Ecology and Evolution of Giant Clams
A number of molecular genetic studies in the last decade have shown that giant clam biodiversity is much higher than previously known and a number of species have been re-discovered or newly described, providing valuable information for conservation. The centre of giant clam biodiversity, and for marine life in general, is the Coral Triangle (CT). The three main biogeographic models trying to explain the evolutionary mechanisms that produce this megadiversity in the CT are: 1) centre of speciation, 2) centre of overlap, and 3) centre of accumulation. The first model suggests that the CT is exporting species to the peripheral areas of the Indo Pacific. The second model assumes that distinct faunas from the Indian and Pacific Oceans overlap in the CT. During sea-level low stands of up to 130 m during the Pliocene and Pleistocene glacials the exposed shelves in Southeast Asia and Australia formed the Indo-Pacific Barrier (IPB), separating populations. The third model suggests that species evolved in the periphery and have been transported towards the CT by favourable large-scale current patterns. Fine scale genetic differentiation among giant clam populations in the CT suggests a centre of speciation, while strong genetic differentiation across the IPB supports the centre of overlap model. Additionally, endemics and divergent genetic lineages of widespread species in peripheral areas, such as the Red Sea and Western Indian Ocean, point towards a centre of accumulation. It is obvious that not only one biogeographic model can explain the megadiversity in the hotspot of the CT, all three mechanisms are operating in concert. This is summarised in the new biodiversity feedback model proposed by Bowen et al. (2013), in which the biodiversity centre exports species to peripheral regions, where these eventually evolve into new species due to isolation, which in turn can be exported back to the biodiversity centre.