Recruitment of hard coral communities on giant clam shells (Cardiidae: Tridacna) differ from surrounding reef habitats at a tourist destination in the Gulf of Thailand

Giant clams are organisms of significant ecological importance on coral reefs. They are known to help maintain oligotrophic conditions on reefs, to act as potential reservoirs of symbiotic dinoflagellates and as a consequence of their large, dense shells, to contribute to reef structural complexity. Despite this, little work has been done to investigate the utility of giant clam shells for coral recruitment. Here, we quantify scleractinian coral growth on living and dead giant clams around the Thai island of Koh Tao, a site with a history of environmental and anthropogenic pressure related to tourism. Transect surveys at 15 reefs around the island yielded 739 clams, of which 120 were found to support coral recruits. A total of 270 distinct corals were recorded between depths of 2.1 and 8 m (none were found at depths shallower than 2 m), with 29% found on dead shells and 71% on living clams. Differences were found in the community structure of hard corals found on clam shells vs those found in surrounding reef environments, with mushroom corals (Fungiidae) and Stylocoeniella being significantly more abundant on clam shells than on the natural reef. In contrast, corals abundant on surrounding reefs (e.g. Acropora and Porites) were disproportionately better represented on the reef than on clam shells. This suggests that giant clam shells may play a significant role in the succession of coral reefs with certain corals being preferentially supported over others.