Same-same but different: habitat use and partitioning of three cryptobenthic coral reef fishes of the genus Trimma

Abstract Partitioning critical resources can alleviate competition among species, allow coexistence, and contribute to biodiversity. The mechanisms driving habitat partitioning within the small-bodied and highly diverse genus Trimma remain unclear. The main aim of this study was to uncover habitat use and partitioning of T. benjamini , T. capostriatum , and T. yanoi that may enable their coexistence on coral reefs in Kimbe Bay, Papua New Guinea, using a combination of active visual search transects and behavioural observations. The spatial distributions of each species were distinct. Reef aspect played a significant role, with T. yanoi and T. benjamini being most abundant on the reef wall, while T. capostriatum predominantly favoured the reef slope at offshore locations. Generally, there were variations in abundance among depths in all species, though there were exceptions for specific reef aspects. Reef location (inshore vs. offshore) influenced the distribution of T. capostriatum , but not T. benjamini or T. yanoi . Population clustering was also examined. T. yanoi formed the largest clusters of 9 individuals on average, followed by T. benjamini with 7 individuals on average. T. yanoi clusters had 1.5 times the population density per m 2 quadrat compared to T. benjamini . In contrast, T. capostriatum exhibited the smallest cluster sizes and densities, with the highest proportion (30%) of solitary individuals. At small spatial scales, coexistence was observed in 34.5–52% of m 2 quadrats. Within 1 m 2 quadrats where species coexisted, there were differences in the microhabitat that each species commonly occupied, resulting in minimal overlap of microhabitats (9.2–8.2%). Behavioural assessments revealed that interactions between Trimma species were rare, involving less than 1% of individuals. These findings exemplify how small reef fish can effectively partition habitats even when closely related, with almost identical body size and morphology.