Depth influences coral-dwelling faunal symbiont communities in the Caribbean, independently of colony size

The characteristics of stony corals and their environmental conditions play a critical role in shaping the abundance and composition of their associated faunal symbionts. Studies have consistently shown a positive correlation between symbiont community assemblages, number of symbionts and the size of branching coral colonies, as well as habitat depth. However, symbiont assemblages associated with non-branching corals, such as plating or massive species, remain underexplored. To address this gap, we conducted visual surveys of coral-dwelling faunal symbionts on three non-branching coral species—Agaricia agaricites, Porites astreoides, and Siderastrea siderea—at two depths (6 and 15 m) along the leeward coast of Curaçao. Symbiont species and abundance were recorded for 585 coral colonies, yielding a total of 4,969 records representing over 15 species. Observed symbiont species included barnacles, polychaete worms, gall crabs, boring bivalves, and blennies. We examined the relationship between overall symbiont abundance (irrespective of species), colony size, and depth, as well as the relative proportion of colonies hosting varying numbers of symbiont taxa. For all three coral species, the number of individual symbionts was uncorrelated with colony size. Depth significantly influenced symbiont abundance in the massive corals P. astreoides and S. siderea, but not in A. agaricites. Across all species, most coral colonies hosted one or two symbiont taxa, and this reduced symbiont composition became more prevalent at greater depths. These findings underscore the complexity of coral-symbiont assemblages and highlight the important role of environmental factors, such as depth, in structuring these communities.