Tuesday, 18 December 2012

Unhappy as Clams

One of the smaller tanks in the aquarium has a new exhibit – a small member of one of the largest bivalves on earth, the Southern Giant Clam Tridacna derasa. With a maximum size of 60cm, it exceeded only by T.gigas, which can reach nearly 1m across. While they are of course exceeded in mass by several species of squid, for a mollusc that is still a pretty respectable size, and the weight of the heavy shell increases their bulk. This giant size is all the odder when it turns out they are actually most closely related to the standard small cockles, Cerastoderma, that can be found around the shores of the UK. This is not due to an especially long life, but rather rapid growth rates – they can be 30cm across within 10 years in some species.

Tridacna clams are currently classed in two subgenera with eight species between them, but this may well change. Many bivalves are now known to conceal numerous cryptic species, diagnosable only with DNA analysis, and some work has indicated that the giant clams may follow this pattern, which might help to explain why the various forms introduced into the marine aquarium trade are so variable in appearance and differ so in response to aquarium conditions. Another feature of Tridacna clams is the fairly short larval period – usually around 10-14 days after fertilization the embryonic clam will settle down on a reef. This gives only a short window for dispersal over any distance, and means that most of the clams on a given reef will be descended from parents on the same reef.

Tridacna derasa is a widespread ‘species’ with a distribution centred on Indonesia and surrounding waters, where it lives on the outer edges of reefs at up to 10m depth. It cannot live at greater depths owing to the symbiotic algae in its tissues, which require sunlight to photosynthesize. Unlike in corals, where the zoxanthellae are contained within the cells of the coral animal, in clams there is a special system of tubules connected to the digestive system where the algae are housed. Infant clams acquire their symbionts through the normal process of filter feeding, probably from nearby adults after they settle. The algae themselves however do not provide the vivid colours of the mantle of the clam – these come from special iridophores in the clams tissues.
Clams and other bivalves are often thought of as eyeless, but in fact they have hundreds of eyes bordering the mantle. This enables them to respond to the shadows of approaching predators such as fish or crustaceans, at which point they can close their shells and seal themselves behind their defences.

Unfortunately for the clam, this does not do any good when the predator is a human being looking for an easy meal, and over-exploitation has severely impacted many populations. It has now been discovered how to culture them artificially in the same way as temperate zone shellfish like oysters and mussels are, and this has enabled restocking of areas where they have been eliminated. This may cause some problems down the line though, as the cryptic species effect may mean that animals adapted to quite different conditions and local environments are being distributed well outside their natural range, where they could potentially hybridise with native stock.
Human overexplotation of giant clams has a very long history. Only recently discovered, the Red Sea T.costata currently forms only around 1% of living giant clams in the area where it is found, but constitutes over 80% of fossil shells. The collapse seems to have begun as long ago as 125,000 years ago, which is suspiciously close to the appearance of modern humans in the region. It most likely suffered as a result of its special habitat preferences – it lives only on the top of the reef where it would be easy to locate by people wading in shallow water.

Next week, part one of a round up of the year

(images from wikipedia)

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