Prying open the life of a giant clam

T. squamosa, one of the five giant clams native to Singapore

Giant clams’ iridescent colours make for pretty photos but they are more than just eye candy. They are valuable contributors to a reef’s ecosystem, whether in the form of food, reef building material, shelter or as a filtration system, according to NUS researchers. These findings provide giant clam conservationists with the ammunition they need to fight for the animals’ preservation.

NUS’ Tropical Marine Science Institute Research Fellow Dr Neo Mei Lin and her co-authors from the Department of Biological Sciences spent five years combing through more than 400 journal articles and conducting their own studies to quantify how these bivalves support the ecosystems around them. Their research was published in Biological Conservation in January 2015.

These charismatic creatures play many important roles in the marine ecology. They are a significant source of food due to their sizable soft tissue masses, while their discharge of photosynthetic algae, gametes and faeces are eaten by opportunistic feeders.

They also offer shelter to reef fish and act as hosts to other living organisms. These megafauna sustain marine habitats by filtering out excessive nutrients that could otherwise cause algal blooms, which can be harmful to sea life. For example, a population of more than 430 Tridacna gigas (T. gigas) at the Great Barrier Reef in Australia can filter about 28,000 litres of water per hour.

The clams continue to be useful even when they are dead, as their massive shells—which are made of a form of calcium carbonate—contribute towards the build-up of coral reefs.

Dr Neo (right) and Asst Prof Todd are working to restore the local giant clam population

In a related study, Biological Sciences Assistant Professor Peter Todd and his former student Ms Pamela Soo made some interesting observations about the giant clams’ behaviour including their surprising mobility. In research published recently in Marine Biology, they showed how fast giant clams crawl towards each other and highlight how clumping together may lower individual risk of predation, increase the probability of successful reproduction and reduce the effect of wave energy on individual clams. Knowing this, people restocking clams might allow the creatures to congregate, as they are inclined to do, instead of dispersing them.

Only three of the five giant bivalves native to Singapore have been sighted in the Republic’s waters. Giant clams, in general, populate coral reefs in the Indo-Pacific, from Southeast Africa to Australia and Japan.

Dr Neo has found 56 individuals of Tridacna squamosa (T. squamosa), 47 Tridacna crocea and one Tridacna maxima at 29 reef sites around Singapore, including those at Sisters’ Islands Marine Park, Pulau Semakau and Raffles Lighthouse. Two species indigenous to Singapore—T. gigas and Hippopus hippopus—are already extinct locally.

The local giant clam population is highly threatened and current numbers are unable to support reproduction, said the researchers. This is because the clam population needs to meet a certain threshold for successful reproduction, explained Asst Prof Todd.

“The densities in Singapore are too low to be self-sustaining. Basically, they’ll die off in time if we don’t do anything,” he said.

Over the last few years, NUS’ giant clam hatchery at St John’s Island has been nurturing three cohorts of T. squamosa and more than 50 sub-adult T. gigas individuals, which the researchers intend to use to restock local reefs with.