Habitat morphology constrains the depth distribution and growth rate of a coral-associated reef fish

Abstract

The distribution of coral reef fishes is often closely linked to the availability of preferred habitats. However, even in specialized gobies associated with a single coral species, their distribution does not always match that of the coral host. The coral goby Paragobiodon xanthosoma is primarily restricted to shallow water ( 30 m. Here, we use observational and experimental approaches to test whether depth-associated changes in morphology and partial mortality of the coral decrease habitat quality. Quantitative surveys from the surface to 30 m found pronounced changes in colony morphology over depth. S. hystrix colonies most likely to be occupied by P. xanthosoma were those with shallow-water characteristics, especially lower partial mortality and a more complex branching structure. In a binary choice experiment, P. xanthosoma individuals preferentially selected colonies from shallow water (5 m) over deep water (30 m) and with high complexity over low complexity when colonies were collected from a standardized depth (15 m). Finally, a transplant experiment examined the effect of depth-associated changes in the coral habitat on patterns of growth of P. xanthosoma. When the depth of placement was standardized, the growth of P. xantho soma was 8-fold lower on colonies collected from deep water (30 m) compared to colonies from shallower water (5 and 15 m). Together, these patterns suggest that depth-related changes in the morphology of the coral host are responsible for driving patterns of distribution over depth in their associated reef fish.