By: T.V. Padma
It’s time they came out of their shells. It seems the world’s largest molluscs, the giant clams of the Indo-Pacific coral reefs, have been doing a huge amount of good work we knew little about.
These sea creatures turn out to be multitasking ecosystem engineers. They are reef builders and shapers, food factories, shelters, reservoirs of algae, and water filters, all rolled in one.
Giant clams have been around coral reefs since about 38 million years, the largest of them growing to 1.2 metres long and weighing more than 200 kilograms.
But, their role in the ecosystem is poorly understood, says Peter Todd, a marine ecologist at the National University of Singapore.
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Todd and his colleagues have examined the clams’ roles, and hope the findings will reinforce the case for conserving the molluscs. Giant clams are under great pressure from threats such as overfishing and global warming.
The team found that the 13 species of giant clams are food factories for coral reef inhabitants. They host food-making algae known as zooxanthellae, serve as food for predatory crabs, lobsters, and even their spawn and faeces attract opportunistic feeders and scavengers such as small snails, crabs and lobsters.
A place to hide
Giant clams are also nurseries for fish, serving as refuges for juveniles escaping predators, and the shell ridges provide privacy for adults laying eggs.
Their shells also help build reefs. Dense populations of clams mean that some species produce 80 tonnes of carbonate shell material per hectare each year, which is available as housing for soft corals, sponges, sea squirts and large algae.
But these benefits are likely to continue only if giant clam populations are healthy, making their conservation paramount, the team concludes.
“There is a pan-global decline in clams,” says Deepak Apte, a marine ecologist at the Bombay Natural History Society, who is running an Indian government-backed programme on clam conservation.
“They are a vital indicator species of coral reef health and their ecological contributions are innumerable,” he says. Once gone, restoring giant clams will be a great challenge because the highly specialised niches in which they occur are continuously deteriorating, he adds.
And in India, he says, there are other challenges.
“Our emphasis on megafauna in India makes looking at lesser known species almost impossible,” he says. “Even within megafauna, Indian conservation approach is almost entirely terrestrial and is too tiger-centric. The study puts forward the case for conservation of these lesser-known species.”
Journal reference: Biological Conservation, DOI: 10.1016/j.biocon.2014.11.004