Stable isotope analysis of the Hypoplectrus species complex reveals no evidence for dietary niche divergence

The polymorphic coral reef fish genus Hypoplectrus (hamlets) provides an excellent system for examining the initial stages of natural biological divergence in the expansive marine environment. Despite close genetic similarities, hamlets occur in assortatively mating colour morphotypes. In this study, we determined whether ecological differences exist between morphs that could reinforce the assortative mating pattern within morphs. We compared the carbon and nitrogen stable isotope dietary signatures of 6 hamlet morphotypes from 5 geographically distant locations. Across 364 individuals, and with the exception of fish sampled in the Gulf of Mexico, no significant isotopic associations with morphotype were detected. Our results therefore provide no evidence that different hamlet morphs are associated with distinct dietary niches, despite finding highly significant geographical differentiation for both isotopes. We argue that tight assortative mating without ecological divergence could be maintained through the demands of reciprocal cooperation within a reproductive pattern of simultaneous hermaphroditism which characterises all hamlets.