Dense Aggregations of Three Deep-Sea Holothurians in the Southern Weddell Sea, Antarctica
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MARINE ECOLOGY PROGRESS SERIES Vol. 68: 277-285, 1991 Published January 3 Mar. Ecol. Prog. Ser. Dense aggregations of three deep-sea holothurians in the southern Weddell Sea, Antarctica Alfred Wegener Institute for Polar and Marine Research, ColumbusstraRe, D-2850 Bremerhaven. Germany Institute for Polar Ecology, University of Kiel, OlshausenstraRe 40, D-2300 Kiel 1, Germany ABSTRACT: Spatial distribution patterns of elasipod holothurians from the southern Weddell Sea were studied by analysing underwater photographs and trawl-catch data from 7 stations, ranging in depth from 240 to 1200 m. The 3 investigated species (Elpidia glacialis, Achlyonice violaecuspidata, Scoto- planes globosa) occurred in dense aggregations of differing spatial scale. For the first 2 species, median photographically determined densities within the patches were 17 and 6 ind. m-2 respectively. The size- frequency distributions of all species were relatively broad. A behavioural communication between individuals assuring the formation and persistence of these aggregations is proposed. INTRODUCTION uniform. Faunistic investigations in various areas clearly show that there are different assemblages, each The benthos of the Antarctic continental shelf and with typical features (Bullivant 1967, Gruzov & Pushkin slope is generally known as an ecosystem of high 1970, Hedgpeth 1977, Everitt et al. 1980, Kirkwood & species diversity and high epifaunal biomass (Holdgate Burton 1988, VoR 1988). For photographic observations 1967, Dayton et al. 1970, Lowry 1975, Knox & Lowry see Bullivant (1961),Oliver (1978),Hempel (1986),and 1977, Jazdzewski et al. 1986, Miihlenhardt-Siege1 Gutt (1988). 1988). The largest proportion is made up of sessile During 2 German Antarctic expeditions into the suspension feeders, e.g. sponges, bryozoans, ascidians, Weddell Sea, underwater photography as well as a and certain echinoderms (Belyaev & Ushakov 1957, dredge (Agassiz trawl) and a bottom trawl were used to Bullivant 1967, Propp 1970, Dayton et al. 1974, Arnaud study the sparsely investigated bottom fauna of the 1977). At some locations on the continental shelf, which shelf and slope (see Drescher et al. 1983, Hempel 1983, is extremely deep (5 800 m) in Antarctica (Picken 1985). VoR (1988), on the basis of the faunistic data 1984), the sea floor is completely covered by such obtained by those expeditions, described 3 species organisms (Dayton et al. 1974, Gutt 1988) and biomass assemblages. The 'Eastern Shelf Assemblage' and the estimates may reach 6 kg wet wt mP2(White & Robins 'Southern Shelf Assemblage' correspond to the general 1972). In terms of species composition, some authors picture given above, i.e. they are dominated by suspen- have noticed an affinity between the Antarctic shelf sion feeders, contain intermediate to high species num- and the deep-sea (Broch 1961, Bullivant 1967, Dayton bers and low evenness. The results presented in this & Oliver 1977). Different hypotheses have been pro- paper, however, refer to a region whose bottom fauna posed to explain this phenomenon in relation to the belongs to the 'Southern Trench Assemblage' (sensu colonization history of both these biotopes (Menzies et VoR 1988). It is poorer in numbers of species as well as al. 1973, Lipps & Hickman 1982). individuals, and significantly less diverse. Elasipod For many Antarctic species a circumpolar distribu- (grazing holothurians are the most abundant taxon. tion is assumed (Bullivant 1967, Dell 1972). The com- Their zoogeography and ecology were investigated in position of the Antarctic benthos, however, is not more detail by Gutt (1988). For 2 selected species, C3 Inter-Research/Printed in Germany 278 Mar. Ecol. Prog. Ser. 68: 277-285, 1991 Elpidia glacialis Theel, 1876 and Achlyonice violaecus- A cyclonic circulation of very cold water (-l.g°C) pidata Gutt, 1990, underwater photography was used known as 'Western Shelf Water' (Carmack & Foster to quantify densities, small-scale dispersion patterns 1975, see Hellmer & Bersch 1985 for review) leads to a and length-frequency distributions, and to gain infor- southward flow under the ice shelf east of the Filchner mation on their behaviour. For another abundant Depression and a northward outflow west of it. Robin et species, Scotoplanes globosa (Theel, 1879), densities al. (1983) described another cyclonic circulation west of and size-frequency distributions were obtained by the Gould Bay which is superimposed by tidal currents analysis of bottom trawl samples. with velocities of up to 0.4 m S-'. They calculated a net All 3 selected species belong to the order Elasipodida northward outflow from areas under the ice shelf for Theel, 1882 which is generally composed of typical the region between ca 40 and 49" W (see also Carmack deposit-feeding deep-sea holothurians (Pawson 1966a, & Foster 1975, Foldvik et al. 1985). Due to these b). On the basis of present knowledge, Achlyonice oceanographic conditions there are only weak bottom violaecuspidata is considered to be endemic to Antarc- currents in the Filchner Depression and, subsequently, tica. It has a longish, flattened body of gelatinous con- the sea bottom at Stn 307 consists mainly of soft, fine sistency with no ossicles in the body wall. The body of substrate. Coarser sediments, however, were found at Elpidia glacialis is oviform and covered by a thin layer the edges of the Filchner Depression. The shelf area of of filigrane calcareous ossicles. Scotoplanes globosa is the Gould Bay is generally characterized by sandy similar in shape, but considerably larger. Information bottoms (Melles pers, comm.). on the worldwide distribution of the 2 latter species Square photographs were taken with a 70 mm was compiled by Hansen (1975). The main distribution underwater camera (Gutt 1988). The camera was trig- area of E. glacialis is the Arctic Ocean. This species is gered at a constant distance to the sea floor; therefore known as an extremely eurybathic deep-sea form the size of the vertically photographed area (1.2 m2) which also occurs in shallow water (Zenkevitch 1963). could be calculated (see also Hersey 1967, Heezen & S. globosa has been found mainly in the southern Hollister 1971, George et al. 1985).A small variation in hemisphere. There are several references on the area photographed may have occurred owing to the patchy distribution of these two and other related trigger weight sinking deeper into softer sediments. species, which can be best detected by means of under- However, pictures in which the weight was photo- water photography (Lemche et al. 1976, Pawson 1976, graphed show that the camera was also triggered Billet & Hansen 1982, Ohta 1983, Smith & Hamilton immediately on soft bottoms. It is thus assumed that the 1983). area1 underestimate is 10 % at most (i.e. 5 0.12 m'). The distance between the positions of the first and the last STUDY SITE, MATERIALS AND METHODS pictures in a run depended on the drift speed of the ship. Since successive pictures did not overlap, and the Specimens were collected in the southernmost part time intervals between shots varied from 12 to 20 S, the of the Weddell Sea during the German Antarctic distance must be at least 80 m for a film of 60 Expeditions ANT I/2 and ANT III/3 with RV 'Polar- exposures. Assuming a maximum horizontal drift of stern' in 1983 and 1985 (Fig. 1). Underwater photogra- 0.5 m S-' the maximum distance between the first and phy was used at 4 stations; one (307) is situated in the the 60th photograph is 600 m. central part of the Filchner Depression in a depth of On each photograph, individuals of the 3 selected 1175 m, the others (301, 302, and 303) in Gould Bay in species were counted. It was easy to identify Achlyo- 240 to 445 m. Bottom trawl catches were made at 5 nice violaecuspidata due to the typical violet colour of stations on the western edge of the Filchner Depression its tentacles and foot tips. In the study area it was also at 640 to 700 m depth. found in considerable numbers in trawl catches. The Fig. 1. Station map. Depth contours after Melles I....,...I.I 1 I-' Stations with trawl catch data; .. JLaUuLIJ with photographic observations Gutt & Piepenburg: Holothurians in the Weddell Sea 279 holothurians visible on the photographs from Stn 307 in opening was ca 22 m. The along-bottom haul time was the Filchner Depression were identified by B. Hansen 30 min except for Stn 180b (60 min). (Zoological Museum, Copenhagen, Denmark) as Elpidia glacialis. This species, however, was not found in any of the numerous trawl catches obtained during 3 RESULTS expeditions of RV 'Polarstern' (ANT I, ANT 11, and ANT 111) in the Weddell Sea. Only one specimen of the third The numbers of specimens per photograph arranged species, Scotoplanes globosa, which differs from E. in exposure sequence are presented in Fig. 2. In glacialis in having 2 instead of 3 or more dorsal papillae Table 1 the specimen counts per photograph are con- and in the shape of the anterior body, was found in the verted to density figures (ind. m-'). photographs. This species was present in high num- bers, however, in the 5 trawl catches at the western edge of the Filchner Depression. Achlyonice violaecuspidata In the following the term 'small-scale' refers to the spatial extent of one station (see above), i.e. each This species was present with 231 specimens on the photograph is regarded as one sample unit. For the first 30 photographs from Stn 301 (Fig. 2). No further statistical description of the spatial pattern we used the specimens were found on the remaining photograph of measure of patchiness m'/m proposed by Lloyd (1967) the sequence (66 photos). Counts range from 1 to 32 where m denotes the sample mean and m' the index of specimens per photograph; the maximum represents a mean crowding (the mean number, per individual, of density of 26 ind.