Ecology of intertidal and sublittoral cryptic epifaunal assemblages. I. Experimental rationale and the analysis of larval settlement

199

J. Exp. Mar. Biol. Ecol., 1986, Vol. 99, pp. 199-231 Elsevier

JEM 709

ECOLOGY OF INTERTIDAL ASSEMBLAGES.

AND SUBLI’ITORAL

I. EXPERIMENTAL

RATIONALE

CRYPTIC

EPIFAUNAL

AND THE ANALYSIS

OF LARVAL SElTLEMENT

CHRISTOPHER D. TODD Gatty Marine Laboratov,

and

STEPHANIE J. TURNER

Departmeni of Zoology and Marine Biology, Universi(v of St. Andrew& St. Andrews. Fi$e, KY16 8LB. Scotland

(Received 27 January1986; revision received 7 April 1986; accepted 10 April 1986)

Two principal factors are predicted to be of overriding importance in the development and structure of under-stone cryptic epifaunal assemblages in both the intertidal and sublittoral: these are (I) inter-substrata] distances (i.e., the gap-sizes between boulder under-surfaces and the bedrock substratum) and (2) timing (in relation to seasonahty of larval settlement) of availability of primary space. A long-term panel study of such assemblages has been undertaken on the west coast of Scotland, and the present paper reports our observations and analyses oflarval settlement. Slate panels, z 15 cm square, were bolted together horizontally, but separated by “spacers” of 25, IO- or S-mm height. These were held in plastic-coated steel frames. The under-surface of the upper panel in each pair mimics a boulder undersurface, while the upper-surface of the lower panel represents the substratum. The long-term analyses will concern only the upper, or “top”, panels although settlement on both “top” and “bottom” panels is evaluated in this paper. The settlement data presented embrace a continuous 35-month period, and concern three panel pairs (one of each gap-size) placed in each habitat for usually 4, but occasionally 6 wk. On removal from the frames the panels were replaced on each of 32 occasions by a new set, which had been previously conditioned in sea water for up to 7 days. A total of 41 species/taxa were separable and some 26000 individuals enumerated. The qualitative similarity of intertidal to sublittoral settlement was particularly striking, but this was overwhelmed by quantitative contrasts in addition to specific differences in subsequent survival. Overall, approximately seven times as many individuals settled in the sublittoral as in the intertidal, a marked contrast, even after allowing for littoral emersion. The seasonahty of particular taxa is considered in some detail, as are the effects of gap-size and surface (“top”/“bottom”) orientation. In general, most species settled “preferentially” on the “top” panels and on the wider gap-sizes; some notable exceptions are, however, identified. The data in the present paper are largely descriptive, but observations of particular taxa will be more fully evaluated when subsequently analysing the dynamics of these assemblages. Abstract:

Key words: intertidal; sublittoral; hard substratum; epifauna; larval settlement; panel

In contrast to the recent interest in the ecology of sublittoral hard-substratum epifaunal assemblages (e.g. Sutherland, 1974, 1978, 1981; Jackson, 1977a,b, 1979a,b; Sutherland & Karlson, 1977; Karlson, 1978; Buss, 1979a,b; Buss & Jackson, 1979; Goren, 1979; Rastetter & Cooke, 1979; Woodin &Jackson, 1979; Dean, 1980; Dean & Hurd, 1980; Russ, 1980, 1982; Schoener & Greene, 1980, 1981; Mook, 1981; Schoener & Schoener, 198 1; Smedes & Hurd, 1981; Vail & Wass, 198 1; Greene & 0022-0981/86/$03.50 0 1986 Elsevier Science Publishers B.V. (Biomedical Division)

200

CHRISTOPHER

D. TODD AND STEPHANIE J. TURNER

Schoener, 1982; Green et al., 1983) only little attention (e.g. Osman, 1977, 1978) has been paid to controlled studies of intertidal assemblages of cryptic suspension-feeders. The present paper is the first of a series reporting on a comparative study of the development and stability of intertidal and sublittoral under-stone (“cryptic”) epifaunal assemblages. The analysis of epifaunal structure and development necessarily divides into the study of either natural (undisturbed) substrata, or the introduction to the habitat of artificial substrata for subsequent colonization. Recent studies of undisturbed epifaunal assemblages include those by Dayton et al. (1974), Vance (1979) and Svane & Lundalv (198 1). Of greater relevance to studies involving the introduction of artificial substrata, however, are those concerning cleared (e.g. Kitching, 1937; Fahey & Doty, 1949; Luckens, 1976) or otherwise manipulated (e.g. Vance, 1979) areas of natural substratum. Moreover, of especial interest are the relatively few studies of assemblages developing on recently introduced substrata, other than small panels. For example, Moore (1939) and Moore & Sproston (1940) reported on the colonization of a rocky shore comprised of clean new stone, created during the construction of a swimming pool at Plymouth, and Grigg & Maragos (1974) and Gulliksen et al. (1980) have examined colonization and assemblage development on lava flows of varying ages in Hawaii and Jan Mayen, respectively. In addition to epilithic faunas a number of studies have specifically considered biotic substrata. Thus, for example, Conover (1979), Stachowitsch (1980), and Karlson & Shenk (1983) provided observations on epifaunal associations with hermit-crab shells, whereas Wells et al. (1964), Vance (1978), Butler & Brewster (1979), Keough (1983, 1984a,b), and Kay & Keough (1981), among others, have examined bivalve shell epifaunas. Jackson (1979a) has studied the assemblages developing on the undersurfaces of (live) foliaceous Caribbean corals and Fishelson (1973) has reported on the development of an epifaunal community on (dead) coral skeletons. In addition, Palumbi & Jackson (1982) have detailed the settlement of larvae into lesions made in encrusting bryozoan colonies. Despite the numerous pitfalls (see p. 226) associated with the experimental use of artificial substrata in studying the ecology of natural epifaunal assemblages, the majority of investigations have involved the immersion of panels of one form or another. The placement of relatively uniform (if not actually identical) panels attempts to eliminate the major variable - that of substratum heterogeneity; all natural hard substrata comprise a multitude of microhabitat variations, often on a small scale which, by their very intricacy, are not subject to practicable sampling. Moreover, in order to permit detailed and informative analysis the ability to place and retrieve the required number of replicate habitat units renders the use of (small) panels expedient. Artificial substrata have been utilized in essentially three types of study in the marine environment. (1) The substrata are immersed for a relatively short time (perhaps up to 1 month). The major aims of such approaches are to determine the variations in rate and pattern

ECOLOGY

OF CRYPTIC EPIFAUNAL

ASSEMBLAGES

201

of larval settlement (e.g. Sutherland, 1974, 1981; Mook, 1980, 1981; Keough, 1983). (2) The substrata are immersed for longer periods (e.g. Sutherland, 1974,1978,198 1; Withers & Thor-p, 1977; Goren, 1979; Moran, 1980). There are two approaches in this kind of study. (a) The substrata are first immersed at intervals and finally removed simultaneously after a period (e.g. Nair, 1962; Osman, 1977); or (b) the substrata are first immersed simultaneously and a proportion removed at pre-determined intervals (e.g. Richards & Williams, 1944; Cover & Harrel, 1978; Field, 1982). Types 2(a) and (b) study highlight the effects of seasonality of larval settlement patterns on the subsequent development of assemblages, and also the effects on subsequent recruitment exerted by already-established organisms. Studies of type 2(a) are especially powerful in distinguishing these features, particularly where assemblage sampling is non-destructive, as in our definitive experiments. The present study is of types 1 and 2(a) above and is a comparative analysis of the intertidal and sublittoral under-stone epifauna on the western coast of Scotland. From inspection of boulder under-surfaces it is apparent that the organisms associated with peripheral rock surfaces (e.g. arborescent bryozoans, solitary ascidians) frequently differ from those of more or less central disposition (e.g. encrusting bryozoans, colonial ascidians) in being of rather higher profile. Inter-substratal distance between the bedrock and boulder under-surface is likely to be a major factor in determining, for example, the water-flow regime and hence the settlement, establishment, and subsequent growth and development of individuals under given circumstances. In particular, it is likely that low-relief situations may provide refuges for certain competitively inferior species which would be otherwise excluded by (in this case, higher-profile) dominants. Above all, the timing of primary space availability (one resource for which competition is intense), in relation to settlement of the dominant organisms is held to be of prime importance to the establishment and subsequent development and stability of these communities (e.g. Sutherland, 1974, 1978, 1981; Osman, 1977: but see also Connell & Sousa, 1983). By using pairs of panels (bolted together horizontally, but separated by various gap-sizes), and by first immersing replicated panels at different times of year we aim to distinguish the respective importance of these two factors on the development and stability of epifaunal assemblages in these particular habitats. This study was initiated in March 1981 and comprised three distinct but complementary parts. The first was continuous observation of short-term settlement patterns over 3 yr. The second was an extensive study in which panel pairs of the various gap-sizes were first immersed in every month for 1 yr; observation of these panels over 12 months enabled us to select four separate months of the year on which to commence the third part. This last concerns the definitive triplicated experimental design for the analysis of assemblage structure and dynamics: these will be reported subsequently. The present paper describes the experimental rationale and presents our observations of larval settlement. It is important to emphasize that the present data concern all observed post-larvae, irrespective of whether or not they had survived. While we undoubtedly

202

CHRISTOPHERD. TODD AND STEPHANIEJ. TURNER

have not recorded all settlement, most taxa (with the notable exception of scyphistomae and ascidians) leave a recognizable trace or skeleton. We are, therefore, evaluating potential, rather than actual, recruits. AREA STUDIED AND FIELD SITES The two field sites are located on the shores of Seil Island, Argyll (56” 17’N : 5”37’W) in the Firth of Lorn, on the western coast of Scotland (Fig. 1). The two sites are separated by a direct distance of 6 km, with the sublittoral observations being made at Clachan Seil (hereafter referred to as Clachan) and the intertidal observations at Cuan Ferry (hereafter referred to as Cuan) (see Fig. 1).

I

Fig. 1. Sketch

15km

I

map of the British

Isles showing

the area studied

on the western

coast of Scotland.

ECOLOGY

OF CRYPTIC

EPIFAUNAL

203

ASSEMBLAGES

Clachan is a very sheltered tidal narrow of some 30 m width and 1 km length separating Seil Island from the Argyll mainland (see Lewis, 1964, Chapter 1 for a full description) (Fig. 2). There are tidal sills at both the northern and southern extremities of the narrows and as a result the spring tides ebb to the same level between the sills irrespective of open coast fluctuations. Drainage over the sills all but ceases at L.W.S. and thus the enclosed narrows are never emersed. In view of this, and because the narrows provide such a sheltered and protected locality, this site was deemed ideal for the placement of unattached sublittoral frames; the frames bearing the panels were easily retrievable manually and by wading, without resorting to diving.

Fig. 2. Clachan

Seil, looking north:

the field site is located

at the narrowest

part of the sound.

The site at Cuan is rather less protected but is, nonetheless, a very sheltered shore (Todd & Lewis, 1984) (Fig. 3). Protection, from all compass points, is afforded by Seil Island itself, the Argyll mainland and Luing Island (Z 0.5 km distant). (The extreme shelter of these sites is manifest in Figs. 2 and 3; both photographs were taken when the wind blew Force 7 to 9, gusting 10 from the southwest.) The shore at Cuan comprises solid bedrock, permitting the drilling of holes for bolting down the experimental frames. Geologically, Argyll is extremely complex. Clachan Seil is characterized by Black Slate, a line-grained shale that has been metamorphosed. This slate is of the Dalradian sequence of the Upper Proterozoic-Cambrian. By contrast, the shores at Cuan Ferry

204

CHRISTOPHER

D. TODD AND STEPHANIE J. TURNER

Fig. 3. Cuan Ferry, looking southwest: the tield site is located at the end of the promontory.

are of Horneblende-Schist resulting from the igneous intrusion of shale. The use of Cumberland slate as our experimental artificial substratum is, therefore, justifiable in that although this differs from local rocks in mineralogy the artificial and natural substrata are of similar texture and hardness. In the vicinity of Argyll the dominant benthic substrata are rock, sand, and gravel. Mean open coast sea-surface temperatures range from 6.5-13.5 “C, with salinity constant at 33-34x,. The maximum regional tidal currents during mean spring tides are z 3 knots although in the case of the particular field sites these are locally increased. The mean tidal amplitude ranges from 1.1 m (neaps) to 3.3 m (springs) with L.W.S. occurring around midday and midnight. Due to the complexity of the local mainland and island topography, however, all of the above factors are subject to local variability. For example, at Clachan for brief periods on spring ebb-tides the surface water temperature may rise slightly during the summer months, and surface salinity may be temporarily depressed by increased terrestrial run-off during the winter (unpubl. obs.). These field sites thus provide fully marine habitats which are ideal for regular, long-term field work in being maximally protected from wave action and easily accessible by road (but nonetheless secluded, and free from human interference), and only 25 km from marine laboratory facilities (S.M.B.A. Laboratory, Oban).

ECOLOGYOF CRYPTICEPIFAUNALASSEMBLAGES

205

MATERIALSAND METHODS All data and observations relate to slate panel pairs held in plastic-coated steel frames. The panels were of dimensions x 12 mm thickness and 15 cm square; these were machine cut to a plane finish. In order to mimic the rock under-surface/bedrock apposition and its variation in vertical relief, panels were bolted together horizontally in pairs and separated by Perspex spacers of either 25-, lo- or 5-mm height (Fig. 4). The frames were const~cted of welded angle-steel to form the sides, and spaced strips across the bottom held the attaching studs on which the panel-pairs could be accommodated. Each was of dimensions 75 x 55 x 10 cm and held 12 panel-pairs

Fig. 4. The panel-pair

experimentaldesign,showing(left) stained “blue” panels and (right) the panel-pair

as placed in the shore frames. Fig. 5. One of the intertidal frames at the commencement ofthe study: within a few weeks the mesh screens became covered by algal epiflora. Fig. 6. The original, “normal” panel design showing the etched and outlined central 10 x 10 cm area: this particular panel bears a variety of solitary and colonial ascidians, bryozoans, sponges, anomiids and serpulids. Fii. 7. The definitive, “blue” panel design showing the etched and stained central 10 x 10 cm area: this particular panel bears a similar range of taxa to Fig. 6, and the clarity of definition of even small organisms is apparent, by comparison with the “normal” substratum.

206

CHRISTOPHER

D. TODD

AND

STEPHANIE

J. TURNER

(Fig. 5). All anchoring (intertidal only) and actual frame materials were dip-coated in a non-toxic plastic by Vyflex Ltd. (Cheshire, England), prior to placement on the shore, to prevent corrosion. Each frame assembly was covered by a heavy gauge 2 mm plastic mesh screen. Frames in the sublittoral (Clachan) were placed and retrieved by hand and were otherwise unanchored; frames in the intertidal (Cuan) were resin-bolted to the shore, by means of 30 cm x 15 mm steel bolts in holes drilled in the bedrock. These latter bolts secured the anchoring “foot” and “leg” attachments which themselves permitted the actual frames to be held horizontally. Owing to the presence of basal retaining nuts beneath the bottom panels (holding these x 3 mm clear of the frame floor itself), and the plastic mesh screen above, water-flow in the vertical plane through the frames was relatively unimpeded. Nevertheless, in view of the close arrangement of the panel-pairs, it was deemed necessary to encourage lateral through-flow by the drilling of 12-mm holes, spaced at 2 cm, in the sides of the frames. Immersion of panels commenced in April 1981 and the long-term elements of the study continued until February 1986. The present paper considers only the settlement data, which cover the period July 198 l-May 1984. These panels were identical to those immersed for long-term assemblage analyses. In all cases the under-surface of the upper panel is held to mimic a rock under-surface while the uppermost surface of the lower panel of each pair corresponds to the bedrock substratum. These are referred to as “top” and “bottom” panels, respectively, hereafter. In order to obtain continuous data of short-term settlement patterns one pair of panels of each of the three gap-sizes (25,lO and 5 mm) was immersed for z 1 month (two tidal cycles) in each of the two habitats. To facilitate consistency of the present larval settlement data the l-month immersions occupied the same three panel slots in the same frame on each occasion. The long-term panels were, however, placed in slots within their allocated frames in an ad hoc manner. On removal from the frames the panel pairs were replaced by a new set which had been previously conditioned in sea water for 7 days. Panel pairs were then preserved in 4% neutral formalin and returned to the laboratory at St. Andrews for analysis. Originally we etched, and then outlined (by means of permanent felt marker pens), a 10 x 10 cm square on the central area of each panel using the same template from which the bolting holes were drilled (Fig. 6). This ensured that the two 10 x 10 cm analytical areas for each panel pair overlay one another precisely, irrespective of the slight differences in panel shape. Delineation of the central analytical area on the (15 x 15 cm) panels permitted observation of an area away from the outer edges, which for long-term panel pairs were possibly subject to inadvertent handling or damage during retrieval - replacement and transportation. Moreover, this area was also away from the four vertical Perspex spacers. Our initial intention was to analyse epifaunal development on the natural slate surfaces of the panels. It soon became clear that monochromatic photography (for non-destructive sampling) of the long-term panels would not permit reliable or precise

ECOLOGYOFCRYPTICEPIFAUNALASSEMBLAGES

207

recognition or delimitation of individual organisms and colonies against the mottled grey background of the slate (Fig. 6). We were obliged, therefore, to alter the panel design and stain the slate substratum. For the definitive long-term experimental design, we therefore used panels with the central area stained blue (hereafter referred to as “blue”) (Fig. 7) as distinct from the initial design of outlined, but otherwise unmodified, natural slate (referred to as “normal”). In order to determine whether the marker pen staining of the substratum would exert an effect on larval selectivity we ran a brief series of parallel immersions of “normal” and “blue” recruitment panel pairs between July 198 1 and February 1982. The immersion of “blue” settlement panel pairs (one of each of the three gap-sizes in each habitat for w 1 month) was continued until May 1984. The long-term immersions of “blue” panels commenced in February 1983. On retrieval to the laboratory at St. Andrews the preserved panels were inspected for established organisms under a Wild M8 stereomicroscope. Panels were scanned in a routine manner using a fine monofilament nylon grid overlay divided into l-cm squares. The position and identity (to species where possible) of all established individuals were noted. Due to the lack of replication of the present observations statistical analyses, where appropriate, have been confined to G-tests (Sokal & Rohlf, 1981) in order to ascribe significance to particular features in the data.

RESULTS

Table I lists all the species and taxa recorded from the panels over the 35-month period considered. Accompanying each taxon is a subjective evaluation (discussed in detail below) as to whether or not the “top” or “bottom” panels were, in general, preferentially settled, and to which site particular organisms were especially attributable. Of the 41 recognizable taxa, four were exclusive to Cuan (intertidal) and four others to Clachan (sublittoral) with 33 co-joint. Thus, there is a considerable qualitative similarity in potential recruits to these distinct assemblages, although quantitative differences were considerable. Such contrasts were further amplified by marked differences in the subsequent survival and longer-term establishment of the species in the two habitats, although these cannot be evaluated from the present data. It should also be appreciated that unequivocal identification to species was not always possible - especially for bryozoans and ascidians. Thus, for example, Schizomavella linearis (Hassall), Escharella immersa (Fleming), Smittoidea reticulata (J. Macgillivray), Porella concinna (Busk), and Callopora rylandi Bobin & Prenant, in addition to Diplosoma listerianium (Milne-Edwards), Aplidiumpallidum (Verrill), Molgula manhattensis (De Kay), Ascidia mentula Mtiller, Ascidia obliqua Alder, Ciona intestinalis (L.), and Corellaparalfelogramma (Miiller) do not feature in the settlement data, but were quantified on the long-term immersions. This reflects the frequent intractability of specifying ancestrulae, tadpoles, and post-larvae. Nevertheless, we feel confident that

TABLE I Species and/or taxa enumerated in the analysis of settlement: each is subjectively coded for “preferred” site (CL, Clachan; CN, Cuan; = ,no “preference”; *, exclusive to that site), and whether the “top” (T) or “bottom” (B) panel was preferentially settled at Clachan and Cuan, respectively; ?, inadequate data; - , no data. Site

CL

CN

CL CL CL

B T ?

B B ?

CL

T

T

= CN CL

T T T

T T T

CL CL

T T

T T

=

T

T

=

B T T B

B T T B B T

CL = = CN* CN* CL* CL CL* CL* CN CN CL = CL CL CN* CL CL =

T T ? T T ? T T T =

CL* CL

T T T ? B T ? T

ZZ

B

CN ZZ =

CL

T T T T T B ? ? ? T T B T T B B

CN* CL CL =

T

T T B T

T T B T

Species Porifera Halisarca dujardini Johnston Leucosolenia botryoides (Ellis & Solander) Unidentified poriferans Cnidaria Aurelia aurita (L.) (Scyphistoma) Annelida Pomatoceros triqueter (L.) Jerpula sp. “Spirorobis” spp. Arthropoda Semibalanus crenatus (Bruguibe) Verruca stroemia (O.F. Mtiller) Mollusca ‘Anomia” spp. Bryozoa Aetea angubta (L.) Alcyonidium spp. (Cheilostome) “Ancestrula” ( + unidentified spp.) Bowerbankia imbricata (Adams) Bicellaria ciliata (L.) Bug& spp. Cauloramphus spiniferum (Johnston) Celleporella hyalina (L.) Conopeum reticulum (L.) Cribrilina annulata (Fabricius) Cribrilina cryptooecium Norman Cribrilina punccata (Hassall) Crisis eburnea (L.) Electra pilosa (L.) Escharoides coccineus (Abildgaard) Flustrellidra hispida (Fabricius) Haplopoma sp. Membraniporella nitida (Johnston) Microporella ciliata (Pallas) Phaeostachys spiniferum (Johnston) Schizoporella unicornis (Johnston) Scruparia chelata (L.) Scrupocellaria scruposa (L.) Smittina sp. “Tubulipora” spp. Entoprocta Pedicellina sp. Echinodermata Antedon bifida (Pennant) (Pentacrinoid) Chordata Botryllus schlosseri (Pallas)/Botrylloides leachii (Savigny) Didemnum candidum Savigny Sidnyum turbinatum Savigny Unidentified “Ascidians” - mostly Ascidiella scabra (Miiller)

ECOLOGY

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209

ASSEiiBLAGES

sufficient resolution is provided by the present data set to permit realistic qualitative and quantitative comparisons to be made for the two habitats prior to evaluation of the assemblage dynamics. “NORMAL”

AND

“BLUE”

PANEL

COMPARISON:

BOTH

SITES,

JULY

1981-FEBRUARY

1982

The data discussed in detail throughout this paper concern the “blue” panels. For comparison of the effects of staining the panel surfaces of one panel pair of each of the three gap-sizes (25,lO and 5 mm) were prepared as both “normal” and “blue” substrata; these were immersed in each of the two habitats for two (or occasionally three) tidal cycles on eight occasions between July 1981 and February 1982. Tables II and III summarize the eight months’ data for both sites. Data are for all three gap-sizes combined, but have been partitioned, in the tables, into “tops + bottoms” and “tops” only. This allows us to isolate any effect of excessive scouring or sedimentation of the “bottom” panels. The two subsets of data for each field site show essentially similar patterns. More individuals and taxa generally settled on the “blue” rather than the “normal” panels and more settled on the “tops” than the “bottoms” in most months. Those species absent from the “normal” panels were generally rare bryozoans and this pattern is probably accountable simply by the enhanced numbers of larvae settling on the “blue” substrata. Note also that a very high proportion of both species and taxa which had settled on the “normal” substrata were contemporaneously co-joint on the “blue” panels. Thus, it is perhaps justifiable to predict that more species-rich (denser) assemblages would subsequently develop on the “blue” rather than “normal” substrata. This will not, however, markedly bias the outcome of our long-term dynamics analyses, because we believe that the “blue” substrata provide a closer approximation, than do the “normal” panels, of the local (black slate) substrata. Certainly, it is no surprise to find more larvae settling on these darker surfaces (e.g. Yule & Walker, 1984). 2 x 2 x 8 G-tests of independence for these data sets are shown in Table IV. In view of the highly significant interaction the data were then partitioned for pair-wise R x C G-tests (Table V). These all show highly significant rejection of the null hypotheses of homogeneity. Inspection of Tables II and III reveals the inconsistency of total settlement on the “blue” compared with “normal” substrata from one month to the next, both within- and between-sites. Nevertheless, both the qualitative and quantitative trends are generally apparent for both subsets of data from each site. More important, none of the species recorded as exclusive to “normal” panels in any particular subset failed to settle either on the “blue” substrata in the extensive survey of settlement (see below), or to the long-term assemblage panels themselves. Indeed, Schizoporellu unicornis (which failed to settle on these eight sets of “blue” panels) proved to be of major importance among the long-term intertidal assemblages.

July 1981 August 1981 September 1981 October 1981 November 1981 December 1981 January 1982 February 1982 Grand totals

July 1981 August 1981 September 1981 October 1981 November 1981 December 1981 January 1982 February 1982 Grand totals

27 30 46 27 28 48 30 29

Days immersed

5 5 13 10 2 2 3 1 17

25 17 71 41 97* 23 5 280

217 167 382 114 74 61 32 36 1083

16 12 17 10 3 4 3 2 26

72 25 107 46 113* 38 9 3 413

150 87 331 97 60 28 21 29 803

“Tops” only

Individuals

Taxa

“Blue”

Individuals

“Normal”

11 13 23 10 10 7 4 3 27

20 22 28 13 11 7 4 4 30

Taxa

6.0 5.1 4.7 2.4 0.6 1.2 4.2 29.0 2.9

3.0 6.7 3.6 2.5 0.7 1.6 3.6 12.0 2.6

Individuals

2.2 2.6 1.8 1.0 5.0 3.5 1.3 3.0 1.6

1.3 1.8 1.6 1.3 3.7 1.8 1.3 2.0 1.2

Taxa

“Blue”/“Normal”

“Tops” + “Bottoms”

19 (76.0) 15 (88.2) 68 (95.8) 29 (70.7) 39 (40.2) 18 (78.3) 4 (80.0) 1 (100.0) 193 (68.9)’

58 (80.6) 23 (92.0) 98 (91.6) 37 (80.4) 41 (36.3) 30 (78.9) 9 (100.0) 3 (100.0) 299 (72.4)’

Individuals (% )

“Co-joint”

13 (100.0) 6 (60.0) 2 (100.0) 2 (100.0) 3 (100.0) 1 (100.0) 16 (94.1)

5 (100.0) 5 (100.0)

15 (93.8) 10 (83.3) 17 (100.0) 6 (60.0) 3 (100.0) 3 (75.0) 3 (100.0) 2 (100.0) 25 (96.2)

Taxa (%)

Summary of total individuals and taxa settling on “normal” and “blue” panels (all three gap-sizes summed) at Cuan (intertidal) over the 8 months’ preliminary comparison: + these values are for contemporary co-joint individuals and taxa; * “Spirorbis” spp. settled in large numbers on “normal” panels.

TABLE II

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CHRISTOPHER

D. TODD AND STEPHANIE J. TURNER TABLE IV

Three-factor G-test of independence between the total number of individuals settling on the “normal” and “blue” panels (Colour), at Cuan and Clachan (Sites) over the S-month (Months) comparative period (see Tables II and III); the hypothesis of joint independence between the three criteria is rejected, as are the hypotheses of pairwise independence; *** P < 0.001.

Months Months Months Months Months

x x x x x

Site Independence Colour Independence Site Independence Colour x Site Interaction Colour x Site Independence

“Tops” + “Bottoms”

d.f.

“Tops” only

d.f.

269.2*** 404.6*** 19.0*** 43.0*** 735.8***

1 7 1 I 22

215.4*** 329.0*** 38.2*** 40.2*** 682.8***

7 1 I 22

1

TABLE V R x C G-tests of independence for pairwise comparisons of settlement on the two colours of substrata (“normal’

and “blue”) at the two sites over the 8-month comparative period: in all cases the hypothesis of independence is rejected; *** P 4 0.001. “Normal” vs. “Normal” “Tops” + “Bottoms”

“Tops” only

Within sites Cuan Clachan Between sites Cuan versus Clachan Within sites Cuan Clachan Between sites Cuan versus Clachan

“Blue” vs. “Blue”

“Normal” vs. “Blue”

G = 137.0*** G = 144.6***

G = 97.0***

G = 215.2*** G = 145.8*** G = 223.4***

G = 108.2***

G = 207.6*** _-

TOTAL NUMBERS OF INDIVIDUALS

AND OF SPECIES AND/OR TAXA

Figs. 8 and 9 summarize the overall settlement data for the entire observational period; they present the grand totals of individuals (Fig. 8) and taxa (Fig. 9) observed for both sites. Plots are for the three gap-sizes (“tops” + “bottoms”) summed, although Fig. 8 includes additional plots for the “top” panels only. The data are plotted as individuals *day- ’ in order to adjust the abundances for the varying periods of immersion. In seven cases (arrowed in Figs. 8 and 9), panels were immersed for three tidal cycles (~46 days) although most (25) were immersed for two cycles ( z 29 days). In the absence of replication of the three gap-sizes immersed at each site on the 32 occasions it is not possible to place error terms on these data. Perhaps the most striking feature of Figs. 8 and 9 is not so much the expected strong seasonality of settlement, but more the consistency of pattern and the close qualitative

ECOLOGY OF CRYPTIC EPIFAUNAL ASSEMBLAGES 1OOr 50.

CLACHAN

213

SEIL

7 $10. 0

S-

II 3 e

‘-

CUAN

FERRY

s z 1

1

11

11

i

‘J’A’S’O’N’D~J’F’M’A’M’~~lgJ~A’S’O’N’D~J’F’M’A’M’~~B?JIA’S’O’N’D~J’F’M’A’M’

Fig. 8. Plots of the total number of individuals of all species, for all three gap-sizes combined, on the 32 sampling occasions throughout the period studied: the numbers are standardized for immersion period (numbers . day- ‘) and are plotted on a logarithmic scale; the upper (thicker) plot for each site refers to the “top” and “bottom” panels combined, while the lower (thinner) plot for each site refers to the “top” panels only; arrowed dates show the 6-wk immersions (see text).

CLACHAN

SElL

Fig. 9. Plots of the total number of species and/or taxa recorded for all three gap-sizes (“tops” + “bottoms”) at each site.

correspondence between the intertidal and sublittoral. Total individuals settling at any one time in the sublittoral were generally an order of magnitude in excess of those for the intertidal (see also Tables II and III). The sublittoral winter trough (February) showed a minimum settlement rate of ~2.5 larvae. day- ’ on the 10 x 10 cm blue squares, as opposed to the summer maximum (July) of perhaps 100 larvae * day - ’ on the blue squares. Conversely, the intertidal winter minimum was attained rather later (March-April) and represented ~0.3 larvae.day- ’ on the 100 sq.cm blue areas. Indeed, the intertidal summer peak (July-August) of s 8 larvae. day- ’ barely exceeded the sublittoral winter minimum, although the latter were invariably spirorbids. Although the data for Clachan and Cuan correspond closely there was a decidedly lower amplitude in settlement recorded for the intertidal - in the main due to an excess of Ascidiellu scabru at Clachan during the summer of 1983. The “tops” only curve closely follows the “tops + bottoms” plot, but there are suggestions that regular deviations of the two do occur, with the autumn (and possibly spring) being occasions when the “bottom” panels have markedly reduced settlement.

214

CHRISTOPHER

D. TODD

AND

STEPHANIE

.I. TURNER

In the absence of error terms such observations remain only tentative, but may be explicable by the seasonality of particular taxa (see below). For the intertidal, annual fluctuations in total individuals settling were found to be relatively minor, while for the sublittoral only summer 1983 appears exceptional during the 35month period. Attention should, however, be drawn to the seven immersion periods (arrowed) during which panels were immersed for three, rather than two, tidal

100 -

Leucosolenia

botrylloides

30 10 52l-

0.1-

10 5lT > SO.+

r: 230 10 25 2:

E

1

1982

Semibalanus

crenatus

1983

Fig. 10. Seasonal settlement data for Leucosolenia, Aurelia, Spirorbis, and Semibalanus: abundances standardized for immersion period and plotted logarithmically; hatched plot refers to Cuan (intertidal), infilled plot to Clachan (sublittoral).

are the

ECOLOGY

OF CRYPTIC

EPIFAUNAL

ASSEMBLAGES

215

cycles. It would appear that these particular panel sets accumulated disproportionately large numbers of individuals. This apparent effect is perhaps corroborated by the numbers of recorded taxa (Fig. 9); here, increases in species number with immersion time are expected, if only as a result of the larger number of larvae “sampled” by the panels and of growth and development of post-larvae to specifiable size. The species and/or taxa data (Fig. 9) show, by contrast to the total individuals plots, considerable absolute similarity: between the sublittoral and intertidal. There is,

0.1

Scruwria

cheiata

J’A’S’O’N’D~J’F’M’A’M’J’J’A’S’OiN’D~J’F’MiA’M’J’J~A’S~O’N’D~J’F’M’A’M 1983 1982

Fig. 11. Seasonal

settlement

data for Verrucu, Anomia, Crisis, Afcyonidium, and Scruparia: details Fig. 10.

as in

CHRISTOPHER

216

D. TODD AND STEPHANIE J. TURNER

however, an apparent phase shift for the intertidal with a slower decline in autumn, a later winter minimum, and delayed spring rise in settling species. This is due, at least in part, to ~tween-habitat species dierences, especially for Bryozoa. SEASONALITY

OF SELECTED

TAXA

The overall seasonal patterns of settlement previously illustrated are evidently the composite outcome of differing behaviour among the large number of species. The present section considers data for particular taxa, for which both identi~cation and the

1

0.1

50

’ Tubulipora’

spp.

10

1

0.1

T

1

Exhcuoides

coccineus

fJ’A’S’O’N’D~J’F’~fA’M’f’J’A’S’O’N’D~J’F’M’A’An’J’J1A’S’O’N’D~J’F’~A’Nf

1982

Fig.

12.

1983

Seasonal settlement data for Scrupocellatia, Tubulipora, Electra, Escharoides, and Botrylhzs: details as in Fig, 10.

ECOLOGY

OF CRYPTIC

EPIFAUNAL

217

ASSEMBLAGES

numbers of individuals were deemed satisfactory. These data are necessarily underestimates in most cases; early post-larval bryozoans and ascidians, for example, were not always specifiable. In all cases the figured data are derived (standardized, for days immersed), and represent the sums of all six panels for the immersion period. Seasonality

Most taxa, with the notable exception of Spirorbis spp., showed seasonality of settlement, marked to a greater or lesser degree. The apparently extended settlement season for Tubulipora is, however, almost certainly artifactual. The majority of individuals in this grouping are Tubulipora lobigera Hastings and Plagioecia patina (Lamarck). Although easily distinguishable as small colonies the ancestrulae of these species could not be reliably separated. In addition, other rarer species - notably Diastopora spp., Lichenopora radiata (Audouin) and Lichenopora verrucaria (Fabricius) - have been observed sublittorally within this grouping. The year-round settlement of Spirorbis spp. is similarly compounded by the fact that several species are present. By contrast, Verruca stroemia, an asymmetrical barnacle which is distinguishable even at the cypris stage, showed decidedly erratic seasonality, between-habitat and withinhabitat settlement. Inter-habitat differences

Most of the species shown in Figs. lo-13 displayed denser settlement in the sublittoral than in the intertidal. While this is a representative general observation it is

20 10 5-

Didemnum

‘Ascidiansj

p -

candidum

Ascidiella

scabra

)

i0.1 -

‘I,,,, ,I, J A S 0 N D’J F MA’M

Fig. 13. Seasonal

settlement

I I I J’J A’S O’N’DIJ’F’M’A’M’J’J’A’S 1982 1983

data for Didemnum and ascidians:

0, ’ 0 N D’J I

details

as in Fig. 10.

CHRISTOPHER

218

D. TODD

AND

STEPHANIE

J. TURNER

by no means exclusive. Schizoporellu unicornis for example, is clearly an intertidal species which rarely settled in the sublittoral. Yearly variation

Acknowledging that settlement is, in general, both markedly seasonal and annual in pattern, it is important to note the year-to-year variations observed. Between-year differences in settlement of structurally important species such as the solitary ascidian Ascidiella scabra proved to be of considerable influence on the long-term development of these assemblages. In the particular case of A. scabru sublittoral settlement can be seen to have increased over the period studied. The parallel increases noted for Didemnum were not, however, of such importance belying the competitive inferiority of this colonial ascidian.

COMPARISON

OF SETTLEMENT

ON THE “TOP”

AND

“BOTTOM”

PANELS

In order to evaluate the settlement of particular taxa with respect to the “top” and “bottom” panels, data have been summed for all months, and all three gap-sizes. Because of the overall pattern of markedly reduced settlement on the narrower gap-sizes shown by almost all species (see below), these data are essentially a measure of the outcomes for the 25-mm panels. Nevertheless, it must be acknowledged that “top” or “bottom” preference may interact with gap-size. The sums are for non-standardized abundances and the “top” : “bottom” ratios are presented in Table VI, together with the results of the significance tests. These latter comprised G-tests for goodness-of-fit. of the “top” : “ bottom” ratios for the tabulated species to a standard 3 : 1; this was the overall “top” : “bottom” ratio of total individuals settling on all three gap-sizes summed over the period studied for each site. Clachan Seil

Most species settled predominantly on the “top” panel. Only Scruparia chelatu (Table VI) appears to show any preference for the “bottom” panels. Nevertheless, Leucosolenia, Alcyonidium spp., Flustrellidra, Halisarca, and possibly also Verruca, appear to display indifference with respect to the two surfaces. Of the remaining taxa nine (Escharoides to Crisiu incl.) showed markedly preferential settlement on the “tops”, with ratios significantly in excess of the overall 3 : 1. In general, the “preference” for the “top” panels was less exaggerated for the narrower gap-sizes (see below, and Table VIII), indicating perhaps the importance of flow-rates across panel surfaces in determining larval selectivity. Nevertheless, this effect on ratios will also be due, at least in part, to the absolute numbers of larvae of particular taxa settling: Scrupariu, for example, showed only one out of 308 individuals settling on the “top” panels at Clachan. As a rule the lo- and 5-mm panels were settled by fewer individuals and hence ratios are likely to be lower than for the 25-mm panels. The term

219

ECOLOGY OF CRYPTIC EPIFAUNAL ASSEMBLAGES TABLE

VI

“Preference” for the “top” and “bottom” panels for the more abundant specifiable taxa: ratios are “top” : “bottom” for the respective sites and are accompanied by the number of individuals in parentheses; the significance of the G-tests of observed ratio against the 3 : 1 standard (see text), for each taxon, are also indicated; *P < 0.05; **P < 0.01; *** P < 0.001; ns = not significant. CllCill

Clachan Ratio Escharoides coccineus Electra pilosa

35.4 1 13.3 1

“Unidentified ascidians” (Ascidiella scabra)

11.8 9.0 8.6 8.5 1.4 6.4 5.6 4.2 3.6 3.2 2.7 2.6 2.0 1.8 1.2 1.2 1

Scrupocellaria scruposa Didemnum candidum Amelia aurita, scyphistoma “Spirorbis” spp.

“Unidentified cheilostome ancestrulae” Crisia ebumea Botryllus schlosseri/Bottylloides leachii “‘Anemia” spp. “Tubulipora” spp. Semibalanus crenatus Celleporella hyalina Schizoporella unicornis Verruca stroemia Leucosolenia botryoides Alcyonidium spp. Flustrellidra hispida Halisarca dujardini Scruparia chelata

1 1 1 1 1 1

1 1 1

1 1 1

1 1 1 1.2 1 1.4 1 307.0

G

Ratio

(582) *** (57) *** (1071) ***

6.3 : 1 3.7: 1 3.5 : 1 8.0 : 1 8.0 : 0 2.6 : 1 6.0 : 1

(100) *** (1007) (1017) (7691) (483) (439) (73) (78) (5074)

*** *** *** *** ***

ns ns ns (313) ns (51) ns (3) (148) ** (3234) *** (277) *** (56) *** (157) *** (308) ***

10.8 : 1 2.0 : 1 6.3 : 1 2.8 : 1 2.9 : 1 12.0 : 1 8.0 : 0 6.1 : 1 4.0 : 1 1 : 37.8 1.4: 1 1 : 3.5 1 : 5.3 1 : 31.5

G

(66) *

(56) ns (163) ns

(108) *** (8) (18) (1323) *** (424) *** (155) * (44) ns (83) ns (257) ns (13) (8) (54) * (5) (155) *** (167) *** (18) (50) *** (155) ***

“preference” should, however, be applied cautiously, if only because the “bottom” panels were subject to a certain degree of cover by sediment and, on occasion, scouring by fine shell gravel. Thus, apparent “top” panel preference may largely reflect a reduced availability of the “bottom” primary substratum, and/or selective abrasion of previously settled (but not recorded) post-larvae. This latter possibility is probably less important here because most dead post-larval forms (except especially Porifera and Ascidiacea) left an identifiable trace or skeleton. Cuan Ferry

As for Clachan Seil most species-taxa (in Table VI) settled preferentially on the “top” panels. Leucosolenia and Halisarca did, however, clearly select the “bottom” panels while Scruparia behaved similarly at the two sites. The between-site contrast in settlement patterns is also notable. While specification of post-larval Bryozoa was frequently impossible the overah pattern of intertidal preference of this phylum for the “top” panel was particularly marked; this, indeed, is reflected in the subsequent development of intertidal assemblages. It is also apparent that some species

220

CHRISTOPHER

D. TODD

AND

STEPHANIE

J. TURNER

demonstrated less extreme ratios of settlement than in the sublittoral, although to what extent this arises from the reduced number of intertidal individuals is difficult to evaluate. A three-factor G-test of independence (Table VII) of settlement was undertaken for the data summarized in Table VI, with the 2 1 species, the two sites and the two surfaces (“top” and “bottom”) as the criteria. The hypothesis of joint independence of the three factors on settlement is clearly rejected, as are the hypotheses relating to pairwise independence. (The negative overall interaction is not an unusual outcome for this analysis.) Evidently, there is considerable variation in settlement responses for these taxa both within and between sites. TABLE VII

Three-factor G-test of independence between the total number of individuals settling on the “top” and “bottom” panels (Surface), at Cuan and Clachan (Sites) for the 21 taxa (Species) shown in Table VI: the hypotheses of joint and pairwise independence of the three criteria are all rejected; *** P -c0.001.

Species x Site Independence Species x Surface Independence Species x Site Independence Species x Surface x Site Interaction Species x Surface x Site Independence

d.f.

G

20 20

4701.0*** 5729.2*** 13.4*** - 1725.2*** 8718.4***

I 20 61

The data for Alcyonidium spp. perhaps merit particular attention. This grouping undoubtedly includes at least two species. It was therefore noteworthy to record that over the 35-month period, for the intertidal, settlement ratios ofAlcyonidium statistically favoured the “top” in September-October, the “bottom” in December, and the “top” again in February (G-test of 1 : 1 ratio). Similarly, for the sublittoral, settlement in December was statistically significant to the “bottom” with a switch to the “top” for January-February and a reversion to the “bottom” in March-April. The indications are that the composite Alcyonidium spp. differ in their settlement behaviour and that the unavoidable lumping of these ancestrulae has resulted in an apparent indifference in selectivity of the respective surfaces. EFFECTS

OF GAP-SIZE

ON SETTLEMENT

- “TOP”

PANELS

ONLY

The consideration of effects of the three gap-sizes on settlement of particular taxa will be confined to the “top” panels since it is these upon which the definitive analyses of assemblage dynamics are based. As for the “top” and “bottom” evaluation, the data for all months have been summed, in order to demonstrate the overall trends. The data in Table VIII summarize the outcomes and are scaled to illustrate particular species’ responses in relation to the 5-mm panel, and Table IX shows the three-factor G-test

221

ECOLOGY OF CRYPTIC EPIFAUNAL ASSEMBLAGES TABLE VIII

Settlement ratios for the more abundant specifiable taxa at the two sites on the 25-mm, IO-mm, and 5-mm “top” panels (except where indicated): values for the 25-mm and IO-mm panels are standardized on the numbers settling on the 5-mm panels; the 25-mm : 5-mm ratios were compared against a standard 6 : 1 (Clachan)or 5 : 1 (Cuan) ratio by G-test; * P < 0.05; ** P < 0.01; *** P < 0.001; ns = not significant; t these values are for the “bottom”, not the “top”, panels. Cuan

Clachan 25-10 : 5 G

n Semibalanus crenatus

“Unidentified ascidians” (Ascidiella scabra) Verruca stroemia Didemnum candidum Escharoides coccineus

BohyBus~Botrylloides “Unidentified cheilostome ancestrula” “Spirorbis” spp. ScrupoceNaria scruposa Aurelia aurita, scyphistoma Electra pilosa “Tubulipora” spp. Crisia eburnea Leucosolenia botryoides Scruparia chelata Alcyonidium spp. “Anemia” spp. Schizoporella unicomis Celleporella hyalina

209 801 90 686 427 48 316 5272 70 554 36 2746 259 1388 217t 119 41

51.3-4.8 32.4-7.8 29.0-2.0 24.4-8.0 9.4-3.4 8.6-2.4 8.3-2.9 7.4-2.4 5.4-1.8 4.8-3.7 4.1-2.6 3.9-2.0 3.8-2.1 3.8-1.3 3.4-1.8 3.1-1.1 3.1-1.9

24

: 1 *** : 1 *** : 1 *** : 1 *** : 1 *** : 1 ns : 1 ns : 1 *** : 1 ns : 1 ns : 1 ns : 1 *** : 1 ** : 1 *** : 1 ** : 1 ** : 1 ns

n

25-10 : 5 G

85

6.1-3.5 : 1 ns

45 31 295 809 72

3.5-1.2: 1 ns 5.2-1.4 : 1 ns 4.7-1.8 : 1 ns 6.3-2.9 : 1 * 3.0- 1.4 : 1 ns

32 133 80 70+ 44t 71 42 32

3.6-1.7 : 1 ns 3.3-1.9 : 1 * 3.0-1.2 : 1 ns 1.1-2.4: 1 *** 5.3-2.4 : 1 ns 4.5-2.1 : 1 ns 2.5-1.6 : 1 ns 1.9-1.4: 1 *

1.2-1.2 : 1 ***

TABLE IX Three-factor G-test of independence between the numbers of individuals of each taxon (Species) settling on the 25-mm, IO-mm, and 5-mm panels (Gap-size), at Clachan and Cuan (Site), for all species in Table VIII: data for “top” panels only; the hypotheses ofjoint and pairwise independence are all rejected; *** P < 0.001.

Species x Site Independence Species x Gap-size Independence Gap-size x Site Independence Species x Gap-size x Site Interaction Species x Gap-size x Site Independence

d.f.

G

18 36 2 36 92

2051.4*** 785.0*** 42.0*** 115.6*** 2994.0+**

(Species, Site, Gap-size) of independence for the numerical frequencies. The hypotheses ofjoint and pairwise independence ofthe three criteria are all rejected and the significant interaction indicates the absence of homogeneity among the ratios. It must be emphasized that the 25 : 5 mm and 10 : 5 mm ratios presented are not invariate; just

222

CHRISTOPHER

D. TODD

AND

STEPHANIE

J. TURNER

as there were considerable month to month variations in “top” and “bottom” settlement, so too there were for gap-size responses. As illustrative of the point Table X provides the numbers of Escharoides settling on the three “top” panels for those occasions when all three were settled contemporaneously. TABLE X

Effects of gap-size on settlement of the bryozoan Escharoides coccineus at both sites: the months shown are those for which the (“top”) panels (only) of each of the three gap-sizes were settled contemporaneously; note the considerable variation in response between months. Clachan 25mm July 1981 September 1981 October 1981 July 1982 August 1982 June 1983 July 1983 August 983 September 1983 October 1983

1O-mm

Cuan 5-mm

11 6 2

3 6 1

1 2 1

20 15 68 128 64 15

2 8 16 35 29 13

2 5 3 14 9 1

25-mm

IO-mm

5-mm

19

6

2

1

1

2

By observation the flow-rate of water through the gap-sizes differs, with 5 < 10 < 25 mm as expected. By measuring through-flow rates under varying ambient conditions we hoped to obtain an independent comparative predictor for the effects of gap-size on approximate water volume (= larval numbers) passing through the panel pairs in unit time. Such a measure would have enabled tests of “preference” of specific taxa for particular gap-sizes. Unfortunately, however (see p. 226), this proved intractable and we have, therefore, selected 25 : 5 mm ratios of 6 : 1 (Clachan) and 5 : 1 (Cuan) as the breakpoints of selection for the wider panels. (These ratios were the overall outcomes for all individuals at each site over the period studied.) No statistical evaluation of settlement on the lo-mm panels is given because the low abundances may provide misleading outcomes. On this basis (see Table VIII), it is evident that Semibalanus, Ascidiella, Verruca, Didemnum, Escharoides, and “Spirorbis” spp. for the sublittoral, but only “Spirorbis” spp. for the intertidal, settled disproportionately on the widest gap-size. Conversely, “Tubulipora” spp., Crisis, Leucosolenia, Alcyonidium, and Celleporella, for the sublittoral, and “Tubulipora” spp., Leucosolenia, and Schizoporella in the intertidal appear to be either indifferent, or perhaps even “prefer” the narrowest gap-size. It is likely that the apparent lack of intertidal “preference” for the 25-mm panels is largely attributable to the low species abundances; nonetheless, avoidance of wider gap-sizes, which are presumably more desiccation-prone in the intertidal, may in fact pertain.

ECOLOGY

OF CRYPTIC

EPIFAUNAL

ASSEMBLAGES

223

It is, perhaps, to be expected that the high-relief solitary Ascidiella scabra (which dominates the “Ascidian” grouping) should positively select for the wider gap-size, and the responses of barnacle cyprids to surface flow-regime are well-documented (Crisp, 1974; Rittschoff et al., 1984). Of more interest, however, are the data for Didemnum, a low-profile, small, competitively inferior (unpubl. obs.) colonial ascidian. Of all species we would have expected Didemnum to settle perhaps selectively in the narrower gap-sizes. ANALYSIS

OF “EDGE

EFFECTS”

ON SETTLEMENT

Recording of individuals in each of the 100 1 cm2 units of the grid overlays has permitted the evaluation of whether or not larvae tended to settle in the marginal or central regions of the “blue” squares. x2 tests have been undertaken for the outer ring of 36 squares versus the inner 64, and for the outer two rows and columns (64 squares) versus the inner 36. x2 tests were restricted to the “top” panels only and included only those with at least 30 individuals. Where appropriate densities were available, particular taxa were also tested independently of all other organisms. For Clachan, 19 out of 25 x2 tests (for all individuals summed) showed a significantly greater marginal than central settlement (36 : 64), and for these same 25 panels 16 showed significant increases when comparing the outer two rows and columns with the centre (64 : 36). There was no significant correlation between deviation from homogeneity and total density of individuals. Tests for particular taxa showed little discernible pattern, with the exception of “Tubulipora”, which consistently (8 out of 16, 36 : 64; 9 out of 16, 64 : 36) showed significantly (P ~0.05) increased marginal settlement. Nevertheless, this grouping does include several species of cyclostome bryozoans. For Cuan fewer panels were settled in abundances adequate to test, but there were 4 out of 9 (36 : 64) and 2 out of 9 (64 : 36) deviations from homogeneity for total individuals on “top” panels. No species displayed any consistent response and the above deviations presumably result from the summed effects of several to many species.

DISCUSSION

These field sites are located in extremely sheltered areas and neither the present settlement observations, nor the subsequent (unpubl.) long-term analyses of assemblage dynamics, should be interpreted as characteristic or typical of all rocky coasts throughout the British Isles. Moreover, it must be emphasized that the above data provide a picture only of potential recruits to otherwise unoccupied habitat “patches” (panels) free of well-established encumbents. It is well-documented that established (filter-feeding) epifaunal invertebrates may exert inhibitory effects on setting larvae simply by their occupation of primary space and/or their feeding activity (reviewed by Connell dc Slatyer, 1977; Sousa, 1984; but see also Dean dc Hurd, 1980; Day & Osman,

224

CHRISTOPHER

D. TODD AND STEPHANIE .I. TURNER

1982). Furthermore, the grazing activities especially of prosobranch molluscs may exert marked effects on epifaunal establishment and subsequent survival (unpubl. pers. obs.). More individuals of more species settled on the experimental “blue” stained panels, by contrast to the untreated “normal” substrata over an &month period. Certainly this result would be expected (see Yule & Walker, 1984) but precisely how this relates to the local natural (black shale) substrata remains unresolved. In view of the dark colour of the field substrata we feel it is likely that the “blue” panels do provide the closer approximation. Settlement on both the intertidal and sublittoral “blue” panels was strongly seasonal (Figs. 8 and 9), and while there were marked between-site contrasts in absolute numbers settling at any one time, there was a striking consistency of correspondence. Over the 35-month period of this initial study, panels were immersed and retrieved on 32 occasions and a total of 25 644 organisms were quantified, and identified to species wherever possible. These individuals were attributable to 41 taxa although perhaps up to 60 are found in well-established panel assemblages. Many bryozoan and ascidian larvae and post-larvae are simply not identifiable on these panels. A considerable degree of species similarity was recorded for the two sites (Table I) - 33 of the 41 species and taxa were co-joint, with four others exclusive to each habitat. Nevertheless, this apparent similarity was overwhelmed by major quantitative differences, and additional contrasts in the subsequent survival of species at the two sites. In total 22 260 individuals were recorded sublittorally (Clachan) and 3384 intertidally (Cuan). (This ratio of 6.6 : 1 for absolute numbers was matched by a value of 6.8 : 1 when comparing the derived abundance data, standardized for immersion duration). Due to the variations in tidal amplitude it is not possible to deduce the precise duration of air-exposure for the intertidal panels. The retaining frames were located at the upper margin of the Laminaria digitatu (L.) zone and estimates suggest an average of two periods of 2-h emersion on each of 5 days over a spring tide period. This approximates to 40 h per month, or z 6% of total time. Clearly, therefore, the reduction in intertidal settlement cannot simply be accounted by reduced “exposure” to potential recruits, but reflects differential selectivity by settling larvae. It is likely that substratum microfloral contrasts provide settling larvae with cues indicative of future survival potential (Strathmann & Branscomb, 1979; Strathmann et al., 1981). Such responses are, perhaps, most clearly seen in the seasonal data for particular taxa (Figs. 10-13) whereby certain species were shown to settle predominantly in one or other habitat (usually Clachan) while, for example, Antedon blJrda displayed exclusively intertidal settlement. With regard to seasonality of epifaunal settlement Keough (1983) differentiated three categories of pattern for a sublittoral south Australian assemblage. First, species (such as spirorbids, as in the present study) which demonstrated year-round settlement and little or no seasonality (see also Chalmer, 1982); secondly, species which settled regularly and consistently at restricted times of year, and thirdly, species which displayed erratic or irregular peaks of settlement. This third pattern appears absent from

ECOLOGY OF CRYPTIC EPIFAUNAL ASSEMBLAGES

225

Fig. 14. Seasonal settlement data (Cuan only), for the primarily intertidal bryozoan Schizoporella unicornis, July 1981-November 1985: the vertically hatched plot refers to the numbers of individuals on all three gap-sizes, “tops” and “bottoms” combined; the infilled plot represents those on the 25-mm panels, i.e. in 1982 all settlement was on the lo- and 5-mm panels only; for 1984 and 1985 only 25-mm panels were immersed; see below for further details.

our data although some “regular” species showed occasional failure during their respective seasons. As might be expected, most species settled preferentially on the (lower surface of the) “top” panels; this is the surface least subject to sedimentation or scour. Nonetheless, certain species - especially the bryozoans Scruparia chelata and Aetea anguina - proved to be “bottom” panel specialists, as indeed, was the pentacrinoid of Antedon bifda. Scruparia and Aetea are well-adapted to overgrow (and bind) shell-gravel fragments, and we suggest that this disposition of individuals results from specific larval selectivity rather than passive settlement. The data for Antedon are, perhaps, of additional interest, for this echinoderm provides evidence of the reliability and reproducibility of our (unreplicated) observations. This species always settled in the intertidal only, invariably on the “bottom” panels, but was always scarce and only observed in June-July; we could, in fact, predict more or less precisely, numbers, orientation and date for this species. It is also of importance that the various species, with some notable exceptions, responded similarly to the “top” and “bottom” panels in the two habitats. Nonetheless, the likelihood of marked local patchiness of the larval plankton, and its effect on realized recruitment (Keough, 1983; Young, 1985), must be acknowledged. A related factor affecting local occupation of newly-available space might be the proximity of species of reduced dispersal capacity. Fig. 14 presents the observed settlement of the bryozoan Schizoporella unicornis in the intertidal; this species is of major importance in such assemblages. The figured data show the numbers settling on the three gap-sizes combined (shaded) but also distinguishes those on the 25mm panels alone (black). From May 1984 to November 1985 the lo- and 5-mm panels were discontinued; clearly, in August-September 1985 we might have expected perhaps 5 individuals. day- ’ in a complete set. This rapid rise in settlement is undoubtedly due to the reproduction of colonies established on other panels within the frame, or in adjacent frames, and was

226

confirmed

CHRISTOPHER

by (unpubl.)

D. TODD AND STEPHANIE J. TURNER

observations

of dense

settlement

around

reproductive

Schizoporella colonies. This does, however, appear to be the only marked occurrence

of this behaviour in the present study. The consideration of settlement on the three gap-sizes remains less than satisfactory. Despite concerted efforts to obtain reliable and reproducible laboratory flume measures (using heated-bead thermistors) of water-flow between panels, the outcome was equivocal. Our intention was to obtain such an independently derived measured of flow-rates, at realistic tidal stream velocities, in order to test statistically preferences of species for particular gap-sizes. Eckman .(1983) and Nowell & Jumars (1984) have emphasized the importance of hydrodynamic effects on benthic settlement and urge that these are accounted when attempting to identify mechanisms which affect recruitment itself and assemblage structure in general This we have failed to do. If it is assumed that panel-pairs of the three gap-sizes (25, 10 and 5 mm) present essentially similar profiles to a laminar water-flow, then through-flow should approximate the square of the inter-panel distance; thus, the 25mm panels should be subject to an internal through-flow 25 x that of the 5-mm panels. Even accepting this assumption we would then have to account for the total water volume ( = larval numbers) passing through each pair (in a variable tidal regime), and the speed at which larvae traverse the panel. Moreover, in the wider gap-sizes, larvae would have to move greater vertical distances (in less time) in order to penetrate the boundary layer and contact the substratum. Clearly, the hydrodynamic facets are extremely complex and we can at present only acknowledge these causal factors and rely simply upon subjective judgements as to the apparent selectivity of particular species. While some species, such as Celleporella hyalina, showed indifference or even perhaps preference for the narrower gap-sizes, most settled disproportionately on the wider panel-pairs. In particular certain structurally important or potentially dominant taxa (e.g. Escharoides, Didemnum, Semibalanus and “Ascidians” - mostly Ascidiella scabra) displayed unquestionable selectivity for the 25-mm panels. Different species will exhibit differing preferenda for a number of local habitat features; surface flow-rate is probably of overriding, but not exclusive, importance in this context. It has, however, recently become apparent that not only established colonies (e.g. Young & Chia, 198 1) but even post-larvae of certain competitively superior species may discourage settlement of diverse other taxa (Grosberg, 198 1). Thus, even short-term observations of settlement (such as the present data) may not be without bias. Even so, the smoothness and inter-site correspondence of our summed (but unreplicated) data (Figs. 8 and 9) lead us to believe that reliable observations have been obtained. Perhaps the major pitfalls associated with all experimental epifaunal studies concern the nature and size of the panels themselves. A review of the literature over the past 45 yr has shown that at least 27 different forms of artificial substratum have been used. These range from plastics to cement, ceramic tiles, glass, metals, and wood. Natural slate has been employed on numerous occasions (e.g. Crisp & Barnes, 1954; Luckens, 1976; Osman, 1977,1978; Scheltemaet al., 1981). Our use of a slate similar to the local

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natural substrata minimized this variable but the size of panels was a subjective logistic decision. The most frequently used panel sizes have ranged from z 100-600 cm’, although some very much smaller units have been investigated. Osman (1977), for example, used 14.5 cm’ panels, among others, and Grosberg (198 1) used glass plates of 25 cm’. The effects of panel size on the number of settling individuals and species, as well as concomitant effects on subsequent assemblage development, have been extensively evaluated (e.g. Jackson, 1977b; Osman, 1977). In essence, larger panels will, in a given time, accumulate more individuals of more species - the latter to an asymptotic level. The variance in number of species settling will also decline with increasing panel size (Keough, 1983; Keough & Butler, 1983). But variations in panel size do not exert only areal influences; Osman (1977) has pointed to the changes in current velocity and turbulence which accompany differences in panel size. Thus, larval responses to these factors alone may result in qualitative and/or quantitative shifts in settlement on panels differing only in size. Our 225-cm2 panels are small but, we feel, not unrepresentative in terms of the situation they mimic. Several studies have shown that there is an effect of distance from panel edges on the dispersion of individuals across the substratum, presumably due to variations in water-flow regime. Vandermeulen & DeWreede (1982) have reported “edge effects” for Obelia and Balanus, while Olson (1983) found that Didemnum molle tadpoles settled in a distinct band around the outer edges of the undersides of panels. Like others (e.g. Foster, 1975; Day, 1977; Moran, 1980) we chose to ignore the edges of our 15-cm square panels and considered only the (10 x 10 cm) central squares. Nonetheless, there was still some suggestion of an edge effect even within the area analysed, although no particular species (other than the aggregate “Tubulipora” spp.) displayed consistent significant patterns of settlement. This initial paper thus summarizes the qualitative and quantitative features of settlement, and thereby potential recruitment, to epifaunal assemblages on the coast of Argyll, Scotland. Subsequent reports will be concerned with the long-term development and “stability” of these intertidal and sublittoral assemblages. ACKNOWLEDGEMENTS

We thankfully acknowledge the assistance of the following in our fieldwork: S. Dar-rig, P. Elvin, S. Hall, J. Havenhand, H. Penrose, and E. Pollock. Technical assistance was unfailingly provided by P. Baxter and R. Jack. The conception and execution of this programme owes much to discussions with (and advice from), numerous colleagues in this, and other laboratories, but especially J. R. Lewis. Vyflex Ltd. (Cheshire, England) kindly assisted in the corrosion-proofing of the experimental frames. We also thank the Director and technical staff for the provision of space and facilities at the DunstafTnage Marine Research Laboratory of the Scottish Marine Biological Association, Oban. Alan Butler kindly offered his constructive criticism of an earlier draft of the manuscript. Financial support derives from GR3/4193“A” awarded to C.D.T. by the Natural Environment Research Council; to all we are grateful.

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