An overlooked giant

Giant clams are among the most conspicuous marine invertebrates on coral reefs due to their large size. Some species can grow up to 230 kg. Species of the genus Tridacna also show a brilliantly colored mantle that contains photosynthesizing symbionts (dinoflagellates of the genus Symbiodinium).

Many of those clams have been harvested for their meat and their shells They are also in demand on the ornamental aquarium market. All that led to widely depleted stocks. Consequently, most Tridacna species are listed by CITES and the IUCN Redlist. A crucial part of conservation management of those giant clams is the proper identification of different species and an estimation of the size of their breeding populations. Many of them are so called broadcast spawners which means eggs and sperm meet freely in the water column. This in turn means that a critical mass of individuals in an area is needed to ensure that eggs and sperm are likely to meet.

A new species of giant clam, thought to have been another well-known species (Tridacna maxima), has now been discovered on reefs at Ningaloo in Western Australia and the Solomon Islands. Individuals looked almost identical to the common species Tridacna maxima, but were actually genetically quite different. The Australian researchers used DNA Barcodes and other DNA regions to study gene flow of different Tridacna species and came across this cryptic species by chance.

This could turn out to be a very important discovery as chances are high that scientists have been overestimating the population of related species of clams. If this new species was mistaken and counted for Tridacna maxima the population size of that species would likely be estimated to be larger than it actually is. Given the conservation status of these majestic animals this is potentially bad news as population estimates need to be adjusted downwards.