Evolution, diversity, distribution and the endangered future of the giant clam-Symbiodiniaceae association

Giant clams are found in a mutualistic association with Symbiodiniaceae dinoflagellates. While clams are economically important for fisheries, Symbiodiniaceae are responsible for most of the primary production in coral reefs. This review addresses key issues regarding the giant clam-Symbiodiniaceae holobiont: diversity and distribution; functional traits; evolution and coevolution; and consequences of climate change. Findings show that holobiont distribution is partitioned by host species and geography. So far, giant clams have been reported in association with only 30 Symbiodiniaceae phylotypes, all within genera Symbiodinium, Cladocopium and Durusdinium. Species and functional diversities are highest in the Coral Triangle. The association evolved in the Eocene-Oligocene transition; there is evidence for coevolution, including host organs developed for housing symbionts, change in host habitat and multiple symbiont phylotypes found exclusively in giant clams. Clam bleaching events have been recorded throughout the Indo-Pacific. These clams may be particularly vulnerable, as they rarely associate with stress-tolerant symbionts and are heavily targeted by fishing. The Great Barrier Reef is particularly at risk, as clams in that area do not associate with thermally tolerant, low-irradiance-adapted and opportunistic symbiont phylotypes. Further reduction in giant clam populations may serve as an important indicator of the increasing decline of coral reef biodiversity.