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Giant clam's true identity revealed

giant clam

The clam has an opening of about 40 centimetres (Source: Eric Treml)

Clammy discovery A colourful new species of giant clam, thought to have been another well-known species, has been discovered on reefs in Western Australia and the Solomon Islands.

The discovery could mean that scientists have been overestimating the population of related species of clams, says researcher Jude Keyse, who helped find the clam as part of her PhD at the University of Queensland.

"If we're missing giant clams like this, what other marine organisms are we missing," says Keyse.

Giant clams of the genus Tridacna grow up to 230 kilograms and are very noticeable on the coral reefs they inhabit because of their colourful photosynthetic mantle.

In their adult phase, these filter-feeding bivalves don't move around but their genes can still flow from one population to another via drifting eggs, sperm or larvae.

While studying the gene flow of different Tridacna species in the eastern Indian and western Pacific Oeans, Keyse and colleagues noticed something strange.

On reefs at Ningaloo in Western Australia, and Solomon Islands, a specimen that looked almost identical to the relatively common Tridacna maxima species, was actually genetically quite different.

"It was only when we looked at the DNA that we discovered it was very different to anything that we had collected," says Keyse.

The findings are published in a recent issue of PLOS ONE.

Keyse says when she and colleagues looked at the CO1 gene - or so-called "barcoding gene" commonly used to differentiate species - they saw the new clam differed genetically from other known species of Tridacna, as much as those other species differed from each other.

Species identification

Most Tridacna species are protected by international conservation agencies because they have been extensively harvested and depleted.

They clams have traditionally be used to make tools, containers and ornaments, and many have been harvested for meat, shells or the ornamental aquarium trade.

Keyse says the clam's adductor muscle "would be just like eating a massive scallop" - not that she herself has ever tried it.

A crucial step in conserving giant clams is the proper identification of different species and an estimation of the size of their breeding population, says Keyse.

"This particular clam is a broadcast spawner which means the eggs and sperm meet out in the water column," she says.

This means there really needs to be a critical mass of individuals in a particular area to make sure eggs and sperm are likely to meet, says Keyse.

She says if the new species was mistaken and counted for Tridacna maxima the population size of that species would be estimated to be larger than it actually is.

"So you might think they're doing okay but really you've got to cut that number possibly in half."

The next step for the team is to formally describe and name the new species of clams.

Tags: conservation, marine-biology