Abundance and diversity of Mollusca in the Beagle Channel *

From the middle of the 19th century onwards many expeditions have led to the Magellan region. Most of them focused on taxonomic studies (e.g. King and Broderip, 1831; Smith, 1885; Mabille and Rochebrune, 1889; Strebel, 1904, 1905a, b, 1906, 1907, 1908; Soot-Ryen, 1959; Dell, 1971). Chilean and Argentinian scientists like Gallardo (e.g. 1979) or de Castellanos (e.g. 1988-93) focused on reproduction and taxonomy, respectively. Brattström and Johanssen (1983) published ecological investigations carried out in southern Chile. Bastida et al. (1992) worked on benthic community structure including molluscs on the Argentine continental shelf. In the southern summers 1989-90 and 1990-91 joint Chilean-Italian expeditions were carried out in the Magellan region with a focus on the Straits of Magellan and a preliminary investigation of the Beagle Channel (Brambati, 1992). Di Geronimo et al. (1993) worked on molluscan thanatocenoses. The “Joint Magellan “Victor Hensen” Campaign 1994” (Arntz and Gorny, 1996) was the first expediSCI. MAR., 63 (Supl. 1): 391-397 SCIENTIA MARINA 1999

In the southern summers 1989-90 and 1990-91 joint Chilean-Italian expeditions were carried out in the Magellan region with a focus on the Straits of Magellan and a preliminary investigation of the Beagle Channel (Brambati, 1992).Di Geronimo et al. (1993) worked on molluscan thanatocenoses.
The "Joint Magellan "Victor Hensen" Campaign 1994" (Arntz and Gorny, 1996) was the first expedi-SCI.MAR., 63 (Supl.1): 391-397 tion that focused on benthos studies in the Beagle Channel.The present study is based on molluscan samples collected during the "Joint Magellan" expedition by means of an epibenthic sledge.
This epibenthic sledge (EBS) collects small epiand suprabenthic macrofauna.One taxon in focus were the molluscs collected with this gear.Bivalves were most abundant and diverse with 26,461 individuals and 52 species, followed by Gastropoda with 5,075 individuals and also 52 species.Aplacophora occurred with 1,456 individuals and 9 species, Polyplacophora with 1,515 individuals and Scaphopoda with 579 individuals and 5 species (Linse, 1997).The aim of the present investigation is to describe the distribution of molluscs in the Beagle Chanel.

MATERIAL AND METHODS
On a transect from off the eastern entrance through the Beagle Channel to the western entrance (Fig. 1) macrobenthic Mollusca were collected.The samples were taken by means of an epibenthic sledge (EBS) (Rothlisberg and Pearcy, 1977) which was improved by Brandt and Barthel (1995) in order to catch both the epibenthic and the benthic-boundary-layer fauna.The sledge carries a sampling box with an opening of 100 cm width and 33 cm height.A 0.5 mm plankton net is attached to each sampler, the cod end is enclosed by a 0.3 mm mesh net.When the sledge touches the seafloor a shovel fixed to the box door of the epibenthic sampler opens the box.The doors are closed when the sledge leaves the bottom.The sledge was hauled over the ground for 10 min at a mean velocity of 1 knot.The haul distances were calculated on the basis of the GPS-derived positions of the ship at the start and end of the haul distance in after Brattegard (pers.comm.) and Brandt and Barthel (1995).The haul length varied from 132 to 608 m (Table 1), therefore the numbers of individuals are calculated for standardized 1000 m hauls.16 stations were sampled at 11 sample locations (Table 1).The result at station 33-1261 might be an artifact due to a failure, because the vessel steamed with 3 knots instead of 1 knot and turned in the channel; therefore the sledge might have left the bottom for some time.At station 28-1279 the plankton net was slightly damaged but as the cod end was intact the sample was considered to be complete.When the samples reached the deck of the vessel, they were washed on a 300 µm screen.The complete samples were fixed in 4 % buffered formaldehyde and later transferred into 70 % ethanol.All molluscs from the samples were analyzed for the comparison of the 16 stations.
Information about the feeding mode was taken from the literature (e.g.Hain, 1990) or personal dissection of stomach contents.Most species with unknown feeding modes are bivalves, and most of these will be filter feeders.
In this paper various components of "species  diversity" are used: species richness, i.e. the number of species sampled at a station (Hurlbert, 1971); the Shannon-Wiener index of diversity H' (using log e ; Shannon and Weaver, 1949), and the evenness J (Pielou, 1966) were estimated for each sampling station.

RESULTS
In total 35,084 specimens belonging to 118 species of 86 genera and 58 families were collected.For comparisons between stations samples were standardized for 1000 m 2 hauls, yielding a total of 107,208 individuals (Table 2).
The molluscan abundances differed strikingly between hauls ranging from three to over 10,000 individuals/1000m 2 (Table 2, Fig. 2a).The highest abundances were found at the deepest station 31-1263, at station 37-1246 in the inner channel and at station 41-1213 at the eastern entrance of the Beagle.Abundance was low at the other stations; only the stations off the eastern entrance (Stns.48-1200, 49-1206) showed a higher number of specimens (Table 2).
At stations at and off the eastern entrance herbivorous, detritivorous and filter feeding taxa were very abundant (Fig. 2a) compared with bivalve taxa with an unknown feeding mode such as species of Cyamiidae, Neoleptonidae, and Carditidae.At stations in the inner Channel substrate feeding taxa were more important in terms of specimen numbers.The high abundance of taxa with an unknown feeding mode refers to 3 very abundant species of the families Thyasiridae and Erycinidae (Fig. 2a).
Species richness varied between stations, ranging from 2 to 69.It was highest at Stn 41-1213 off the eastern entrance and also quite high at Stn. 37-1247 in the inner channel (Table 2, Fig. 2b).Polyplacophora and Gastropoda were most frequently sampled down to 200 m.Scaphopoda, however, occurred predominantly at deeper stations (200->300 m).Higher abundances of Aplacophora and Bivalvia were also found at deeper stations but these taxa occurred in shallow water too (Table 3).
At and off the eastern entrance the Shannon-Wiener diversity H' varied between 1.0 and 4.2 and J between 0.5 and 1.0, in the inner channel H' varied between 1.9 and 3.6 and J between 0.5 and 1.0.

DISCUSSION
The abundance and species richness of the molluscs varied significantly (Figure 2, Table 2) among the stations on the longitudinal transect.This patchy distribution observed in the Beagle Channel could be caused by food availability, feeding mode or sediment composition.Brandt et al. (1997) found that stations in the Beagle Channel and off the eastern entrance were characterized by great abundances of small taxa such as Mollusca and Peracarida whereas larger epibenthic macrofauna such as Echinodermata, Anthozoa or Porifera is often absent (Gutt and Schickan, 1996;Gutt, pers. comm.;Witte, 1996).The absence of large filter feeding taxa could increase the presence of small taxa, because, as in the Magdalena Channel (Gutt and Schickan, 1996;Linse and Brandt, 1998) large filter feeders might reduce the amount of particulate organic matter in the benthic boundary layer available for other benthic taxa.Food competition could occur between vagile taxa such as Amphipoda or Mysidacea that can feed in the water column and epi-and endobenthic Mollusca that are restricted to feeding on particles at the sediment surface.At stations (Stns.37-1253, 39-1237) with high peracarid abundance especially of vagile Amphipoda (Brandt et al., 1997) molluscan abundance was low.The few occurring molluscs were mostly substrate feeding bivalves like Ennucula grayi (Orbigny, 1846) and Yoldiella granula (Dall, 1908) or Lucinoma lamellata (Smith, 1881).Conversely, off the eastern entrance and in the eastern entrance both taxa (Peracarida and Mollusca) were highly abundant.At these stations (Stns. 41-1213(Stns. 41- , 48-1200(Stns. 41- , 49-1206) ) herbivorous, detritivorous and filter feeding molluscs like Colpospirella algida (Melville and Standen, 1912), Margarella violacea (King and Broderip, 1831), Calliostoma sp., Crenella sp. and Limatula pygmaea (Philippi, 1845) occurred in high numbers, as well as other dominant species with unknown feeding mode, such   (Shannon and Weaver, 1949) and evenness J (Pielou, 1966) per station from west to east through the Beagle Channel.
A possible reason for the high densities at these stations might be a more favourable food supply.Witte (pers. comm.;Linse, 1997) found that Chlorophyll a and especially phaeopigment concentrations are favourable in the upper centimetres of sediment.Gutt and Schickan (1996) and Witte (1996) mentioned "a thick layer of fluff" (deposited organic matter) occurring on the sediment of the sample locations in the Beagle Channel.Food availability does not seem to be a limiting factor in the Beagle Channel during southern spring.
Different depth ranges of the molluscan species may also be related to their feeding mode.Herbivorous taxa such as the gastropod families Trochidae (e.g.Calliostoma spp.) or Cerithiidae were highly abundant down to 100 m depth (Table 3).Deeper than 100 m they occurred only rarely, with two to eight specimens at some stations.
FIG. 2. -(A) abundance of molluscs and (B) number of species with a specific trophic mode, calculated per station from west to east through the Beagle Channel.hd = herbi-or detritivores; sb = substrate feeders; ff = filter feeders; pa = parasites; ca = carnivores; ?= feeding mode unknown.