Comparative analyses of sexual size dimorphism (SSD) across species have led to the discovery of Rensch’s rule. This rule states that SSD increases with body size when males are the largest sex, but decreases with increasing size when females are larger. Within-species comparisons of SSD in fish are rare, yet these may be a valuable tool to investigate evolutionary patterns on a fine scale. This study compares SSD among closely related populations of three species of Mediterranean blennies (Blenniidae): Microlipophrys canevae (Vinciguerra, 1880), Parablennius incognitus (Bath 1968), and Aidablennius sphynx (Valenciennes, 1836). SSD varied more among populations than among species and Rensch’s rule was confirmed within two species. It is not likely that the variation among populations in SSD mirrors genetic variation, as many of the populations were in close proximity of one another, with a high potential for genetic exchange. This study complements larger scale analyses of other taxa and demonstrates the fine scale on which evolutionary processes responsible for Rensch’s rule may be operating.

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.