Metabolic rates mirror morphological and behavioral differences in two sand-dwelling coral reef gobies

Morphology, physiology, and behavior are the primary axes of organismal diversification, and phenotypic differentiation among species in any of these dimensions can be indicative of where, when, and why species can occur and coexist. However, nuanced adaptations in superficially similar species can be difficult to pinpoint, especially for mobile animals in highly diverse ecosystems, such as coral reef fishes. In this context, morphology and behavior are often investigated to assess potential differences, but interspecific variation in physiological traits is not frequently considered. Here, we use field surveys, morphometrics, behavioral observations, gut content DNA metabarcoding, and metabolic rate trials to investigate phenotypic differentiation in 2 small sympatric species of sand-dwelling cryptobenthic coral reef fish. We show that the gobies Fusigobius neophytus and Gnatholepis cauerensis co-occur in sandy habitats throughout a coral reef lagoon in French Polynesia. While superficially similar, the 2 species differ in the length of their gastrointestinal tracts, their ingested prey, and their foraging rates. These differences are also reflected in the species' standard and maximum metabolic rates: G. cauerensis has a longer intestinal tract and ingests more animal prey, has a more active foraging style, and displays markedly higher standard and maximum metabolic rates than F. neophytus. These results indicate clear differences in the functional niches of the 2 species, which are detectable across these 3 major organismal axes. Given the well-documented direct links between metabolic rate and organismal energy expenditure, we suggest that physiological traits may offer a useful additional dimension in the assessment of sympatric species and their functional roles.