Molluscs are soft-bodied invertebrates possessing (usually) an external shell for protection, a ventral foot for locomotion and, in aquatic species, gills for respiration. The digestive and reproductive tissues are located together to form the visceral mass dorsally. An extensive fold of tissue, known as the mantle, not only produces the shell but also acts as a protective sleeve for the head and gills.
Classes of Mollusca. Top row: Gastropoda (turban snail), Bivalvia (venus clam); Bottom row: Scaphopoda (tusk shell), Polyplacophora (chiton), Cephalopoda (tail-light squid shell)This phylum – or major animal group – consists of 6 subdivisions known as Classes:
Gastropoda - marine, freshwater and land snails (mostly with coiled shell) and slugs (lacking shell)
Bivalvia - marine or freshwater; two-shelled molluscs including clams, oysters, scallops, mussels
Cephalopoda - marine; shell internal (squid, cuttlefish), lacking (octopus) or external (Nautilus)
Polyplacophora - marine; ‘chitons’ – molluscs with an 8-plated shell
- marine; ‘tusk’ or ‘tooth shells’ – with a single curved shell
Aplacophora - marine; worm-like molluscs without a shell
Each Class contains many families, genera and species. The entire phylum may contain as many as 100,000-200,000 species worldwide, of which probably 10,000-15,000 occur in Australia.
Molluscs of Moreton Bay
Moreton Bay has a wide variety of habitats suitable for different species of molluscs such as mud whelks, trochus shells, octopus and clams (view of bay islands from Wellington Point) Moreton Bay is home to at least 1000 recorded species of marine molluscs, with over 950 species of gastropods (snails and slugs) and 350 species of bivalves (clams, oysters, scallops etc). The exact number is unknown as much more research needs to be carried out, especially on the numerous micro-molluscs (species of 5mm or less in length). New species are still being described and it is likely that most of them will be small and previously overlooked.
There are a number of factors which determine the number of marine species occurring in any locality, such as habitat types and suitability, availability of larvae, water temperature and quality. In the case of Moreton Bay, the region lies at the meeting point of tropical (south-moving) currents and temperate (north-moving) currents and hence there is great opportunity for planktonic larvae (of molluscs and other animals) to settle and grow. Moreton Bay is also special in that it offers a wide range of habitats for different types of molluscs to occupy, including mangroves, sea grass areas, rock platforms and rocky reefs, surf beaches and even hard coral reefs. In addition the sediment bottom of the bay is extremely diverse ranging from fine clay through sand and sandy mud to pebbles to rock. Dead mollusc shells themselves may form grit bottoms and whole beaches (eg Shelly Beach on the Sunshine Coast) or intertidal and subtidal shell ‘reefs’. Molluscs will exploit almost any available habitat and their rich diversity in Moreton Bay well illustrates this fact.
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