Trophic niche partitioning in giant clams.

Ecosystems are influenced by competition for limited resources, a driver of niche partitioning. Over time, the emergence of novel traits facilitating new resource exploitation can reduce competition. However, additional layers of complexity, like symbiosis, complicate our understanding of the patterns shaping reef communities. Therefore, empirical evidence of niche partitioning reducing competition in symbiotic benthic communities is limited. Using a unique common garden experiment, we examined the nutritional strategies of six giant clam holobionts and characterized their symbiont assemblages. Variation in trophic strategies confirmed trophic niche partitioning, as species fell along a continuum from highly heterotrophic to highly autotrophic. Tridacna gigas and T. derasa, listed as critically endangered and endangered, respectively, were the most autotrophic and fast-growing species. We found significant phylogenetic signals in trophic niche scores, growth rate, and shell length, indicating the role of natural selection in shaping giant clam nutritional ecology. We conclude that niche partitioning is a driver of giant clam evolution with benefits and costs; high autotrophy reliance results in greater growth rates yet may increase vulnerability to disturbances. Given the impact of human activities on giant clams, conservation efforts should focus on these ecosystem engineers, especially highly autotrophic and geographically constrained species.