Records of the Western Australian Museum Supplement No. 59: 29-46 (2000). MOLLUSCS OF THE MONTEBELLO ISLANDS F. E. Wells, S. M. Slack-Smith and C. W. Bryce Western Australian Museum, Francis Street, Perth, Western Australia 6000, Australia Summary . various types of coral reef, intertidal rocky and Six hundred thirty-three species of molluscs are sandy shores, and mangroves. The centre portions recorded from the Montebello Islands, north­ of the archipelago are characterised by an extensive western Australia. This is the second highest system of protected channels between the islands. diversity of molluscs recorded on any of the surveys Collecting was done both in the channels and the of tropical reef systems conducted by the Western exposed areas outside of the islands. Australian Museum; only the expedition to the In addition, the senior author collected molluscs Muiron Islands and eastern Exmouth Gulf recorded during a short visit to the Montebellos from 19 to 21 more species of molluscs (655). It is comparable to August 1986. Species collected during that trip are three surveys of the central area of the Indo-West included on the present list as station numbers Pacific conducted by Conservation International, preceded by a W. The stations collected were: W1. though the Cl surveys included only one West side Trimouille L; W2. East side Trimouille L; malacologist. Of those species recorded from the W3. Tide Pole Bay, Trimouille I.; W4. Northwest Montebellos whose distributions are known, the corner, Trimouille I.; W5. Northwest I.; W6. great majority are widespread Indo-West Pacific Stephenson's Passage; W7. Bay on southern end of species. A single warm temperate Australian Hermite L; W8. Mangroves near southern end of species, Thais orbita, was collected. The molluscan Hermite L; W9. Northern Hermite L assemblage at the Montebellos is characteristic of the continental coastline, not the offshore atolls. Drupella comus, a coralliverous species known to Results and Discussion cause considerable damage to corals along the The 12,000 km coastline of Western Australia can Ningaloo Reef Tract in Western Australia, is present be divided into three components. The tropical at the Montebellos, but no damage to coral north coast, which extends northeast from North populations was found. West Cape, is part of the vast Indo-West Pacific province. The south coast, east of Cape Leeuwin, is continuous with the remainder of the southern Introduction Australian warm temperate province. The west Over the last 25 years the Western Australian coast, between Cape Leeuwin and North West Museum has conducted a number of surveys of the Cape, is an area of overlap between the south coast marine fauna inhabiting coral reefs on the north warm temperate and north coast tropical faunas. coast of the state. These include reefs along the About 10% of the shallow water marine molluscs of inshore coastline such as the Dampier Archipelago, Western Australia are endemic to the State; most of North West Cape, Muiron Islands and eastern shore these occur on the west coast for at least part of of Exmouth Gulf, and Bernier and Dorre Islands in their distribution (Wilson and Gillett, 1971; Wells, Shark Bay, the Houtman Abrolhos, and offshore 1980; 1997; Wilson and AlIen, 1987). atolls such as the Rowley Shoals, Scott and Being on the north coast, the Montebellos have Seringapatam Reefs and Ashmore Reef. The an essentially tropical molluscan assemblage. One molluscan component of the August 1993 survey of surprise during the survey was the collection of a the Montebello Islands survey is reported here. recently dead specimen of the southern Australian Thais orbita, found outside its previously known range of as far north as North West Cape. This was Methods the only southern temperate species of mollusc In order to obtain as many species of molluscs as collected during the survey. Many of the ranges of possible of >5 mm in shell length, collecting was species are poorly known. Wells (1980) based his undertaken from 8 to 26 August 1993 in a variety of analysis of molluscan distribution patterns in ways, including extensive SCUBA diving and reef Western Australia on the 20 families of gastropods walks at low tide, and less extensive dredging and for which there are the best data. In the sorting of drift algae. All of the major marine Montebellos 217 species of these families were habitats of the Montebellos were collected: the identified at least tentatively to species. Ten of 30 F.E. Wells, S.M. Slack-Smith, C.W. Bryce these (4.6%) are known only from Western Mangrove assemblages are simple and small, without Australia. much species diversity. As a consequence, the The 633 species collected during the survey molluscs characteristic of mangroves are depauperate. demonstrated a far higher molluscan species For example, only one record of an ark shell of the diversity than was found on any of the previous genus Anadara (family Arcidae) was made, although surveys in Western Australia except for the some fossilised shells were found on dry salt pans. expedition to the Muiron Islands and the eastern This genus is well represented in Pilbara mainland shores of Exmouth Gulf (Tables 7 and 8). Diversity intertidal and shallow sublittoral sand and mud in the Montebellos was similar to that recorded by habitats by generally large populations of a few three subsequent expeditions to the coral triangle of species. On the other hand both species of Terebralia the Indo-West Pacific undertaken by Conservation which are associated with Pilbara mangroves were International: Milne Bay, Papua New Guinea; found; Telescopium, which occurs in a similar habitat, Calamian Islands, Philippines; and Toggian and was not recorded in the Montebellos. The oyster Banggai Islands, Indonesia. There are several Hyotissa numisma is abundant under intertidal rocks in possible reasons for the increased diversity in the the Dampier Archipelago but only one specimen was Montebellos. The Montebellos expedition was obtained during this survey. longer than some of the previous surveys; the Diversity of molluscs in the Montebellos varies additional time clearly allowed some additional considerably between the various classes. species to be collected. In addition there were three Gastropods typically comprise about 80% of the people collecting molluscs on the Montebello species in the phylum, and 401 species were Expedition, compared to only one on the recorded in the Montebellos, 63.3% of the total. This Conservation International trips. was a smaller proportion than found on some of the However, the major reasons for the high mollusc previous expeditions. Bivalves were more diverse diversity in the Montebellos appear to be biological. than usual, with 223 species (35.2%). The Montebello Islands and their associated reefs Polyplacophorans (4 species), cephalopods (3) and shelter an expanse of relatively shallow waters with scaphopods (2) were only minor elements of a mixture of hard and softsubstrates cmd with molluscan diversity. generally good water circulation because of the Duritlgthe late 1980s and early 1990s population tidal range and the physiography of the bottom. The explosions of the neogastropod Drupella cornus extent of these sheltered waters is unusual along occurred along the Ningaloo Reef Tract on the west the Western Australian coast and unique in the side of North West Cape, causing considerable Pilbara region. damage to the corals (summarised in Turner, 1992). As a result, in the waters of the Montebellos there Hilliard and Chalmer (1992) undertook a survey of is a large concentration of habitat types otherwise population densities of the species along the Pilbara uncommon in the area. This range of habitats s~ems coast. Their study included the Lowendale Islands, to be associated with an unusually high diversity in just south of the Montebellos, but did not include the Montebellos of some groups of molluscs. The the Montebello Islands themselves. Hilliard and large expanse of semi-sheltered sandy lagoon Chalmer found feeding aggregations of D. cornus, supports a high diversity of filter-feeding bivalves particularly in the western half of the study area, living infaunally in soft substrates. Some of the which caused localised damage to coral species have much higher population densities than populations, but no major outbreak. Drupella cornus are found in other parts of the Pilbara. Included was found at 7 stations during the Western among these are species of the families Australian Museum Montebellos survey. While no Spondylidae, Ostreidae, Gryphaeidae and attempt was made to quantify the populations of D. Chamidae which may be so dense as to actually cornus, only a few individuals were seen and there cover some of the semi-sheltered reefs to which they was no apparent damage to corals. are cemented. Sand-burrowing scaphopods of the Moyer (1982) recorded Drupella frugal as one of genus Laevidentalium and pyramidellid gastropods two species causing coral damage in Japan and the are also unusually abundant. Philippines, and Fujioka and Yamato (1983) In comparison to the abundance of some bivalve reported damage by D. frugal in the Ryukyu Islands. groups is the relative sparsity of other species that The taxonomy of this genus is difficult, and was , are common in various parts of the Pilbara. Among recently discussed by Wilson (1992), who concluded these are species of wing shells of the genus Pteria that the species identified as D. frugal in the two (family Pteriidae) which live byssally-attached, studies is actually D. rugose. Drupella rugose was mainly to gorgonians. collected during the Montebellos survey, but only However the absence of some species is due, as at stations 17 and 30, and there was no apparent would be expected, to the absence or rarity of some damage to corals. types of habitats, such as those associated with run-off Isolated individuals of two additional coralliverous of freshwater bearing terrigenous sediments.