidal areas. Surface recognition of the burrow is difficult since the aperture has similar dimensions and general appearance to that of Callianassa sp.2 burrows. In the specimen collected (Plate II, 4), the aperture was a well-defined circular opening 2.5 cm in diameter, lying almost flush with the surface. The shaft expands below to 4-5 cm diameter, so that in the cast the entrance appears as a constriction. The burrow is a simple cylindrical tube 74 cm in length. Descending as a vertical shaft for about 15 cm, it curves to lie almost horizontal 20 cm below the surface, rising again slightly in the last 10-15 cm. The termination is smoothly rounded and slightly inflated. No mud lining is present, but sediment adhering to the cast, particularly on the lower surface, is rather nodular. Nodules are roughly ovoid and about 1 cm in length; they are not well defined and are probably not faecal.
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A few fragments of Thalassia leaves are included within the immediate wall. These may be significant since Taylor (1968) has noted the presence of large numbers of Thalassia leaves and similar organic material on the walls of Pseudosquilla burrows. DISCUSSION A N D C O N C L U S I O N S
Contemporary burrows The burrows described above fall into two broad morphological groups. Crab burrows are in general large-diameter simple curves lacking branches, while shrimp burrows are usually of smaller diameter, multibranched, and with frequent directional changes. This architectural separation reflects a divergence in function, the crab burrows are largely dwellings, while those of the shrimp serve both as shelter and as a means of sediment exploitation. The palaeoecological significance of this grouping is that it corresponds to an imprecise but nevertheless real boundary between terrestrial and marine environments. If the specific identity of the burrow were known further refinement should be possible, for example, the Cardisoma/Sesarma group, the Uca/Macropthalma group or Ocypode spp. could be used to differentiate between supra-tidal environments, upper intertidal low-energy areas or upper beaches. Distribution of burrows is summarized in Fig.2 but it has not been possible to test their value in precise boundary definition. In the sub-tidal zones a more diverse infauna is present including Callianassa spp., Alpheus and Pseudosquilla. These have an overlapping distribution which bears little relationship to the biofacies zonation of the reef. Some environmental information may, nevertheless, be gained from them. The mounds associated with Callianassa sp.2 indicate that the burrow was formed in a low-energy environment, while burrows without mounds suggest a higher energy but still stable condition. Preserved mounds have not been observed during this study but are certainly present in some ancient sediments such as the Oxfordian of southern England. The disposition of occupied and vacant regions within the burrow and their relationship to the sediment surface provides evidence of erosive or depositional variation of that surface. Gross morphology can give some indication of the thickness of workable sediment.
Preservation Adequate preservation is a primary necessity for crustacean burrows or body fossils to be of geological value. In contrast to their high density in many modern environments, neither decapods nor stomatopods are common as fossils. Research on the Pleistocene limestones of Aldabra has so far produced few crabs and no shrimps. The carapace of both is, of course, fragile in comparison to the shells of most common molluscs, but it might be supposed that a burrowing habit would
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provide a potentially favourable environment for fossilization. The advantages are clearly insufficient, and remains have presumably either been fragmented beyond recognition or destroyed by diagenetic changes. That being so, even greater emphasis must be placed upon their trace fossils. Some burrows, by virtue of their situation or construction, seem more likely to be preserved than others. For example, the great depth of penetration of some Callianassa burrows might place them beyond the limits of normal sediment removal and reworking, but the depth of penetration is itself a function of sediment thickness and a lack of reworking, so the relationship is not a simple one. The linings to Callianassa sp.2 burrows are often quite hard, even in occupied systems, and are thus relatively resistant to early compaction, so that Shinn (1968) was able to demonstrate open burrows at more than twice their normal inhabited depth. Linings, by their differing porosity and (probably) chemistry, may also act as loci for early lithification. This suggestion is based partly on the observations of Taylor and llling (1969), who reported recently-cemented sediment closely associated with burrows in the Persian Gulf, but cemented burrows with multiple linings have been found in the Seychelles. These are unfortunately rare and their origin is uncertain. Crab burrows do not have the advantage of linings, although the walls may be strengthened by surface packing and binding with mucus or similar substances. In addition, the high intertidal and low supra-tidal position which they occupy seems particularly vulnerable to erosion. It seems that to be preserved they must either be filled and buried during a prograding phase in sedimentation, or cemented at a very early stage in order to resist compaction. The instability of the beach environment suggests that preservation of Ocypode burrows should be a particularly unlikely event. Nevertheless, open burrows ascribed to this genus are present in beach-rock on Mab6 (at Bel Ombre) and on Aldabra (Dune D'Messe). It seems probable that these were re-excavated rather than preserved open, but recent fillings are not differentiated from host sediment in ways which might have encouraged selective erosion. Well-preserved burrows, believed to have been formed by crabs, are present at two localities in the Aldabra Pleistocene. Low angle tunnels of 3-5 cm diameter with few branches occur at Passe Horeau. These resemble some Ocypode burrows and the fact that they are overlain by well-sorted sediments containing beachdwelling limpets (identified by J. D. Taylor) lends support to this interpretation. Similar large diameter systems with few branches are present at the eastern end of Picard. These occur in a unit which overlies calcarenites containing large numbers of small diameter burrows showing the same frequent angular directional changes as those formed by the smaller callianassids. The sequence is interpreted as recording the seaward accretion of a beach over shrimp-burrowed sand flats. The picture of an emergent sand cay is completed by the associated sediments which contain terrestrial molluscs, tortoise bones and plant rootlet impressions. Palaeogeogr., Palaeoclimatol., Palaeoecol., I I (1972)265 285
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Other probable crustacean burrows occur east of Passe Gionnet. These consist of a vertically disposed system of tubes about 3 cm in diameter with bulbous turning areas, but because of interference the extent to which they branch is not known. They resemble in some respects the casts of burrows of the mole crab Albunea figured by Farrow (1971) and are unlike those of any living forms described here. Associated with them are smaller burrows of general callianassid form. CONCLUSIONS
The lack of specific identity of individual burrows in this study and the broadcast nature of morphological groups places a limit on their use as environmental indicators. It is, however, clear from our observations on Mah6, tested on a limited scale in the Pleistocene of Aldabra, that crustacean burrows are of value, both individually and conjointly, for general environmental definition and the elaboration of themes suggested by other evidence. ACKNOWLEDGEMENTS
This study forms part of a larger investigation of carbonate sediments in the Seychelles financed by The Carnegie Trust for the Universities of Scotland and the Natural Environment Research Council. The work on Aldabra was sponsored by The Royal Society. The University of Dundee generously granted leave of absence. We are particularly indebted to Mr. G. Lionnet, Director of the Department of Agriculture of the Seychelles, and to his staff for the help given to us in support of our field work. REFERENCES Baker, B. H., 1963. Geology and mineral resources of the Seychelles archipelago. GeoL Surv. Kenya, Mem., 3" 1-140. Braithwaite, C. J. R., 1971. Seychelles reefs: structure and development. In: D. R. Stoddart and C. M. Yonge (Editors), Regional Variation in Indian Ocean Coral Reefs. Symp., Zool. Soc. Lond., 28: 39-63. Crane, J., 1957. Basic patterns of display in fiddler crabs (Ocypodidae: gen. Uca). Zoologica, 42: 69-82. Farrow, G. E., 1971. Back-reef and lagoonal environments of Aldabra Atoll, distinguished by their crustacean burrows. In: D. R. Stoddart and C. M. Yonge (Editors), Regional Variation in Indian Ocean Coral Reefs. Symp., Zool. Soc. Lond., 28: 455-500. Frey, R. W., 1970. Environmental significance of Recent marine lebensspuren near Beaufort, North Carolina. J. Palaeontol., 44: 507-519. Hayasaka, I., 1935. The burrowing activities of certain crabs and their geologic significance. Am. Midland Naturalist, 16: 99-103. Lewis, M. S., 1969. Sedimentary environments and unconsolidated carbonate sediments of the fringing coral reefs of Mah6, Seychelles. Mar. Geol., 7: 95-127. MacGinitie, G. E., 1934. The natural history of Callianassa californiensis Dana. Am. Midland Naturalist, 15: 166-177.
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McMullen, R. M. and Allen, J. R. L., 1964. Preservation of sedimentary structures in wet unconsolidated sands using polyester resins. Mar. Geol., I : 88 97. Piggot, C. J., 1968. A soil survey of the Seychelles. Minist. Overseas Dev. Tech. Bull., 2:1 89. Pohl, M. l., 1946. Ecological observations on Callianassa major Say. at Beaufort, North Carolina. Ecology, 27:71 80. Rosen, B. R., 1971. Principal features of reef coral ecology in shallow water environments of Mah6, Seychelles. In: D. R. Stoddart and C. M. Yonge (Editors), Regional Variation in Indian Ocean Coral Reefs. Symp., Zool. Soe. Lond., 28:163 183. Sauer, J. D., 1967. Plants and Man on the Seychelles Coast. Univ. Wisc. Press, Madison, Wise., pp.l-132. Shinn, E. A., 1968. Burrowing in recent lime sediments of Florida and the Bahamas. J. Palaeontol., 42(4): 879-894. Taylor, J. C. M. and Illing, L. V., 1969. Holocene intertidal calcium carbonate cementation, Qatar, Persian Gulf. Sedimentology, 12: 69-107. Taylor, J. D., 1968. Coral reef and associated invertebrate communities (mainly molluscan) around Mah6, Seychelles. Philos. Trans. R. Soc. Lond., Ser. B., 254(793): 129-206. Taylor, J. D. and Lewis, M. S., 1970. The flora, fauna and sediments of the marine grass beds of Mah6, Seychelles. J. Nat. Hist., 4: 199-220. Weimer, R. J. and Hoyt, J. H., 1964. Burrows of Callianassa major Say. geologic indicators of littoral and shallow marine environments. J. Palaeontol., 38: 761-767.
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