At present the most urgent need is for taxonomic research to establish what species of molluscs live on the GBR and where they live. At present the figure of 3000 species is a best guess and the number of adequately identified species is rapidly increasing as more and more groups of micromolluscs like those shown in Fig. 24.1 are studied. An indication of the richness of these micromol-luscs on the GBR can be obtained from just one family, the curious left-twisted triphoras (Triphoridae; a triphora shell is arrowed in Fig. 24.1), where a single sample of coral sand is known to have contained 80 species of triphoras, many of them undescribed scientifically.
Genetic technology is becoming not only an important tool for taxonomic research, but it is also highly significant for assessing the relationships between the different taxa. Genetics has great power to separate taxa that do not offer sufficient morphological characters to distinguish them, or present a confusing array of morphological characters. The prime candidate for this novel genetic technology would be the beautiful balers of the genus Cymbiola (Volutidae) mentioned above and shown in Fig. 24.2.
Despite the very large number of species of molluscs on the GBR, research on their behaviour and physiology has barely begun. As shown above, we have general knowledge about the feeding types of molluscs, but there has hardly been any research on individual species. For example, although we know some chitons have 'home' sites that they return to after feeding excursions, we do not know how these chitons find their way 'home' or what makes a particularly good 'home' site. Similarly, we know that some members of the mussel genus Lithophaga (Mytilidae) burrow into coral using a chelating agent secreted by pallial glands, but we do not know what turns this agent off when individual mussels attain maturity and stop burrowing, or how they avoid being overgrown by living corals.
As the advent of scuba diving on the reefs themselves has opened our eyes to the nudibranch fauna of the GBR, drift diving in mid-water has just started to reveal some astonishing information about the behaviour of the holoplanktonic molluscs. For example, the very peculiar sea goddess Hydromyles globulosa, which is moderately common on the GBR, has its entire animal encased in a transparent and flexible cuticle. When threatened, an animal can retract completely into this cuticle and seal the slit-like opening with a fold in the cuticle, thus turning itself into a completely impervious sphere.
If one investigates the body of any mollusc closely, one is likely to find it harbours numerous parasites (e.g. ciliated protists, nematodes, trematodes, isopod crustaceans), both externally and internally. For example, on Heron Island, the common clusterwinkle Planaxis sulcatus (Planaxidae) acts as an intermediate host for a number of trematodes, including Austrobilharzia terrigalensis, which is responsible for 'swimmer's itch'.
Nudibranchs have come to symbolise all the molluscs on the GBR because of their fragile bodies, bizarre shapes, bright colours and remarkable behaviours. Nowadays no diver can have a holiday on the GBR without taking at least some digital images of nudi-branchs. Photography is certainly a form of 'collecting', albeit one that does not involve the removal of any live animals themselves, though damage is inadvertently caused to the reef by trampling, boat's anchors and incorrect weighting. In the past, shell collectors received much criticism by outsiders for their activities. One hears anecdotes of the 'devastation' caused by shell collectors and one reads emotive passages like: 'By their thoughtless depredation the whole productivity of that reef flat and the natural habitat of thousands upon thousands of its inhabitants can literally be ruined in a matter of hours' Bennett (1986: 126). But such hysterical statements have never been tested on the GBR. Humans have collected molluscs for ages, for food, trade and decoration, and detailed studies on population ecology have shown many species are highly resilient to collecting. Responsible shell collectors only take a few individuals and never remove immature specimens or females guarding eggs. Furthermore, they are aware of the ecological consequences of not turning over dead coral slabs. It is the shell collectors who have provided us scientists with the specimens for our comparative studies and museum reference collections. In recognising shell collectors as the least threatening of a host of (natural and unnatural) processes that could affect the GBR—human trampling, bottom trawling, commercial fishing, collection for consumption by Aboriginal and non-indigenous people, eutrophication from land-derived nutrients, cyclones and rising sea levels—the Great Barrier Reef Marine Park Authority currently allows recreational collection of molluscs on certain reefs on the GBR and along the adjacent coast without a permit. This collecting is limited to five individuals of a species (i.e. a total that comprises living animals and/or their shells, but not more than five of any species) to be taken in one day. For all other purposes, a permit is required from GBRMPA (see Chapter 12).
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