Records of the Western Allstralian Mllselllll Supplement No. 66: 247-291 (2004). Diversity and distribution of subtidal benthic molluscs from the Dampier Archipelago, Western Australia; results of the 1999 dredge survey (DA2/99) John D. Taylor and Emily A. Glover Department of Zoology, The Natural History Museum, London SW7 5BD, U.K. email: [email protected] [email protected] Abstract - From a dredge survey of the subtidal fauna of the Dampier Archipelago a total of 422 species of macromolluscs was identified, comprising 227 gastropods, 188 bivalves, four scaphopods and three chitons. Most species were uncommon but abundant taxa included the bivalves Melaxinaea vitrea, Corbllla fZlIIlcata and C. crassa and the gastropods Herpetopoma atrata and Xenophora solarioides. Community analysis identified eight molluscan assemblages, reflecting the varied and patchy nature of the substrates that ranged from muds and silts to coarse sands, gravel, rubble and rocks. The most species-rich stations were those located inshore at water depths <10 m. These muddy stations were also notable for the diversity and abundance of suspension-feeding bivalves. Most of the mollusc species identified are distributed widely around tropical Australia and the Indo-West Pacific but a few are endemic to northwestern Australia, including the newly described lucinid bivalve Lamellolllcina pilbara. INTRODUCTION parts of the world. Studies of latitudinal gradients Although the northwestern Australian shelf is of in molluscan diversity usually focus on well­ outstanding biological interest for its suspected high documented continental margins such as the diversity and the relatively high numbers of eastern Pacific coast of North America (Roy, endemic taxa, the subtidal molluscan fauna is Jablonski and Valentine, 2001; Valentine, Roy and poorly known. Surveys have been made at some Jablonski, 2002). For example, in a study of bivalve locations relatively close to the Dampier global diversity patterns, Crame (2000a, b) has Archipelago, including the Montebello and Muiron highlighted Australia as an apparent high diversity Islands and Exmouth Gulf, but the results are held "hotspot" in the southern hemisphere. Although in internal reports (Wells, Slack-Smith and Bryce, published information for northern Western 1993; Slack-Smith and Bryce, 1996). Most of the Australia is generally lacking, our observations of published information on molluscs of the Dampier museum collections and recent fieldwork suggest a region lies in taxonomic accounts (see Wilson, 1993-­ serious underestimation of the true diversity. New 4; Lamprell and Whitehead, 1992; Lamprell and publications are beginning to confirm this idea Healy, 1998; and bibliographies) and there have (Glover and Taylor, 2001; Middlefart, 2002). been no published studies of offshore subtidal The results of the dredge survey (DA2/99) benthic communities. Faunal surveys of subtidal reported here are first steps toward recording the molluscs that have been published concern fauna of a poorly documented area that is of high locations further south along the Western ecological and biogeographical interest. Faunal Australian coast such as Shark Bay (Slack-Smith, surveys such as this provide a baseline for the 1990), the Houtman Abrolhos Islands (Glover and recognition of future changes in diversity, Taylor, 1997; Wells and Bryce, 1997) and Rottnest abundance and distribution of the benthic fauna in Island (Glover and Taylor, 1999). The probable high relation to such factors as climate change, pollution endemism of the molluscan fauna in the Dampier and other types of environmental disturbance. area may be reflected in the number of new species Additionally, the area is being subjected to described with variations on "dampier" as part of increasing levels of environmental disturbance. the name (e.g. Amoria dampieria Weaver, 1960; Conus Factors that may continue to affect the Dampier dampieriensis Coomans and Filmer, 1985). area include: changes in water circulation patterns Recent increasing interest in global diversity induced by the construction of salt extraction patterns is revealing the dearth of data from some ponds; particulate material from iron ore loading; 248 J. D. Taylor, E. A. Glover dredging of shipping channels; pipeline Museum, London. Comparisons were made with construction; development of harbour installations type material where appropriate and reference and increased sedimentation from terrestrial made to original descriptions and illustrations. construction activities. Voucher material is lodged at the Western Although a highly speciose fauna was recorded Australian Museum and the Natural History during this survey, this is just a part of the total Museum, London. molluscan diversity of the area. It does not include Data were analysed using PRIMER 5 (Plymouth the fauna from the extensive and complex intertidal Routines in Marine Environmental Research ­ habitats, most of the rich subtidal rocky and coral www.pml.ac.uk/primer/primer-e.htm). habitats and the deeper burrowing infauna of subtidal soft substrates. Some of this molluscan fauna will be described in other publications from RESULTS the area, including the results of the International The molluscan species identified from the Marine Biological Workshop held in 2000 (Wells, dredge stations sampled are listed in systematic Walker and Jones, 2003). Additionally, most order in Table 1. Molluscs recovered only as dead molluscs of <5.0 mm shell length were not retained individuals are marked with "*". A total of 422 by the dredge (but some were collected by grab) species of molluscs was identified, comprising 227 and are not included in this report. gastropods, 188 bivalves, four scaphopods and three chitons. A single inarticulate brachiopod was also recorded. Out of this total, 321 species were MATERIALS AND METHODS recovered live, with around 25% of bivalves and From 14-28 July 1999, a total of 99 dredge stations 17% gastropods recorded as shells only. The most was sampled in inshore and offshore shallow water species-rich families were the Veneridae with 26 around the Dampier Archipelago covering an species, Turridae (24), Trochidae (21), Muricidae extensive range of habitats. Most of the archipelago (17) and Tellinidae (11). An estimate of species is located within water depths of less than 30 m, with richness was made using cumulative curves depths of 30-42 m seaward of the islands. Refer to calculated with the EstimateS 5 programme the Station Lists section of this volume for the (Colwell, 1999). The slope of the species dredging expedition (DA2/99) station data and map. accumulation curve (Figure 1) shows that the The fauna was sampled with a box-shaped dredge sample was not saturated. Extrapolations of total measuring 1200 mm x 330 mm at the mouth with a species richness (for the size range of mollusc 1.0 cm mesh using either a scooped or raked bottom sampled) gave estimates of between 335-450 plate. All dredge runs were of 10 minutes duration at species. Our list, including both live and dead­ a vessel speed of 2-3 knots except when interrupted collected (422 species) falls within this range. Total by snagging. For some later stations, a sleeve of fly richness is anticipated to be far higher than this screen was inserted into the dredge to enhance because we have not sampled either small species recovery of smaller animals but this made little or those with specialised habits such as commensal difference to mollusc recovery. The dredge samples, and parasitic taxa. Bouchet et at. (2002) showed including sediment, rocks and sponges, were washed that 33.5% of the molluscan species in their through sieves and the molluscs sorted and preserved intensive sampling at New Caledonia have an in 80% ethanol or 8% buffered formaldehyde. Blocks adult size smaller than 4.0 mm. of rocks, corals and sponges were broken up to The distribution of the 321 species of living recover cemented epifauna and endobionts. molluscs at all stations is shown in Appendix 1. Although the sampling time at each station was Most species were rare, with 96 (31%) recorded consistently maintained during the survey, an from single stations and only eight identified from inherent bias in individual mollusc numbers is more than 20 stations. A similar pattern is shown created by the uneven and patchy nature of the for species abundance, with 197 species (63%) substrates, with the sporadic presence of large represented by five or less individuals while 81 boulders, coral blocks, hard bottoms, sponge (26%) of these were recorded only as single gardens and gravel. At some stations where specimens. Fifteen species composed about 50% of sponges and corals were abundant, these were the mollusc individuals recovered, with only eight broken up to search for endolithic molluscs but it of these forming 41% of the numbers. Ranked was not possible to consistently recover all the abundances of the most frequent species are shown species from them. Nevertheless, for molluscs in Table 2, with some of these illustrated in Figures inhabiting soft substrates the relative frequency of 2 and 3. The most abundant mollusc was the species recorded in the dredge samples is likely a shallow burrowing glycymerid bivalve, Melaxinaea reflection of their natural abundance. vitrea, followed by two other shallow burrowing The molluscs were identified using the reference bivalves, Corbula crassa and C. tunicata. The most collections and library at the Natural History abundant gastropod was the hard substrate-living