Distribution of epibenthic megafauna and lebensspuren on two central North Pacific seamounts

Deep.SeaResearch,Vol. 36, No.

12,

pp. 1863-1896,1989.

0198--0149/89$3.00+ 0.00 ~) 1990PergamonPressplc.

Printedin Great Britain.

Distribution of epibenthic megafauna and lebensspuren on two central North Pacific seamounts RONALD S. KAUFMANN,* W . WALDO WAKEFIELD* a n d AMATZIA GENINt

(Received 10 March 1989; in revisedform 26 June 1989; accepted 1 August 1989) Abstract--The abundance, composition and spatial distribution of megafaunal communities and lebensspuren assemblages at three sites on two deep seamounts in the central North Pacific were surveyed photographically using still cameras mounted on the research submersible Alvin. Photographic transects were made on the summit cap (~1500 m depth) and summit perimeter (~1800 m depth) of Horizon Guyot and on the summit cap (~3100 m depth) of Magellan Rise. The summit caps of both seamounts were covered with foraminiferal sand, while the summit perimeter of Horizon Guyot was characterized by numerous rock outcroppings (basalt and chert encrusted with ferromanganese oxides) on which was situated a speciose assemblage of suspension-feeding organisms. The most abundant megafauna at all three sites were large, sedimentagglutinating protists belonging to the class Xenophyophorea. Among the three sites, the Horizon Guyot summit cap supported the highest densities of fishes and lebensspuren and the fewest echinoderms, while the Magellan Rise summit cap was populated by a diverse community of deposit-feeding echinoderms. Megafaunal abundances on Horizon Guyot were lower than those at equivalent depths on the western North Atlantic continental slope, while those on Magellan Rise were higher. The faunal differences observed between the two seamounts were attributed primarily to differences in hydrodynamic conditions, substrate availability and nutrient availability. Most of the lebensspuren on these seamounts appeared to be patchily distributed on spatial scales of 10-1000 m, while xenophyophore distributions were predominantly random on the same spatial scales. Biogeographically the species identified exhibited predominantly widespread to cosmopolitan distributions with Indo-West Pacific faunal affinities, typical of other seamounts in the same depth range and biogeographic province.

INTRODUCTION

SEAMOUNTSare prominent features in all major ocean basins, yet the biota they support have been poorly investigated. Biological data have been reported for more than 100 seamounts worldwide (WILSON and KAUFMANN, 1987), yet most publications on seamount-dwelling organisms deal exclusively with the representatives of a single taxon, and there is relatively little published information concerning either the spatial or ecological structure of seamount communities. The most conspicuous species inhabiting seamounts and other deep-sea habitats are members of the megafaqna (GRASSLEet al., 1975; WILSON et al., 1985). Operationally, "megafauna" may be defined as organisms large enough to be recognized in photographs (typically I>1-2 cm; GRASSLEet al., 1975; REX, 1981; OHTA, 1983; SMITHand HAMILTON, 1983). Megafauna comprise a sizeable fraction (17-50%) of the deep-sea benthic biomass * Marine Biology Research Division, A-002, Scripps Institution of Oceanography, La Jolla, CA 92093, U.S.A. t H. Steinitz Marine Lab., P.O. Box 469, Eilat 1863, Israel. 1863

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(HAEDRICHand RowE, 1977; SIBUET and LAWRENCE,1981; SIBUET et al., 1984), and information on their abundance and distribution is critical to an understanding of deepsea benthic ecology. Megafauna also are thought to be the primary agents of bioturbation in the deep sea (MAuwEL and SIBUET, 1985; LEVINet al., 1986; LEVINand THOMAS,1988) and thus may influence many other biological and geochemical components of the deepsea floor. Data on the distribution of megafauna within and between different areas on a single seamount have been gathered (GENINet al., 1986; LEVINand THOMAS,1986; GENIN, 1987; GRIGG et al., 1987; LEVIN and THOMAS, 1988; GENIN and SMITH, unpublished data), but dispersion patterns of soft-bottom seamount fauna have not been examined quantitatively. This may be due to the general use of non-quantitative sampling gear (WILSONand KAUFMANN, 1987). While the spatial distribution of soft-bottom epibenthic megafauna has been studied on continental slopes (GRASSLEet al., 1975; HAEDRICH et al., 1975; HAEDRICHand RowE, 1977; RICE et al., 1979, 1982) and in bathyal basins (BARHAMet al., 1967; SMITHand HAMILTON,1983), such studies have not been performed on seamounts. In general, there is a paucity of published data on the distribution of deep-sea epibenthic megafauna. There is a similar lack of information about biogenic sedimentary structures, collectively termed "lebensspuren" (HANTZSCHEL,1962; EWING and DAVIS, 1967). These structures are generated through the action of epi- and infaunal benthic organisms, with some of the most distinctive and conspicuous forms being produced by megafauna (YOUNO et al., 1985). Megafaunal densities seem to correlate poorly with lebensspuren densities (KITCHELLet al., 1978; YOUNG et al., 1985; WHEATCROFr et al., 1989), a relationship attributed to the "smoothing out" of lebensspuren by some depositfeeding organisms (YouNo et al., 1985; WHEATCROFTet al., 1989). However, a positive correlation has been reported between the number of megafaunal species and lebensspuren morphotypes within a given area (KITCHELL et al., 1978; YOUN6 et al., 1985), suggesting that lebensspuren may be good indicators of species richness in the absence of or as a complement to megafaunal observations. This is especially important when considering the community structure of epibenthic megafauna in the deep sea, since such traces are often the only visible signs of certain species' presence (HEEZENand HOLLISTER, 1971; HOLLISTERet al., 1975; MAUVIELet al., 1987). Biogeographically, seamounts can be likened to islands separated by large areas of deep ocean. They have thus been seen as possible "stepping stones" for species dispersal and as potential isolating mechanisms for speciation (HAMILTON, 1956; HUBBS, 1959; RAO and NEWMAN, 1972). Consequently, seamounts may serve as oceanically isolated distributional centers for many species restricted to relatively shallow-water environments and as refugia for relict populations of species that have disappeared from the greater portions of their former ranges (Hvaas, 1959). In March 1987, D.S.R.V. Alvin and its support ship, R.V. Atlantis II, were used to make observations and conduct experiments on two deep seamounts in the central Pacific, Horizon Guyot and Magellan Rise. In conjunction with these studies, a series of photographic transects was performed to provide quantitative data on the abundance, spatial distribution and community structure of the megafauna and lebensspuren on these two seamounts. With this information we sought to answer the following questions. (1) How do the abundance, spatial distribution and community structure of epibenthic megafauna and lebensspuren differ between Horizon Guyot and Magellan Rise? (2) How do these parameters on the two seamounts compare with those in areas at

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similar depths on continental margins? (3) Are there differences between these parameters on the summit cap and summit perimeter of a single seamount (Horizon Guyot)? (4) What are the biogeographic affinities of the megafauna from these two seamounts? MATERIALS AND METHODS

Study site description Horizon Guyot (19%0'N, 169°00'W) is an elongated, fiat-topped seamount rising --3500 m above the surrounding sea floor with a summit at ~1440 m depth (Fig. 1). Its summit, as delimited by the 2000 m depth contour, is approximately 135 x 40 km and is covered by a cap of carbonate sediments as much as 160 m thick (KARIO et al., 1970; LONSDALEet al., 1972; KAVENet al., 1989). The summit perimeter of Horizon Guyot, in particular the northern perimeter, is characterized by exhumed rock terraces (principally basalts) and outcrops of middle Eocene chert, most or all of which are encrusted with black ferromanganese oxides (LONSDALEet al., 1972; HEIN et al., 1985; KAYEN et al., 1989). In contrast to many other seamounts, Horizon Guyot has been extensively sampled and, prior to this study, was known to support at least 29 megafaunal species (RAO and NEWMAN, 1972; WILSONet al., 1985; WILSONand KAU~iANN, 1987). Magellan Rise (07°00'N, 176°00'W) is a large plateau with a summit at -3100 m and a base at -5500 m depth (Fig. 2). The summit is capped by carbonate sediment nearly 1200 m thick (WINTERERet al., 1973) and is roughly 150 × 100 km at the 3500 m depth contour (MAMMERICr,X and SMITH,1985). Sample collections Biological samples were obtained on both seamounts using thermally insulated baited traps, a dip net operated by the starboard manipulator arm of Alvin, and the grasping "hand" of Alvin's starboard manipulator arm. On Magellan Rise, additional samples

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Seabeam bathymetric map of Magellan Rise, showing locations of photographic transects from Alvin dives 1815, 1816 and 1818,

were collected using a 15.2 m semi-balloon bottom trawl with 3.8 cm mesh (stretch measure) in the wings, body and codend, and a 1.3 cm mesh codend liner. Animals collected by these means were identified to species whenever possible and were used to assist the identification of organisms in photographs. Photographic transects Photographic transects were performed on both seamounts using Alvin's E G & G Model 20t.B survey cameras, equipped with 28 mm Nikkor lens6s. These cameras were mounted as a stereo pair on Alvin's sponson and were each capable of taking ca 800 photos without reloading. Photographic transects suitable for quantitative analysis were made on five dives: two on Horizon Guyot, covering - 3 7 , 0 0 0 m 2, and three on Magellan Rise, covering -105,000 m 2 (Table 1). Throughout each transect, except one (Horizon Guyot summit perimeter), the submersible followed a constant heading and trim at a speed of 0.9 kn (46 cm s-1) and an altitude of 0 m (sub's skids on the sediment surface). Transects were from 47 to 110 rain long, with photographs taken at 4-15 s intervals, resulting in distances of 1.9-7.4 m between centers of consecutive photographs. During each transect, the submersible's inclination was measured by holding an inclinometer against a horizontal strut within the sub. This strut was parallel to the submersible's runners, at an angle of 27 ° to the optical axes of the survey cameras. Thus, when the submersible's inclination was 0 °, the optical axes of the cameras were inclined at 27 ° below horizontal. The height of the cameras above the substratum (209 cm) was

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Table 1. Dates, locations, photographs taken and bathymetric ranges of quantitatively analysed Alvin photographic transects on Horizon Guyot and Magellan Rise Dive

Date (1987)

Site

1809HG 1810HG 1815MR 1816MR 1818MR Total

6 March 7 March 16 March 17 March 19 March

HG HG MR MR MR

summit perimeter summit summit summit

Photos

Depth range (m)

680 420 735 497 611 2943

1482-1532 1754-1854 3113-3121 3112-3142 3124-3152

Total area (m 2) 22,850 -14,000 47,000 16,700 41,550 - 142,100

HG, Horizon Guyot; MR, Magellan Rise.

measured while the submersible was stationary on the deck of Atlantis II. Once the submersible's trim had been established, repetitive inclination measurements during the course of a single photographic transect (except dive 1810) did not display any variation. Additional photographic information was collected during each dive with an Osprey video camera mounted on the starboard arm of Alvin. The resulting video tapes, totalling 80+ h, were viewed to observe the behavior of animals encountered by the submersible. Other natural-history information was gleaned by viewing survey-camera photographs that were not quantitatively analysed (see below) and by studying records kept by observers on each dive.

Photographic analysis The use of photogrammetric techniques for the analysis of oblique, non-stereoscopic pictures of the sea floor has been well documented (GRASSLE et al., 1975; SMnna and HAMILTON, 1983; YOUNG et al., 1985; WAKEFIELD and GENIN, 1987). This involves superimposing a perspective grid over each photograph, permitting rapid and precise calculations of the sizes and relative positions of objects within the field of view. When using this method we assume: (a) a flat substrate with no appreciable relief, and (b) that the horizontal axis of each photograph is parallel to the sea floor (WAKEFIELD and GENIN, 1987). Our initial examination of the transect photographs produced a subjective classification scheme to encompass the range of recognizable features (organisms and lebensspuren, collectively) on the two seamounts. The resulting categories were defined taxonomically, when possible, and morphologically, otherwise. This classification scheme was used to identify organisms and lebensspuren recorded during analysis of the transect photographs. Because of the uncertainty associated with identifying megafaunal species from photographs, we use the term "species" only in instances where positive identifications were possible. In all other cases, organisms and lebensspuren were classified morphologically, and we refer to these interchangeably as "forms" or "morphotypes". A perspective grid was constructed for each transect from measurements of camera altitude and inclination, along with the horizontal and vertical angles of view for the lenses. Using a modified Beseler model 23C-II enlarger, the film from each transect was projected onto a digitizing pad (Science Accessories Corporation), to which the appropriate grid had been affixed (one grid per transect). Within a given photograph, the classification and relative position of each object was recorded with a computer digitizing program developed for the analysis of oblique photographs (GENIN and WAKEFIELD, unpublished computer program). Replicate measurements of grid lines with lengths

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equivalent to 20 cm on the sea floor yielded standard deviations of +0.95 and +1.22 cm (N = 5) in the horizontal and vertical directions, respectively, for the center of the grid. These calculations assume a submersible inclination of 0° and represent the mechanical precision of the digitizing apparatus. Linear dimensions of megafauna and lebensspuren were measured whenever possible (i.e. when an entire organism was in contact with the substrate, for megafauna). Very abundant features, specifically single mounds, common xenophyophores (large, sediment-agglutinating protists), sediment columns and "stalks", were either recorded from a subset of regularly spaced photographs within a transect (dive 1809) or not recorded at all for replicate transects from a single area (dives 1816 and 1818). Species which are demersal (typically at rest on the substrate but capable of excursions into the water column) or benthopelagic (seldom, if ever, make contact with the substrate but live in association with the bottom) present censusing problems not associated with obligately benthic forms. First, because of their swimming ability, these organisms are capable of avoiding certain environmental disturbances (e.g. camera-bearing submersibles), thus decreasing apparent densities. Second, because these organisms were an indeterminate distance above the substrate, it was not possible to ascertain their exact positions relative to our grid on the sea floor. Thus, there is a degree of uncertainty associated with density estimates for benthopelagic species, and our calculated densities for species capable of avoidance (shrimps and most fishes) must be regarded as minimum estimates, while densities of non-evasive benthopelagic species [e.g. scyphozoans, pelagic holothuroids (Peniagone)] possess an intrinsic error not associated with benthic and demersal forms. Because of the unevenness of the substratum on the flank of Horizon Guyot, data on the inclination and altitude of the submersible were unavailable for A l v i n dive 1810. Thus, we were unable to digitize this transect, and features instead were counted and recorded on a per-photograph basis. Densities were calculated using an estimate of the area covered by each photograph, based on the assumption that the submersible's inclination was approximately 0° (~33 m2 per photograph). The densities generated by this method are probably accurate only to within a factor of two and are likely to be underestimates, since, when photographing hard substrate obliquely, features on the opposite side of or in the shadow of rock outcrops are not visible. The extent of rock cover in the area surveyed during dive 1810 was estimated by projecting the transect film onto a perspective grid based on a submersible inclination of 0°. Ten regularly spaced points (on the sea floor) were marked on this grid, and the number of points impinged upon by rock was noted for each frame (cf. GENIN,1987). A certain amount of error is inherent in the use of a perspective grid for the analysis of oblique photographs. The magnitude of this error is directly proportional to the distance of an object from the nadir point [defined as the point on the substratum directly beneath the nodal point (center) of the camera lens] and to the camera's inclination (WAKEFIELD and GENIN, 1987). This error may be reduced a posteriori by limiting the portion of each photograph used in subsequent calculations (see below). Another potential source of error is the variability in the slope of the bottom and in the submersible's inclination and altitude during a transect, which may affect the measurement of features within a single photograph. Based on measurements of bottom slope and submersible inclination during our transects, we believe our inclination values to be within +0.5 ° , resulting in density estimates that are accurate to within +7.5%.

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Before initiating the analysis of data from the photo transects, rarefaction curves were constructed to examine the minimum area required for meaningful interpretations and comparisons of community structure. These curves were generated by plotting the cumulative number of forms present in each transect against the cumulative number of photographs analysed in that transect. Theoretically, such a curve approaches an asymptote at the cumulative area sufficient to encounter all of the common taxa of megafauna or lebensspuren within a study area (HEssLER and SANDERS, 1967; OI-rrA, 1983).

Data analysis When calculating densities of benthic features seen in photographs, it is important to eliminate from consideration any portions of the photographs in which an accurate census of these features is not possible (e.g. dark areas only partially illuminated by the strobe). One of the benefits of oblique illumination is the ability to discern objects based on the shadows they cast, assuming the objects are not collinear with the axis of illumination. Because of the more nearly vertical angle of illumination close to the submersible, shadows were less evident in the foreground of each of our photographs than in the middle and background. In addition, the high reflectivity of the nearly white carbonate sediments on the two seamounts caused a "washing out" of the entire photograph by glare, particularly in the foreground. These phenomena, along with reduced illumination of objects at greater distances from the submersible, resulted in an optimal zone for analysis in the middle of each photograph. This was particularly evident when attempting to distinguish objects whose texture and color closely resembled that of the sediment itself (e.g. xenophyophores and most lebensspuren). A quantitative determination of the useful area within each photograph was made a posteriori by constructing a plot of density vs distance from the camera for each feature or category of features in question (WAKEFIELDand GENIN, 1987). First, all data points located outside the outermost grid lines were eliminated, yielding trapezoids on the digitizing tablet corresponding to rectangular quadrats on the sea floor. The area within each trapezoid was then divided into transverse swaths with widths corresponding to 20 cm on the sea floor. During preliminary observations of the transect photographs, it was noted that certain types of features were more readily visible than others, due to differences in texture, color and size. Based on their differential visibility, features were divided into five categories: single mounds, discrete lebensspuren (e.g. single pits, fecal casts), multiple lebensspuren (e.g. mound clusters), xenophyophores and non-xenophyophore megafauna. The density of features within each of these categories was calculated for each swath and a plot of density against distance from the bottom (proximal) margin of each photograph was generated (Fig. 3). Using the swath having the highest mean density of objects within a given visibility category as a basis for comparison (the "reference value"; Fig. 3~, each contiguous swath with a mean density not significantly different from the peak density (Mann-Whitney U-test, two-tailed, P > 0.025) was included in the area used for subsequent density calculations. Because trails are not discrete structures and hence inappropriate subjects for density calculations, they were instead tabulated as either present or absent within a given photograph and are reported as the percentage of photographs containing a given trail morphotype. Lebensspuren also were divided into mobile and sedentary forms, based on their association with locomotory or non-locomotory activities, respectively. The relative representation

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0.3 ¸ Reference Value 0.25 P>0.025 e~ 0.2

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100 2DO 300 400 500 600 700 800 900 1(}00 Object Distance (pixels) Fig. 3. Mean density of objects per 20 cm swath on the sea floor plotted against distance (in pixels) from the bottom margin of each photograph. This graph was generated using densities of discrete lebensspuren from Alvin dive 1816. The highest density swath was used as the reference value and all swaths with means below the two horizontal lines were rejected as significantly different, using a Mann-Whitney U-test (two-tailed, P < 0.025), and were not used in abundance calculations. Pixels (X) may be converted to distance on the sea floor (Y) from the nadir point, using the equation Y = AX s, where .4 = 9.36 x 10~, B = 1.97, r2 = 0.99. of these two categories at a given site is thought to indicate the relative significance of sedentary vs mobile deposit feeding, with mobile deposit feeding being more important under conditions of lower nutrient availability (YOUNG et al., 1985). We examined several indices of community structure for the assemblages of megafauna and lebensspuren observed on each of the five quantitative photographic transects. The first, species richness, is defined as the number of species present in a given area and is perhaps the simplest measure of community diversity. It is, however, of limited utility for between-site comparisons, since it does not take into account the distribution of abundances among the species making up a community. To gauge the degree of similarity between communities in two areas on Horizon Guyot and Magellan Rise, we calculated the Similarity Index (WHITFAKER,1952) S

2 ~, min (Xik,Xjk) k=l s

xi~ + Xjk k=l

where Xik is the fraction of the total community at site i represented by the kth feature, Xjk is the fraction represented by the same feature at site j and S is the total number of features present at both sites. This index has been shown to accurately represent the overlap between samples under a variety of conditions (BLOOM, 1981). The variance-to-mean ratio was used as an index of dispersion to examine the spatial distribution of features within and between transects. This ratio approaches unity if the distribution of abundances within a group of quadrats approximates a Poisson distribution (ELLIOTr, 1977). The significance of any deviations from unity may be evaluated by comparison with a chi-squared statistic divided by n-1 and having n-1 degrees of freedom, where n is the number of quadrats in the group under consideration (ANDREW

Megafauna and lebensspuren on Pacific seamounts

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and MAPSTONE, 1987). We considered deviations from a chi-squared distribution significant at the 95% probability level, with P < 0.025 indicating a patchy distribution and P > 0.975 reflecting a uniform distribution. The area within a quadrat was varied systematically by using one photograph as the fundamental unit and grouping photographs in a geometric progression to yield quadrats of 1, 2, 4, 8, 16, 32, 64 and 128 photographs in size. For dives 1809 and 1816, this technique allowed us to examine spatial distributions at scales of 7-896 m, while for dives 1815 and 1818, the scales ranged from 10 to 1280 m. Dispersion indices for dive 1810 were depicted on a per-photograph basis, because of uncertainty involving the area covered by each photograph on this transect. It is difficult to interpret dispersion statistics in situations where the features of interest are sparse (mean density <5 per quadrat; B A T E M A N , 1950). In particular, there appears to be a bias in the direction of randomness at such low densities; thus, the features in our data set that were best suited for dispersion analysis were xenophyophores and lebensspuren, although the rock substrate and some of the megafauna on the summit perimeter of Horizon Guyot (dive 1810) were also present in sufficiently high densities to permit spatial analysis. RESULTS

We quantitatively analysed 2943 photographs, representing more than 140,000 m 2 of sea floor (Table 1), and qualitatively surveyed an additional 4021 photographs with an estimated coverage of more than 200,000 m E. Before beginning the digitization of our transect photographs, we examined the shape of the rarefaction curve from each transect and concluded that, because the curves became nearly horizontal in each instance (Fig. 4), the assemblages of megafauna and lebensspuren observed in each of our photographic transects were representative of the communities on their respective seamounts. Photographs were analysed from two transects on Horizon Guyot, one from the summit cap (dive 1809) and one from the northern summit perimeter (dive 1810) (Fig. 1). The summit transect consisted of 680 photographs covering c a 23,000 m 2, and the perimeter transect was made up of 420 photographs with an estimated coverage of 1 4 , 0 0 0 m E (Table 1). The perimeter was characterized by numerous rock outcrops 70' Dive 1815

~6o

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~ 30 ~ 2o

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0

i 100

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Cumulative Photographs Fig. 4. Cumulative number of summed megafaunal and lebensspuren morphotypes observed as a function of cumulative photographs analysed on each of the five photographic transects.

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(Fig. 5A), covering approximately 23% of the total area visible in our photographs. Ripple marks were observed in 48% of the frames from the summit transect and 79% of the frames from the perimeter transect, although the mean and maximum current speeds measured during 6-9 day current meter deployments on the perimeter were not significantly higher than those on the summit (GEms et al., 1989). Ripples on the perimeter were substantially larger and less eroded than those on the summit, indicative of more recent formation and stronger current activity (LoNsDALEet al., 1972; KAVENet al., 1989). The frequency of our ripple mark observations differs from that of LONSDALE et al. (1972), who reported ripples in 40% of their photographs from Horizon Guyot, including summit, perimeter and flank areas. Three photographic transects from the summit of Magellan Rise were analysed (dives 1815, 1816 and 1818) (Fig. 2). The areas covered during these transects were 47,000 m 2 (735 photographs), 16,700 m 2 (497 photographs) and 41,500 m 2 (611 photographs), respectively (Table 1). Neither ripple marks nor rock outcrops were observed on any of the transects, and current speeds in our study area appeared to be substantially lower than those measured on Horizon Guyot (GENIN et al., 1989; SMITHet al., 1989). Megafauna We identified 57 megafaunal morphotypes, 48 invertebrates, representing six phyla, and nine teleost fishes in our transect photographs from Horizon Guyot (Table 2). Of these, 21 invertebrate morphotypes and five fish species were seen on the summit cap. Xenophyophores were the most abundant and morphologically diverse group on the cap, with eight distinct morphotypes present at densities ranging from 3.1 to 73.5 individuals per 1000 m 2 for a given morphotype. Individual xenophyophores, especially those with fan-shaped tests, were commonly observed on the tops and sides of mounds. Collectively, this group comprised 76% of the megafaunal organisms seen on the summit cap (Table 2). In this same area, the most abundant metazoans identified were pennatulids (Sympodium? sp.) and hexactinellid sponges (Euplectella spp.), with densities of 4.9 and 1.8 individuals per 1000 m 2, respectively. Among the fishes, Synaphobranchus sp. (Synaphobranchidae) and Aldrovandia sp. (Halosauridae) were the most numerous (Table 2). Occasionally, patches of 5-15 short (2-3 cm), vertical twig-like structures were seen. on the summit (Fig. 5B). These structures superficially resemble the "stalks" reported from 4734 to 5064 m depth on the continental rise off Nova Scotia by Swivr et al. (1985) and have not been identified, although it has been suggested that they might be agglutinating protozoans, solitary corals, bryozoans or polychaete tubes (L. LEVIN and O. TENDAL,personal communication). The overall density of these "stalks" was greater than that of any other non-xenophyophore in this area (64.3 per 1000 m2). Only one echinoderm species, an unidentified elasipod holothuroid, was observed on the summit cap of Horizon Guyot. Twenty-nine invertebrate morphotypes and four fish species were recorded from the perimeter transect. As on the cap, xenophyophores dominated the megafauna numerically (54.7%; Table 2), while among the metazoans, hexactinellid sponges, scalpellid barnacles, gorgonians and comatulid crinoids were the most abundant forms (Table 2). Fishes were observed only rarely on the perimeter (one per 70 frames), relative to the summit (one per 17 frames), and sponges and echinoderms were the most morphologically diverse groups in this area, with 10 morphotypes each. Suspension-feeding forms dominated the non-xenophyophore megafauna at this site (99% numerically); xenophyo-

Fig. 5. Photographs taken on the summit and perimeter of Horizon Guyot with Alvin's survey cameras. (A) Ferromanganese oxide-encrusted rock outcrops on the perimeter of Horizon Guyot. Arrows indicate enteropneust burrows and a brown comatulid crinoid. (B) Patch of "stalks" on the summit of Horizon Guyot (lower right comer). Arrows indicate an echiurid burrow and a borrow pit. (C) Unstalked fan xenophyophore on the perimeter of Horizon Guyot (arrow). Note comatulid crinoid in foreground. (D) Mounds on the summit of Horizon Guyot. Arrows indicate calderas (right) and a borrow pit (left).

Fig. 6. Photographs taken on the summit and perimeter of Horizon Guyot and the summit of Magellan Rise with Alvin survey cameras. (A) Borrow pits on the summit of Horizon Guyot. Note the consistent orientation of the mound and pit components. Arrows indicate an echiurid burrow (right), a caldera (middle) and a single mound trail (left). (B) Sitzmarks on the summit of Magellan Rise (arrows), some of which appear to contain flocculent material. (C) Troughs on the summit of Magellan Rise (trough lengths ~90 and 270 cm), both of which appear to contain flocculent material. Arrow indicates a halosaurid fish, Aldrovandia sp. (D) Spatangoid urchin (Cystocrepis setigera) trail (mean width ~19 cm) on the summit of Magellan Rise. Inset depicts an individual C. setigera with a diameter of 22 cm.

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Megafauna and lebensspuren on Paeitic seamounts

Table 2. Abundances of megafauna in transectphotographs from Horizon Guyot and Magellan Rise

Morphotype/Species Protista Class Xenophyophorea Branched xenophyophore Crenelated lamellar xenophyophore Irregular aggregated xenophyophore Irregular lamellar xenophyophore Lamellar cross xenophyophore Lamellar triad xenophyophore Reticulated xenophyophore Spherical xenophyophore Spoked xenophyophore Stalked broad fan xenophyophore Stalked irregular lamellar xenophyophore Stalked narrow fan xenophyophore Unstalked fan xenophyophore Porifera Class Hexactinellida EuplecteUa sp. Euplectellid sp. 1 Euplectellid sp. 2 Euritid sp. Hyalonema sp. 1 Hyalonema sp. 2 Articulated sponge Branched sponge Fluted sponge Irregular sponge Plate sponge Tube sponge Vase sponge Cnidaria Class Hydrozoa Unidentified hydrozoan Class Scyphozoa Borellia sp. Unidentified scyphozoan 1 Unidentified scyphozoan 2 Unidentified scyphozoan 3 Unidentified scyphozoan 4 Class Anthozoa Order Gorgonacea Lepidisis sp. Miscellaneous gorgonians Order Pennatulacea Sympodium? sp. Unidentified pennatulid Order Actiniaria Actinoscyphia sp. Cerianthid anemone Hermit crab anemone Unidentified anemone 1 Unidentified anemone 2 Order Antipatharia Unidentified antipatharian

HG summit HG perimeter MR summit density density density (No./1000 m 2) (No./1000 m 2) (No./1000 m 2) Reference*

30.6 24.5 3.1 73.5 19.9 26.0 29.1 52.1

48.9

0.1 29.5 7.3 259.4 5.1 4.5 0.2 12.5 4.4 6.6 12.5 149.5

cf.2 (Plate 8c) 2 (Plate l l e ) cf. 2 (Plate 8a) 3 (Fig. 2c) 3 (Fig. 2t) cf. 3 (Fig. 21) 3 (Fig. 6) 2 (Plate 2a) 1 (Plate 3) 3 (Fig. 5a) 3 (Fig. 51) 3 (Fig. 50) 3 (Fig. 5h)

1.8 1.8 0.2 0.1 <0.2t 0.5 1.1 0.1 2.0 0.6 0.3 3.4 0.1

0.1 0.2 <0.2

<0.1 0.1 0.1 <0.1 0.1 21.9

4.9

0.4 0.1

1.0 1.2

1.0

1.0 0.1 0.1 0.1 0.3

Continued

1876

R.S. K A ~

et al.

Table 2. Continued

Morphotype/Species Ctenophora Unidentified ctenophore 1 Unidentified ctenophore 2 Unidentified ctenophore 3

HG summit density (No./1000 m 2)

HG perimeter density (No./1000 m 2)

Echinodermata Class Crinoidea Brown stalked crinoid Yellow stalked crinoid Brown comatulid crinoid Yellow comatulid crinoid Class Asteroidea Unidentified asteroid 1 Unidentified asteroid 2 Unidentified asteroid 3 Unidentified asteroid 4 Unidentified asteroid 5 Unidentified asteroid 6 Unidentified asteroid 7 Unidentified asteroid 8 Unidentified asteroid 9 Class Ophiuroidea Unidentified ophiuroid Class Echinoidea Cystocrepis setigera Unidentified echinoid Class Holothuroidea Benthodytes sibogae Deima validum validum Mesothuria megapoda Paroriza prouhoi Peniagone diaphana Elasipod holothuroid Unidentified holothuroid 1 Unidentified holothuroid 2

Reference*

<0.2 0.1 <0.1

Mollusca Class Gastropoda Unidentified gastropod 1 Unidentified gastropod 2 Arthropoda Class Pycnogonida Colossendeis macerrima Class Cirripedia Scalpeilid barnacle Class Malacostraca Order Decapoda Munidopsis subsquamosa cf. Parapagurus sp. Penaeid shrimp Unidentified shrimp 1 Unidentified shrimp 2 Unidentified shrimp 3 Unidentified shrimp 4 Unidentified shrimp 5 Unidentified shrimp 6

MR summit density (No./1000 m 2)

0.1 0.1

O.1 2.9 0.4 0.5 0.2

1.2 <0.2

0.1 0.1 0.2 0.1 0.1

0.4 0.1 1.6 1.1 0.1 0.1 0.1 0.1 <0.1 <0.1 0.2 0.1 0.2 0.5 0.2 0.2

0.1 <0.1 O.1 0.2 0.5 0.4 <0.1 <0.1

Con~nued

1877

Megafauna and lebensspuren on Pacific seamounts Table 2. Continued

Morphotype/Species Chordata Class Osteichthys Acanthonus armatus AMrovandia phalacra Aldrovandia sp. Alepocephalid sp. Bathypterois longipes Bathypterois sp. Bathysaurus mollis Coryphaenoides sp. Ipnops meadi Morid sp. Ophidioid sp. Synaphobranchus sp. 1 Synaphobranchus sp. 2 Unidentified teleost 1 Unidentified teleost 2 "Stalks" Taxon Total Protista Total Porifera Total Cnidaria Total Ctenophora Total Mollusca Total Arthropoda Total Echinodermata Total Chordata "Stalks" Non-protist megafauna Total megafauna

HG summit density (No./1000 m2)

HG perimeter density (No./1000 m2)

MR summit density (No./1000 m2)

Reference*

<0.1 2.7 <0.1 0.2 O.1

0.2 <0.1 0.1 0.2 0.2 0.1 2.5 0.2 0.1 0.1 64.3

46.5 Density (No./1000 m2) HG perimeter

MR summit

48.9 (1) 10.0 (10) 23.1 (7) 0.0 (0) 0.0 (0) 2.9 (1) 4.2 (10) 0.4 (4) 0.0 (0)

491.6 (12) 0.0 (0) 2.3 (7) 0.1 (2) 0.2 (2) 1.8 (9) 1.7 (13) 0.5 (6) 46.5 (1)

81.0 (18)

40.6 (32)

53.1 (40)

339.8 (26)

89.5 (33)

544.7 (52)

HG summit 258.8 2.0 7.3 <0.2 0.0 1.2 0.4 5.8 64.3

(8)~ (3) (5) (1) (0) (2) (1) (5) (1)

HG, Horizon Guyot; MR, Magellan Rise. 1, LEMCHEel al. (1976); 2, TENDAL(1972); 3, TENDALand GOODAY(1981). * References are given in instances where an organism resembling one of the morphotypes seen in our photographs is depicted in the literature. t In cases where large, rare organisms were observed during a transect and subsequently eliminated from density calculations, their densities are listed as less than the density corresponding to one individual per photograph. These numbers are not included in the density totals, except as part of the morphotype count. ~: Numbers in parentheses at bottom of table indicate number of morphotypes included in category. p h o r e s w e r e e x c l u d e d f r o m this figure b e c a u s e o f u n c e r t a i n t y c o n c e r n i n g t h e i r t r o p h i c m o d e (LEVlN a n d THOMAS, 1988). D e s p i t e t h e i r r e l a t i v e l y high d e n s i t i e s , o n l y o n e xenophyophore morphotype, a stalked, broad fan-shaped morphotype resembling the " r e g u l a r p l a t y " f o r m p h o t o g r a p h e d b y TENDAL a n d GOODAY (1981), was i d e n t i f i e d f r o m this t r a n s e c t (Fig. 5C). O f t h e n o n - x e n o p h y o p h o r e i n v e r t e b r a t e s , 9 8 % w e r e r e s i d i n g o n r o c k o u t c r o p s , a n d a p o s i t i v e statistical c o r r e l a t i o n was o b s e r v e d b e t w e e n t h e d e n s i t y o f sessile, n o n - x e n o p h y o p h o r e m e g a f a u n a a n d r o c k s u b s t r a t e in this a r e a ( S p e a r m a n r a n k c o r r e l a t i o n test, r = 0.68; P < 0.001), i n d i c a t i n g t h e i m p o r t a n c e o f h a r d s u b s t r a t e in structuring the megafaunal community. Additionally, a number of comatulid crinoids

1878

R.S. K A ~

et al.

and ophiuroids were observed clinging to gorgonians and hexactinellid sponges, possibly as a trophic adaptation (see Discussion). We identified 52 megafaunal morphotypes, 46 invertebrates, belonging to six phyla, and six species of teleost fishes in our transects from the summit cap of Magellan Rise (Table 2). Xenophyophores were the most abundant group in this area, with densities as high as 259 individuals per 1000 m 2 for a single morphotype. Twelve distinct xenophyophore morphotypes were recognized at this site, numerically comprising 90.3% of the megafauna (Table 2). Echinoderms were also well-represented on the Magellan Rise summit, with 13 morphotypes of asteroids, echinoids and holothuroids identified. Despite their morphological diversity, however, more than two individuals of a single echinoderm morphotype were seldom recorded from a single transect. Among the nonprotist invertebrates, the most abundant forms were cerianthid anemones (unidentified sp.), pennatulids (Sympodium? sp.) and "stalks" resembling those reported from Horizon Guyot (Table 2). As on Horizon Guyot, the density of these "stalks" (46.5 per 1000 m 2) was greater than that of any other non-xenophyophore. Fishes were relatively scarce on Magellan Rise (one per 154 frames), the most abundant species being Synaphobranchus sp. (Synaphobranchidae) and Ipnops meadi (Chlorophthalmidae) (Table 2). All of these species have been reported previously from seamounts in the MidPacific Mountains (WILSON et al., 1985), although our observation of I. meadi at 31003200 m depth represents a new minimum depth record for this species (NIELSEN, 1966). Lebensspuren Thirty-two lebensspuren morphotypes were identified from Horizon Guyot, 25 of which were present on the summit transect (Table 3). Of these, 20 were discrete forms and five were trails. Mounds were the most conspicuous features seen on the summit cap (Fig. 5D). In addition to the four types of mounds identified, all six distinguishable groups of multiple lebensspuren had mound components (Table 3). Single mounds were the most abundant features on the summit (1785 per 1000 m2), with sizes ranging from 2 to 39 cm across (mean + S.D. = 8 _+ 4 cm; n = 1203). The most striking sedimentary features on Horizon Guyot were "borrow pits" (Fig. 6A; YOUNG et al., 1985), each consisting of a relatively large pit (mean diameter = 25 + 9 c m ; n - - 9 5 ) adjacent to one or more mounds (mean diameter = 17 + 6 cm; n = 135). Although previously classified as mobile lebensspuren by YOUNGet al. (1985), we refer to them here as sedentary, since they seem to be associated with burrowing rather than with grazing or movement across the sea floor. The pits often contained flocculent material of the type commonly observed in patches and around sedimentary features on these seamounts (REIMERSand WAKEFIELD, 1989); however, no organisms were observed in association with either the pits or mounds. The relative orientation of the pit and mound components of the borrow pits appeared to be constant on the summit cap. Mound-pit axes were oriented north-south, within a range of +30 °, with the mound southern, although the significance of this pattern is unclear. Fifteen lebensspuren morphotypes were identified on the guyot perimeter, eight of which were discrete forms (Table 3). Single pits were over three times more abundant than single mounds (68 vs 22 per 100 frames) in this area, contrary to the trend observed on the summit cap. Mound diversity was also lower on the perimeter, with only one morphotype present (Table 3), and multiple lebensspuren were absent from this area. In general, lebensspuren were sparse on the perimeter, although trace densities increased as

1879

Megafauna and lebensspuren on Pacific seamounts

Table 3. Abundances of lebensspuren in transect photographs from Horizon Guyot and Magellan Rise

Morphotype Mounds SU Single mound SU Caldera SU Caldera w/pit trail SU Irregular mound SU Ridged mound SU Myriochele? mound Pits SU Single pit SD Spoked pit SD Pit with stalk SU Shallow pit SU Irregular pit SD Borrow pit SU Shallow ellipse SD Deep ellipse SD Trough Burrows SD Enteropneust burrow SD Narrow echiurid burrow SD Wide echiurid burrow SU Sitzmark Fecal casts MU Fecal cast MU Holothuroid cast Miscellaneous SU Mound pair SU Caldera pair SU Ridged mound pair SU Small mound cluster SU Mound cluster SU Ring of mounds SU Small sediment column SU Large sediment column SU Column duster SU Dark ring/clear area Trails MU MU MU MU MU MU MU MU MU MU MU MU MU MU

Broad grooved trail Narrow grooved trail 1 Narrow grooved trail 2 Echinoid trail Dimpled trail Broad dimpled trail Scooped trail Ridged trail Ridged treaded trail Grooved treaded trail Narrow light trail Broad light trail Small mound trail Double mound trail

HG summit density (No./1000 m2)

HG perimeter density (No./1000 m 2)

MR summit density (No./1000 m2)

1785.1 95.0 11.3 13.8

6.7

502.9 0.7

2 (Fig. 6.24)

12.7 16.7 9.2

5 (Fig. 5a) 1 (Fig. 24-16) 6 (Fig. 7)

26.4

1 (Fig. 24-73) 1 (Fig. 24-60) 5 (Fig. 8)

20.9 59.2 1.0 6.6 39.8

20.5 0.4

1.8 3.1 <0. It

68.9 2.0 1.0

Reference*

6 (Fig. 19) 6.8

1.5 3.6 26.0

0.2

1.3

0.4

0.1 0.1

3.9 91.7

0.1 0.1

3.5 9.1

3 (Fig. 4 (Fig. 4 (Fig. 6 (Fig.

lc) 3) 2) 10)

2 (Fig. 5.13) 1 (Fig. 24-19)

6.3 <0.1 1.7 6.4 35.8

1 (Fig. 24-22) 3.6 0.2 86.5

306.4 23.0

0.3 13.5 5.7%:~ 48.7%

10.0% 1.0%

2.5%

1.2% 0.5% 3.3% 0.7% 1.0%

19.3% 7.2%

4.0% 1.5% 55.2% 0.2%

2.5% 0.6% 17.9% 11.0% 10.2%

2 (Fig. 4.26) 2 (Fig. 4.8) 6 (Fig. 17) 2 (Fig. 4.28) 1 (Fig. 24-85) 1 (Fig. 24-97) 1 (Fig. 24-49) 6 (Fig. 4) 6 (Fig. 13) 1 (Fig. 24-92) 1 (Fig. 24-38) 6 (Fig. 12b) 1 (Fig. 24-25) 1 (Fig. 24-26) Continued

1880

R.S. K A ~

Table 3.

Category

HG summit

Total mounds Total pits Total burrows Total fecal casts Total miscellaneous

1926.1 178.5 31.1 0.2 371.6

Total superficialtraces Total deep traces Total sedentary traces Total mobile traces

2398.9 (13) 108.6 (7) 2507.3 (19) 0.2 (1)

Total trails Total lebensspuren

(5)§ (7) (3) (1) (4)

et

al.

Continued

Density (No./1000 m2) HG perimeter

MR summit

6.7 20.9 1.5 0.2 0.0

(1) (2) (3) (2) (0)

542.2 38.1 96.0 12.6 112.1

(5) (5) (3) (2) (8)

27.4 1.8 29.1 0.2

(5) (3) (6) (2)

788.1 (19) 12.9 (4) 788.4 (21) 12.6 (2)

63.5% (5)

16.7% (7)

72.2% (9)

2507.5 (20)

29.3 (8)

801.0 (23)

D, deep; M, mobile; S, sedentary; U, superficial; HG, Horizon Guyot; MR, Magellan Rise. 1, Ewn~G and DAvis (1967); 2, HEEZENand HOLLISTER(1971); 3, MAUVlELet al. (1987); 4, OrrrA (1984); 5, SWIFT et al. (1985); 6, YOUNGet al. (1985). * References are given in instances where a trace resembling one of the morphotypes seen in our photographs is depicted in the literature. t In cases where rare lebensspuren were observed during a transect and subsequently eliminated from density calculations, their densities are listed as less than the density corresponding to one trace per photograph. These numbers are not included in the density totals, except as part of the morphotype count. All abundances are listed as number per 1000 m2, except trails, which are listed as percent occurrence (percentage of photos in which morphotype appeared). § Numbers in parentheses at bottom of table indicate number of morphotypes included in category. the submersible m o v e d upslope during the course of dive 1810 ( S p e a r m a n r a n k correlation test, r = 0.18; P < 0.001). Thirty-two l e b e n s s p u r e n m o r p h o t y p e s were identified f r o m Magellan Rise. A s on the s u m m i t of H o r i z o n G u y o t , m o u n d s were the most a b u n d a n t lebensspuren, with five different m o u n d m o r p h o t y p e s and six groups of " m u l t i p l e " l e b e n s s p u r e n with m o u n d c o m p o n e n t s (Table 3). Single m o u n d s were the m o s t a b u n d a n t l e b e n s s p u r e n at this site (503 p e r 1000 m2), with asteroid sitzmarks (92 p e r 1000 m2; Fig. 6B) and small s e d i m e n t columns (86 p e r 1000 m 2) also present at relatively high densities. T h e single m o u n d s on Magellan Rise were roughly the s a m e size as those on H o r i z o n G u y o t ( m e a n + S.D. = 8 + 5 c m vs 8 + 4 cm), although they s p a n n e d a wider size range (2-71 cm in d i a m e t e r vs 2-39 cm). T h e most r e m a r k a b l e features seen on this s e a m o u n t were elongate troughs, ranging f r o m 40 to 267 cm in length and 6 to 65 cm in width ( m e a n length = 117 + 47 cm, m e a n width = 32 + 13 cm, n = 72), with estimated depths of 10--20 cm (Fig. 6C). T h e s e troughs were frequently b o r d e r e d by n u m e r o u s small m o u n d s and lined with tension cracks, p e r h a p s indicative of recent f o r m a t i o n (MuLLINS et al., 1982), and often contained flocculent m a t e r i a l (LEVlN and NHTROUER, 1987; REIMERS and WAKEFIELD, 1989). While no m e c h a n i s m for the g e n e r a t i o n of these features was r e c o r d e d , in one instance four collinear troughs were o b s e r v e d at regular intervals o v e r a distance of 15-20 m, indicating that a motile epifaunal o r g a n i s m was p r o b a b l y responsible. L a r g e troughs have b e e n r e p o r t e d to result f r o m b u r r o w i n g (ABLE et al., 1987; BOYER et al., 1989) or scraping against the b o t t o m (OrrrA, 1983) by several species of fishes; h o w e v e r n o n e of the fishes k n o w n to p r o d u c e such features were either p h o t o g r a p h e d or collected on M a g e l l a n Rise.

Megafauna and lebensspuren on Pacific scamounts

1881

Another common lebensspuren morphotype on Magellan Rise resulted from the locomotory and foraging activities of the spatangoid echinoid, Cystocrepis setigera (Fig. 6D). These trails were widespread, appearing in 55% of the photographs analysed from this area, and averaged nearly 20 cm across (mean width = 19 + 3 cm; n = 93). The urchins were large (mean diameter = 22 + 5 cm; n = 3) and, while observed at relatively low densities (1 per 6000 m2), appeared to have impacted a large proportion of the sea floor, although the temporal scales associated with their locomotory and foraging activities are not known.

Biogeography Because of the uncertainty associated with identifying organisms from photographs, biogeographic comparisons were restricted to those species identified from our nonphotographic sampling, including organisms collected with baited traps, Alvin's starboard manipulator arm and a semi-balloon trawl (see above). Among the megafauna collected, cnidarians, arthropods, echinoderms and fishes were represented by the greatest number of species, although representatives of several other taxa were also obtained (Table 4). Individuals of 19+ species of megafauna were collected on Horizon Guyot, including at least three species previously identified from this site (WILSOY et al., 1985) and two new species (Table 4). Distributional information was available for six species, of which three were cosmopolitan (spanning two or more ocean basins), two trans-Pacific, and one provincial (restricted to the region in which the seamount is located; WILSON and KAU~, 1987). Representatives of 40+ megafaunal species were captured on Magellan Rise, with minimally four new species. This collection resulted in broadened depth distributions for Distichoptilum verrillii? (Pennatulacea), Trianguloscalpellum cf. elegantissimum (Cirripedia), Styracaster horridus (Asteroidea), Paroriza prouhoi (Holothuroidea) and Ipnops meadi (Osteichthys), as well as the first report of Benthodytes sibogae (Holothuroidea) from the Pacific. Fourteen of the 20 species for which distributional information was available were cosmopolitan, while five more were trans-Pacific. The remaining species, Catherinum cf. rossi, was previously known only from the Mid-Pacific Mountains (RAo and NEWMAN, 1972).

Comparisons between study sites on Horizon Guyot and Magellan Rise Megafaunal abundances on the summit of Magellan Rise were 1.6 times higher than on the summit and more than 6 times higher than on the perimeter of Horizon Guyot (Table 2). Much of this descrepancy was due to differences in xenophyophore abundance at the three sites, since this group comprised 90% of the total megafauna on Magellan Rise, compared to 76 and 55% on the summit and perimeter of Horizon Guyot, respectively. Densities of non-xenophyophore megafauna were higher on the Horizon Guyot summit than at either of the other two sites, although this area supported the lowest number of megafaunal morphotypes (Table 2). Taxonomically, each site was conspicuously deficient in one phylum, relative to the other two sites. The Horizon Guyot summit was virtually devoid of echinoderms, while the Horizon Guyot perimeter supported only one xenophyophore morphotype and none of the "stalks" that were so abundant on both summits. Additionally, the perimeter of Horizon Guyot possessed the highest densities of suspension-feeding forms, notably sponges, gorgonians and crinoids.

1882

R.S. K A ~

et al.

Table 4. Biogeographic distribution of megafauna collected on Horizon Guyot and Magellan Rise during March-April 1987 Species

Horizon Guyot

Protista Class Xenophyophorea Unidentified spp. Porifera Class Hexactinellida Euplectella sp. Unidentified Euritidae sp. Hyalonema sp. Unidentified sp. Cnidaria Class Anthozoa Order Gorgonacea Lepidisis of. olapa Order Pennatulaeea

Distichoptilum verrillii? Virgularia cf. tuberculata

Magellan Rise

Geographic range

Bathymetric range (m)

3t

References*

13

2 1 2 1

2 2

P

1620-2880

23

1

AP

100--4000

2,8

1

P P

1400-1730 1560-1670

21 21

1 1 2 5

AP AIP

1830-4150

25 26

1

AP

1

Order Actiniaria

Actinoscyphia sp. Paraphelliactis? n. sp.

1 1

Unidentified sp. Order Antipatharia

>10

Bathypathes ?lyra

2

Mollusca Class Gastropoda Mohnia ?n. sp. Orectospira n. sp. Class Bivalvia Unidentified sp. Class Cephalopoda Unidentified sp.

1 1 3 1

Arthropoda Class Pyenogonida

Colossendeis macerrima

1

Class Cirripedia

Catherinum of. rossi Trianguloscalpeilum cf. elegantissimum Unidentified Sealpellidae sp. Class Copepoda Unidentified sp. Class Malacostraca Order Decapoda Ethusina sp.

1 1 >100

Glyphocrangon vicaria Munidopsis subsquamosa cf. Parapagurus sp. Order Isopoda

Bathygyge grandis

15

Order Amphipoda

Eurythenes gryllus Orchomene sp. 1 Orchomene sp. 2 Paralicella caperesca Unidentified Lysianassidae spp.

14 >100 >100 > 100 >100

AP

<1000-6200

7,10

AP

1900-5900

11,22

Continued

1883

Megafauna and lebensspuren on Pacific seamounts

Table 4. Species

Horizon Guyot

Brachiopoda Unidentified sp. Echinodermata Class Crinoidea Ptilocrinus pinnatus Thalassocrinus n. sp. Class Asteroidea BrisingeUa sp. Paragonaster stenostichus Paragonaster cf. subtilis Psilaster sp. Styracaster chuni Styracaster horridus Class Ophiuroidea Ophiomusium armature Ophiomusium ?n. sp. Class Echinoidea Cystocrepis setigera Class Holothuroidea Benthodytes sibogae Benthodytes ?n. sp. Benthodytes sp. Deima validum validum Mesothuria megapoda Paroriza prouhoi Pseudostichopus mollis Chordata Class Osteichthys Acanthonus armatus Aldrovandia phalacra Bathypterois longipes Bathysaurus mollis lpnops meadi Synaphobranchus sp. 1 Synaphobranchus sp. 2

Continued Magellan Geographic Rise range

Bathymetric range (m)

References*

3

1 1

P 1 1 1 1 - 10 1

--2900

3

P AP

--1200 2450--4380

5 5,20

AP AIP

2550--4570 4040-5610

14 14

10 7

P

1000-4500

12

1

P

2875-3435

1

1 1 1 1

I

690-2800

6

P P AP AIP

720-4820 ~4500 2670-3020

6 4 9 20

AIP AIP AP AP IP

1920-3270 750-1800 2510-5610 2610--4700 3310-4940

18 16 24 17 19

1 1 1

2 6 1 1 1 3 1

A, Atlantic; I, Indian; P, Pacific. *References pertain to geographic and bathymetric data. tNumbers indicate number of specimens collected at each site. 1, AGGASSlZ(1904); 2, C. A. CHaD (personal communication); 3, CLARK(1907); 4, CLARK(1920); 5, FISHER(1911); 6, HA~SEN(1975); 7, HASEGAWAet al. (1986); 8, HEI)GPETH(1948); 9, HEROUARD(1902); 10 HOPKINS (1985); 11, R. S. KAUFMA~ (unpublished data); 12, KOEriLER(1922); 13, L. A. LEVIN (personal communication); 14, MADSEN(1961); 15, MARKHAM(1985); 16, McDoWELL (1973); 17, MEAD (1966); 18, NmLSEN (1965); 19, NIELSEN(1966); 20, D. L. PAWSOlq(personal communication); 21, RAO and NEWMAN (1972); 22, SCHULENBERGERand BARNARD(1976); 23, STUDER(1894); 24, SULAKand SHCHERBACHEV(1988); 25, WICKSTEN(1979); 26, WILLIAMSand TURNER(1986). T h e s u m m i t of M a g e l l a n Rise was the o n l y site at which m o l l u s k s w e r e o b s e r v e d a n d s p o n g e s were n o t , in a d d i t i o n to which this area d i s p l a y e d t h e greatest n u m b e r of a r t h r o p o d m o r p h o t y p e s . Statistically, the two s u m m i t s w e r e m o r e similar to each o t h e r t h a n e i t h e r was to the p e r i m e t e r site, while the p e r i m e t e r m e g a f a u n a l c o m m u n i t y was m o r e similar to the H o r i z o n G u y o t s u m m i t c o m m u n i t y t h a n to that o n M a g e l l a n Rise ( T a b l e 5).

1884

R.S. KAOFMANNet al.

Table 5. Similarity indices .for assemblages of megafauna and lebensspuren on Horizon Guyot and Magellan Rise

HG summit

HG perimeter

MR summit

'Megafauna HG summit HG perimeter MR summit

0.202 0.435

2 ~ 0.049

12 2

Lebensspuren HG summit HG perimeter MR summit

0.235 0.847

~

15 10 0.270

Valuesin upper rightportionof table are numbersof morphotypesshared betweensites. Valuesin lowerleft portion are similarityindices (see text).

Lebensspuren on the Horizon Guyot summit were 3 times more abundant than on the summit of Magellan Rise and 85 times more abundant than on the Horizon Guyot perimeter (Table 3). This trend was mirrored by densities of meio- and macroinfauna (LEvIN and THOMAS, 1989) and non-xenophyophore megafauna. Magellan Rise displayed the highest abundances of burrows and fecal casts as well as the most frequent occurrence of trails and the greatest number of sedentary lebensspuren morphotypes, while the highest densities of mounds and pits were seen on the Horizon Guyot summit. The proportion of sedentary lebensspuren morphotypes was comparable on the summits of Horizon Guyot and Magellan Rise (76 and 66%, respectively) and much higher than on the Horizon Guyot perimeter (40%), although trails appeared in far fewer photographs from the Horizon Guyot perimeter (16.5%) than from either summit site (63.5 and 72.2%; Table 3). As in the case of the megafauna, the lebensspuren assemblages on the two summits were more statistically similar to each other than to the perimeter site. However, unlike the megafauna, the perimeter lebensspuren assemblage was more similar to the Magellan Rise summit assemblage than to that on the summit of Horizon Guyot (Table 5), possibly as a result of the relative paucity of echinoderms on the Horizon Guyot summit cap. Spatial distribution

On the Horizon Guyot perimeter, a high degree of patchiness was observed among the megafauna, apparently as a result of the patchy distribution of the substrate occupied by the predominantly sessile forms in this area (Fig. 7A). With the exception of scalpellid barnacles, a group whose species are noted for their gregarious settlement and growth patterns (BARNES, 1980), the rock substrate was more patchy than any of the taxa it supported. This observation corroborates the previously noted statistical correlation between the occurrence of hard substrate and sessile, non-xenophyophore megafauna in this area. On the two summits, distributions of individual xenophyophore morphotypes appeared to be random at all spatial scales examined (Figs 7B-E), although a number of morphotypes from dive 1815 exhibited a consistently greater degree of patchiness than those from the other three summit transects (Fig. 7C). Lebensspuren were generally more patchy than xenophyophores (Fig. 8A-E); how

Megafauna and lebensspuren on Pacific seamounts

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Fig. 8. Dispersion indices calculated for individual lebensspuren morphotypes [mobile forms (X), sedentary forms (O)] and presented for a geometric progression of quadrat sizes, with 95% coalidence limits. (A)Dive 1809 (Horizon Guyot summit). (B)Dive 1810 (Horizon Guyot perimeter). (C) Dive 1815 (Magellan Rise summit). (D) Dive 1816 (Magellan Rise summit). (E) Dive 1818 (Magellan Rise summit).

10~0

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ever, mobile forms were distributed randomly in 9 of 11 instances, the exceptions being the large-scale patchiness of single mound trails from dives 1809 and 1816. Since mobile lebensspuren are associated with foraging and movement across the sea floor, usually by deposit feeders, the random nature of their distribution seems to indicate that either the foraging and locomotory patterns of benthic deposit feeders at our study sites are predominantly random on spatial scales of 10-1000 m or that these patterns are directed while the distribution of food resources is random. DISCUSSION

Comparisons with other studies Megafauna. Densities of megafauna on the Horizon Guyot summit were of the same magnitude as those reported for non-seamount communities at similar depths. A photographic survey similar to ours, using Alvin in the western North Atlantic at depths of 1450-1550 m, yielded a total megafaunal density of 705 individuals per 1000 m E (GRASSLE et al., 1975), a value comparable to that obtained in the same area using a lowered camera (790 per 1000 mE; ROWE, 1968) and roughly twice our estimate for the Horizon Guyot summit (340 per 1000 mE; Table 2). Megafaunal densities on the seamount were, in turn, roughly twice those reported at 1380-1562 m in Suruga Bay (164-219 per 1000 m2; OHTA, 1983). However, fish densities on the summit of Horizon Guyot (5.7 per 1000 m 2) were substantially lower than at corresponding depths in Suruga Bay and on the northwest Atlantic continental slope (12-36 per 1000 mE; GRASSLEet al., 1975; OrrrA, 1983). No xenophyophores were reported in any of these non-seamount studies. Xenophyophore densities on the Horizon Guyot summit cap (259 per 1000 m 2) were slightly lower than those measured in sediment-covered areas at 1700-2200 m on seamounts in the eastern Pacific (297-1004 per 1000 mE; LEVIN and THOMAS, 1988). Megafaunal abundances on the guyot perimeter (89 individuals per 1000 m E) were higher than those measured at 1650-1750 m on Cross Seamount (10-30 per 1000 mE; GRIGG et al., 1987) but substantially lower than at comparably deep non-seamount sites; data from the northwest Atlantic at 1705-1852 m depth (RowE, 1968; GRASSLE et al., 1975) and at 1715 m in Suruga Bay (OHTA, 1983) indicated megafaunal densities ranging from 2460 to 12,810 per 1000 m 2. Fish densities on the seamount perimeter were also much lower than in non-seamount areas, with an estimated abundance of 0.4 individuals per 1000 m 2 on the seamount as compared to values of 4-13 per 1000 m E for the northwest Atlantic (GRASSLEet al., 1975; HAEDRICHand ROWE, 1977) and 63 per 1000 m E for Suruga Bay (Orrrg, 1983). Densities of xenophyophores in this area (49 per 1000 m E) were comparable to those reported from 1700 to 2500 m in rocky areas on seamounts in the eastern Pacific (16-59 per 1000 m2; LEVIN and THOMAS, 1988). Megafauna were 20 times more abundant on the summit of Magellan Rise (545 per 1000 m 2) than at 2820-2830 m in Suruga Bay (26 per 1000 m2), while fish densities in the same area of Suruga Bay ~vere 8 times higher than on Magellan Rise (4.0-4.2 vs 0.5 per 1000 m2; OHTA, 1983). The large discrepancy in megafaunal densities may be partially attributed to the abundance of xenophyophores on the seamount and their absence from Suruga Bay, although the density of non-xenophyophore megafauna on Magellan Rise (53 per 1000 m E) was still more than twice the density of megafauna in Suruga Bay. Xenophyophore densities on Magellan Rise (492 per 1000 m 2) were more than twice those reported from 2700 to 2900 m in sediment-covered areas on Sasha Seamount in the eastern Pacific (216 per 100 m E, LEVIN and THOMAS, 1988).

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Lebensspuren. Lebensspuren abundances on the summit cap of Horizon Guyot (2508 per 1000 m 2) were comparable to those measured between 3450 and 5050 m depth in the Venezuela Basin (2175-4138 per 1000 m2; YOUNGet al., 1985) and greater than densities at 1900-4700 m in the Bay of Biscay (217-1361 per 1000 m2; MAUVIELand SmUET, 1985). The number of lebensspuren morphotypes on the guyot summit (25) was similar to the number reported for the Venezuela Basin and Bay of Biscay stations (7-23) as well as for study sites in the Arctic Ocean (7-19; KrrCHELL, 1979). The density of lebensspuren on the summit perimeter (29.3 traces per 1000 m 2) was substantially lower than abundances measured between 3450 and 5050 m in the Venezuela Basin (2175-4138 per 1000 m2; YOUNGet al., 1985) and at 1900-4700 m in the Bay of Biscay (217-1361 per 1000 m2; MAUWELand SIBUET, 1985). As on the guyot summit cap, however, the number of iebensspuren morphotypes on the perimeter (15) was comparable to the number reported for the Venezuela Basin, Bay of Biscay and Arctic Ocean (7-23). Lebensspuren densities on Magellan Rise (801 per 1000 m 2) were up to 4 times lower than those reported for the Venezuela Basin (2175-4138 per 1000 .m2; YOUNG et al., 1985) but comparable to values from the Bay of Biscay (217-1361 per 1000 m2; MAUVIEL and SmUET, 1985). These lower abundances were not indicative of differences in morphotype richness, however, since the number of lebensspuren forms identified from the seamount (32) was 50-250% greater than in either of these two areas (9-23) or in the Arctic Ocean (8-19; KITCHELL,1979). Factors influencing distribution Substrate availability. The differential availability of certain substrate types at our three sites may be responsible for some of the observed differences in megafaunal community composition. In particular, the occurrence of a morphologically diverse assemblage of sessile, suspension-feeding forms on the perimeter of Horizon Guyot was apparently related to the presence of rock outcrops. Suspension feeders have been reported to comprise a larger fraction of communities found on hard substrates than of communities on adjacent sediments (MuLLINEAUX,1987), and some species may, in fact, be specific to hard substrate. These substrates provide sessile megafauna with a solid foundation, relatively free from short-term erosion and instability, as well as a predominantly sediment-free surface that may be more conducive to larval settlement within some taxa (GRIGG et al., 1987). Among sediment-dwelling organisms, differences in sediment quality between the seamount sites do not appear to be responsible for observed distributions of lebensspuren and megafauna. Measurements of sediment grain size, organic carbon and organic nitrogen correlate poorly with measured densities of meio- and macroinfauna (LEvINand THOMAS, 1989) and non-xenophyophore megafauna, although we observed an inverse relationship between xenophyophore densities and mean sediment grain size. Xenophyophores have been reported to preferentially incorporate sand-size particles into their tests, eschewing both coarser and finer materials (LEVIN and THOMAS, 1988); however, all three sites were characterized by sediments composed of >45% sand, with the highest levels (84%) found in the same area as the lowest xenophyophore densities. Hence, while xenophyophores may be selectively removing sand-size particles from the sediments, resulting in smaller mean grain sizes in areas with greater xenophyophore densities (cf. LEVIN and THOMAS, 1988), the converse does not appear to be true in this instance.

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Hydrodynamic factors. Differences between the current regimes at our three study sites may also have influenced the abundance and composition of the observed megafaunal and lebensspuren assemblages. Currents may impact biological communities by affecting rates of food supply to suspension feeders (SEBENS, 1984; LEONAROet al., 1988), larval settlement, assuming the presence of suitable substrates (see above), and sediment transport (CAccmONE et al., 1988). Among our three study sites, we observed an inverse relationship between current intensity, based on current meter records (GENIN et al., 1989; SMITHet al., 1989) and the occurrence and morphology of sediment ripples, and certain groups of megafauna and lebensspuren. Areas characterized by more vigorous currents supported higher densities of sponges and sessile cnidarians and a higher proportion of mobile lebensspuren morphotypes, with lower densities of xenophyophores and total lebensspuren (Tables 2 and 3). Local increases in the proportion of suspension feeders have been correlated with more vigorous current regimes in nonseamount environments (PEOUEGNAT,1964; SEBENS, 1984; SIBUETand SEGONZAC,1985) and with the presence of hard substrate and inferred flow acceleration over small- and large-scale topographic features on a number of seamounts in the central and eastern Pacific (GENIN et al., 1986; LEVlN and THOMAS, 1986; GENIN, 1987; GRIGG et al., 1987; GENIN and SMITH, unpublished data); quantitative flow profiling of such topographic features has not yet been reported in the literature. Since sponges and sessile cnidarians are suspension feeders, it is quite possible that they benefit from increased food supply rates due to more vigorous currents on the perimeter of Horizon Guyot. The "perching" of ophiuroids and comatulid crinoids on sponges and stalked cnidarians may serve to elevate these animals out of the hydrodynamic boundary layer associated with the rock formation or, in the case of a crinoid, to permit the animal to deploy its arms in a planar array perpendicular to the prevailing current direction, facilitating more efficient particle capture (MEYER,1973; MEYER and MACURDA,1980; LEONARDet al., 1988). The same may be true for the ophiuroids (FuJITA and OHTA, 1988), although it has also been suggested that some ophiuroid epibionts may either steal food from or prey upon their hosts (GIsLI~N, 1924). Xenophyophores and lebensspuren seem more susceptible than megafauna to influence by current-induced sediment transport. The presence of large, well-developed sediment ripples on the perimeter of Horizon Guyot, the site characterized by the lowest densities and least number of morphotypes of both xenophyophores and lebensspuren, seems to reflect a high degree of sediment transport in this area, relative to the two summit caps. Similar observations were made by LEVlN and NrrrROUER (1987), who reported that abundances of xenophyophores and other sediment-constituted features were lower in areas with ripple marks than in those without on three deep seamounts (788-3353 m) in the eastern Pacific. It may be that certain xenophyophore species are unable to successfully colonize areas characterized by high rates of sediment transport, perhaps due to periodic inundation, effectively limiting the density and diversity of xenophyophores in such locations. Increased sediment transport rates may decrease lebensspuren abundances by lowering their residence times and by impacting the organisms responsible for their generation. Infaunal forms, particularly sedentary burrowers and species living at or just beneath the sediment-water interface, may be adversely affected by extensive sediment transport, while more mobile forms might be better able to successfully evade or cope with this problem. Thus, the relatively low number of lebensspuren morphotypes and high proportion of mobile lebensspuren on the Horizon Guyot perimeter as compared to the

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two summits (Table 3) may result from changes in the infaunal community due to differences in sediment transport rates between the perimeter and summit locations. The abundances and numbers of taxa represented for both meio- and macroinfauna were lower on the perimeter of Horizon Guyot than on either summit, while a greater percentage of the perimeter macroinfauna was located more than 2 cm beneath the sediment surface (LEvIN and THOMAS, 1989). Since many lebensspuren, especially sedentary forms, are apparently the result of activity by macro- and megainfauna (HINCA, 1981; OrrrA, 1984; MAUVIELet al., 1987; RO~tERO-WETzEL, 1987), a reduction in the density, species richness and proximity to the sediment-water interface of these faunal types, as seen on the Horizon Guyot summit perimeter, might have contributed to the observed decreases in lebensspuren abundance and morphotype richness and the increased representation of mobile morphotypes at this site, although the contribution of physical erasure to these figures cannot be overlooked (WHEATCROFret al., 1989). These factors may also account for the generally lower abundances of lebensspuren on these seamounts, relative to non-seamount areas of the deep sea, which may not be exposed to such vigorous current regimes. Nutrient availability. A positive correlation has been observed between the biomass of benthic macrofauna in the deep sea and rates of primary productivity in the overlying surface waters (RowE, 1971, 1983). Differences in primary productivity may help to explain the occurrence of higher megafaunal densities on the western North Atlantic continental slope (primary productivity = 120 g C m -2 y-l), relative to Suruga Bay (90 g C m -2 y-l), Magellan Rise (35-60 g C m -2 y-l) and Horizon Guyot (15-30 g C m -2 y-l; HOGETSU, 1979; BERGER et al., 1987). Benthic communities on continental margins, by virtue of their proximity to large land masses, may also benefit from additional nutrient fluxes resulting from terrigenous inputs as well as lateral advection of particulate organic material from the continental shelf and upper slope (SANDERSand HESSLER, 1969; WALSH, 1989); this may amount to nearly 40% of the measured primary productivity in these areas (WALSH,1989). The observed trends in megafaunal abundance among these four areas are similar to the pattern of primary productivity, although megafaunal densities in some areas of Surnga Bay were lower than would be predicted on the basis of surface production. Disregarding xenophyophores, megafaunal abundances on the summit of Magellan Rise were still higher than at comparable depths in Suruga Bay (53 vs 26 per 1000 m2), although this was not the case for the Horizon Guyot summit (81 vs 164--219 per 1000 m E in Suruga Bay). Fish densities in Suruga Bay and on the western North Atlantic continental slope were consistently higher than on the seamount summits by a factor of 2-8. Unfortunately, it is not possible to translate our numerical densities into estimates of biomass or community metabolic demand without further, unavailable information, and while it seems reasonable to surmise that nutrient availability differences may account for some of the variability in megafaunal abundance and community composition between our study sites and those on the continental margin, no definitive conclusions are possible at this time. Bioturbation. An important factor influencing lebensspuren residence times, and hence lebensspuren densities, is bioturbation (WHEATCROFr et al., 1989). Logically, surface bioturbation rates should be higher in areas with greater densities of sedimentdisturbing epi- and infauna, particularly areas supporting large, mobile deposit-feeding species capable of effectively reworking the sediment surface (e.g. epibenthic holothur-

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oids). Meio- (COLLEN, 1973; EKDALE et al., 1984) and macroinfauna (LI et al., 1985; RICHARDSONet al., 1985) have also been implicated in the homogenization of the upper layers of the sediment, although their importance as bioturbators, relative to the megafauna, is difficult to assess. Among the seamount sites, lebensspuren abundances were better correlated with densities of macroinfauna and non-xenophyophore megafauna than with densities of deposit-feeding echinoderms, although currents also appear to have had a substantial effect on densities of lebensspuren, particularly at the Horizon Guyot perimeter site (see above). Generally, erasure of traces by mobile deposit feeders appears to be less influential in structuring the lebensspuren assemblages at these sites than trace production (including superimposition) by other faunal components. However, deposit feeders, notably echinoderms, had an important effect on the morphotype composition of the lebensspuren assemblages at each of the three sites, as evidenced by the abundance of echinoid trails, sitzmarks and holothuroid fecal casts on Magellan Rise, relative to the Horizon Guyot sites (Table 3). Depth. The interpretation of faunal differences between the two seamount summits is partially confounded by the difference in their depths. Researchers studying bathymetric zonation of megafauna on the Atlantic (HAEDPaCHet al., 1980) and Pacific (PEARCYet al., 1982) continental slopes of North America and in other areas of the world ocean (VINOGRADOVA,1962) have reported distinct differences in species composition, numerical density and biomass between the faunal assemblages located at <2000 m and >3000 m depth. The reasons for these changes are unclear, although differences in nutrient availability are thought to be at least partially responsible (SANDERSand HESSLER,1969; HAEDRICH et al., 1980). Depth-related changes in community composition may result from a number of factors, including changes in nutrient availability, environmental stability, competition, predation and hydrostatic pressure. While some or all of these parameters differ between our three study sites, a rigorous discussion of depth-related species replacement and its potential causes and effects are beyond the scope of this study. Biogeography The non-xenophyophore megafaunal assemblages on both seamounts were dominated by species with widespread and cosmopolitan distributions. This is in concordance with the observations that shallow seamounts (summit depth <1000 m) tend to support a roughly equal number of provincial and widespread/cosmopolitan species, while deep seamounts (summit depth >1000 m) are typically dominated by widespread/cosmopolitan species (WILSONand KAUFMANN,1987). The degree of endemism observed among invertebrates on Horizon Guyot (11.8%) and Magellan Rise (11.4%) was slightly lower than the average reported by WILSONand KAU~iANN(1987) for more than 100 seamounts worldwide (15.4%). Presumably, as the database on seamount fauna continues to increase, estimates of the extent of endemism among seamount megafauna will decrease. The most complete distributional records were available for species of echinoderms and fishes. Many of these exhibited Indo-West Pacific distributions, corroborating previous data from seamounts in the central Pacific (WILSON et al., 1985); similar biogeographic affinities have also been reported for mollusks (REHDERand LADD, 1973) and cirripedes (RAo and NEWMAN, 1972) from the Mid-Pacific Mountains. Thus, it

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appears that, at least biogeographically, the megafauna on these two seamounts are typical of others in the same depth range and geographic vicinity. CONCLUSIONS

Assemblages of megafauna and lebensspuren differed greatly between the summit cap and perimeter of Horizon Guyot and the summit cap of Magellan Rise, although xenophyophores were the most abundant taxon at all three locations. Non-xenophyophore megafauna on the Horizon Guyot perimeter consisted primarily of hard-substratedwelling suspension feeders, in contrast to the two summit communities. Among our three study sites, the Horizon Guyot summit hosted the greatest abundance of nonxenophyophore megafauna, while the summit of Magellan Rise displayed the highest xenophyophore densities. Megafaunal assemblages on the perimeter of Horizon Guyot and summit of Magellan Rise were characterized by numerous echinoderms, a taxon virtually absent from the Horizon Guyot summit. Several factors seemed to account for the differences between our study sites: substrate availability, hydrodynamics, nutrient availability, bioturbation and depth. Substrate availability appeared to be particularly influential in structuring the non-xenophyophore megafauna, with hard substrate supporting predominantly suspension-feeding morphotypes and sediment-covered areas dominated by deposit feeders. Hydrodynamic factors may also be important, since current regimes influence the settlement of larvae, the amount of food available to suspension feeders and the rate and volume of bulk sediment transport, which may, in turn, inundate certain sessile or infaunal organisms and erase lebensspuren. Nutrient availability was apparently related to both primary productivity in the overlying surface waters and proximity to a large land mass, and benthos-associated megafauna appeared to be more abundant in areas with higher predicted nutrient fluxes. Nutrient availability, in combination with substrate availability and hydrodynamic factors, influenced the trophic modes available to and/or desirable for the species inhabiting a given ecotope. Bioturbation appears to have substantially affected the abundance and composition of lebensspuren assemblages, with higher densities of lebensspuren present in areas characterized by greater abundances of macroinfauna and non-xenophyophore megafauna. Depth differences between the two seamounts may have been responsible for some of the observed faunal differences; however the relative contribution of depthrelated effects is unknown. Megafaunal abundances on the summit cap of Horizon Guyot were comparable to those at similar depths in non-seamount areas, while densities on Magellan Rise were substantially higher than in non-seamount areas and those on the Horizon Guyot summit perimeter were substantially lower. However, all three seamount sites exhibited much lower fish densities than similarly deep non-seamount sites. Xenophyophore densities on the two seamounts were comparable to those reported from similar depths on seamounts in the eastern Pacific. The lebensspuren assemblages at our three sites reflected differences in megafaunal and macrofaunal abundance and community structure, bioturbation rates and physical disturbance. Differences in the abundance and morphology of lebensspuren between areas apparently resulted from variations in the degree of physical and biological disturbance of the surface sediments. The spatial distribution of lebensspuren morphotypes associated with foraging activity indicated that most of the organisms generating such traces on these seamounts were randomly on spatial scales of 10-1000 m. These

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patterns may have resulted from either random foraging activity or directed foraging with randomly distributed food sources. Most of the megafaunal forms on these seamounts appeared to have widespread to cosmopolitan distributions in the deep sea, with biogeographic affinities in the Indo-West Pacific. In terms of their faunal composition, these two seamounts seem typical of others in the same depth range and biogeographic province. Acknowledgements--This study was made possible by an NSF-sponsored Atlantis IUAlvin cruise conducted by K. L. Smith Jr in February-April 1987. The Alvin group assisted tremendously in the collection of data, and this study would have been impossible without them. Helpful discussions regarding this manuscript were provided by J. Enright, D. Foster, A. Leonard, G. Sugihara, E. Venrick and B. Walden. Expert taxonomic and biogeographic information was contributed by K. Baba, F. Bayer, C. A. Child, M. Downey, D. Fautin, R. Lemaitre, L. Levin, S. Luke, W. Newman, D. Pawson, R. Rosenblatt, O Tendal, A. Williams and R. Wilson. We are especially grateful to D. Pawson for his tireless and cheerful identification of echinoderms and for his invaluable and diverse input. We would also like to thank C. DeMoustier, J. Mammerickx and W. Schwab for their assistance with Figs I and 2 and R. Portillo and S. Stultz for their expert technical support in the preparation of this paper. Valuable comments on the manuscript were provided by J. Bernhard, J. Enright, L. Levin, K. L. Smith Jr, R. Wilson and two anonymous reviewers. This research was supported by NSF grant OCE-84-17913 to K. L. Smith Jr and by ONR contract N00014-84-K-0081 to L. A. Levin. REFERENCES ABLE K. W., D. C. TWICHELL, C. B. GRIMES and R. S. JONES (1987) Tilefishes of the genus Caulolatilus construct burrows in the sea floor. Bulletin of Marine Science, 40, 1-10. AGASSIZ A. (1904) The panamic deep-sea Echini. Memoirs of the Museum of Comparative Zoology at Harvard College, 31, 1-243. ANDREW N. L. and B. D. MAPSTONE (1987) Sampling and the description of spatial pattern in marine ecology. Oceanography and Marine Biology: An Annual Review, 25, 39-90. BARHAM E. G., N. J. AVER Jr and R. E. BOYCE (1967) Macrobenthos of the San Diego Trough: photographic census and observations from bathyscaphe, Trieste. Deep-Sea Research, 14, 773-784. BARNES R. D. (1980) Invertebrate zoology, 4th edn. Saunders College Press, Philadelphia, 1089 pp. BATEMAN G. I. (1950) The power of the Z2 index of dispersion test when Neyman's contagious distribution is the alternate hypothesis. Biometrika, 37, 59--63. BERGER W. H., K. FISCHER, C. LAI and G. Wu (1987) Oceanic productivity and organic carbon flux. Part 1. Overview and maps of primary production and export production. Scripps Institution of Oceanography Reference Series, 87-30, 67 pp. BLOOM S. A. (1981) Similarity indices in community studies: potential pitfalls. Marine Ecology Progress Series, 5, 125-128. lOVER L. F., R. A. COOPER, D. T. LONG and T. M. ASKEW (1989) Burbot (Lota Iota) biogenic sedimentary structures in Lake Superior. Journal of Great Lakes Research, 15, 174-185. CACCHIONE D. A., W. C. SCHWAB, M. NOBLE and G. TATE (1988) Internal tides and sediment movement on Horizon Guyot, Mid-Pacific Mountains. Geo-Marine Letters, 8, 11-17. CLARK A. H. (1907) A new species of crinoid (Ptilocrinus pinnatus) from the Pacific coast, with a note on Bathycrinus. Proceedings of the United States National Museum, 32, 551-554. CLARK H. L. (1920) Holothurioidea. Memoirs of the Museum of Comparative Zoology at Harvard College, 39, 121-154. CULLEN D. J. (1973) Bioturbation of superficial marine sediments by interstitial meiobenthos. Nature, 242, 323-324. EKDALE A. A., L. N. MULLER and M. T. NOVAK (1984) Quantitative ichnology of modern pelagic deposits in the abyssal Atlantic. Palaeogeography, Palaeoclimatology, Palaeoecology, 45, 189-223. ELLIOTr J. M. (1977) Some methods for the statistical analysis of samples of benthic invertebrates. Freshwater Biological Association Scientific Publication, Number 25, Titus Wilson and Son Ltd., Kendal, 160 pp. EWING M. and R. A. DAVIS (1967) Lebensspuren photographed on the ocean floor. In: Deep-sea photography, Johns Hopkins Oceanographic Studies, Number 3, J. B. HERSEY, editor, Johns Hopkins Press, Baltimore, pp. 259-294. FISHER W. K. (1911) Asteroidea of the North Pacific and adjacent waters, Part 1. Bulletin of the United States National Museum, 76, 1-419. FUJITA T. and S. OrrrA (1988) Photographic observations of the life style of a deep-sea ophiuroid Asteronyx loveni (Echinodermata). Deep-Sea Research, 35, 2029-2043. GENIN A. (1987) Effects of seamount topography and current on biological processes. Ph.D. Thesis, University of California, San Diego, 169 pp.

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