Molluscs belong to a group of highly sophisticated invertebrates in terms of their morphology, having the greatest biodiversity of any phylum in the marine environment. This diversity is exceedingly high in the tropical waters of the Indo-Pacific Ocean, particularly in coral environments like those of the Great Barrier Reef (GBR), where it has been estimated that approximately 60% of all the marine invertebrate species are molluscs. There are as many as 3000 species of molluscs on the GBR.
On the GBR, molluscs vary in size from the impressive metre-long giant clams (Tridacnidae) down to micromolluscs (Rissoidae, Barleeidae, Stenothyridae) and nudibranchs (Hedylopsidae, Microhedylidae) less than 1 mm long. These tiny nudibranchs spend their lives between grains of coral sand. Molluscs exploit every possible kind of plant or animal as food. Herbivores and carnivores among them use a specialised feeding organ (radula) to scrape food off the substratum into their mouths. Some species graze algae or sponges from rocks, while others hunt down their quarry actively, detecting it either by smell (gastropods) or by acute eyesight (cephalopods). Many molluscs, particularly bivalves, make a living feeding off the soup of plankton suspended in the water by filtering out edible particles using mucous nets or gill sieves. There is also an ecologically important group of molluscs known as detrital feeders that separate as food the minute organic particles from the sediments. In doing so, these detrital feeders recycle nutrients that would otherwise be locked up in the sediments.
Not only are molluscs linked into every food chain on the GBR, but they are also integral for the very formation and destruction of the reef itself. Their shells serve as settlement sites for crust-forming red algae (Corallinaceae) that grow outward to weld the hard corals together, thus building up the reef. Other molluscs (especially Mytilidae, Pholadidae and Gastrochaenidae) actively bore into live (and dead) coral, weakening it, and eventually causing its collapse. Molluscs are fundamental to the economy of the GBR in terms of its productivity, for their eggs are laid in tens of millions and the floating larval stages form a very important component of the zooplankton in the waters over the reef.
During the life of a mollusc, its shell serves as a living space for other invertebrates that also take advantage of the protection afforded by the rightful inhabitant. In doing so, they reward the mollusc by helping to defend the shell from attackers like fishes and crabs. Examples of such commensal invertebrates are poly-chaetes that live inside the shell and hydroids that live on the outside. After the mollusc that manufactured the shell has died, the shell continues to be recycled by other animals like hermit crabs, tanaids and peanut-worms as their 'home' until it finally breaks down completely. One species of hermit crab, Trizopagurus strigatus, is specially adapted to live only in empty cone shells; its body, claws and legs are all flattened to fit inside the narrow-mouthed shell. During its decomposition the shell also serves as a site of attachment for a multitude of other invertebrates, general and specific, like boring sponges, sea anemones, barnacles and foraminiferans (single celled animals enclosed within a tiny calcareous shell belonging to the Phylum Protista; indicated by an arrow in Fig. 24.1).
Molluscs are present in all habitats on the GBR, from coral outcrops and rocky shores, to sandy lagoons, and in the plankton that washes over the reef. Not only are molluscs present in all habitats, but the sheer abundance of some species like those of the Littorinidae can be staggeringly high. Some families (e.g. Pyramidellidae and Eulimidae) are almost all parasitic, on echinoderms in particular, and members of several other families (Can-cellariidae, Galeommatidae, Tergipedidae) live com-mensally with a range of animals from other phyla.
One of the most significant points about the molluscs of the GBR is that there are definitely no non-native (i.e. introduced or exotic) species among them, not a single one. This situation is unique in the world today and it is one reason authorities need to be so wary when dealing with the shipping traffic moving up and down and through the GBR every day, because ships can easily transport non-native 'pests' on their hulls, in ballast water and in internal seawater compartments called 'sea chests'.
The larger gastropods and bivalves with external shells are the best known groups taxonomically on the GBR, so the level of accuracy of identification for them is relatively high compared to the smaller molluscs, particularly the micromolluscs, and those without shells. The basis for the classification of molluscs has shifted over the last century from the shell to the animal itself.
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