Using photogrammetry to examine how coral host size and architecture relate to Gobiodon body morphology and group size

Abstract The size and structural architecture of a microhabitat can influence a wide range of an organism’s biological and ecological traits. In the strongly microhabitat-associated, coral-dwelling genus Gobiodon , body morphology appears as a unique adaptation to their obligate Acropora spp. coral hosts. These fishes often form social groups within a single coral colony, which they can inhabit throughout their adult lives. Using photogrammetric reconstructions of 72 coral colonies, in situ coral health estimates, body morphology measurements and counts of resident gobies, we tested whether the size and structure of Acropora hosts were associated with variation in body morphologies and social group size in three poorly studied Gobiodon species: G. axillaris, G. citrinus , and G. sp. We found strong alignment between coral host structure and the body morphologies in all three species. The larger, more openly branched corals occupied by G. citrinus aligned with its larger and broader body form. In contrast, the smaller-bodied congeners occupied smaller, more rugose and densely branched colonies, with G. axillaris exhibiting significantly greater elongation and lateral compression while occupying densely packed corals. We also found that group size increased with coral surface area in all three species. We found no relationship between coral health and fish group size in any of the focal species. These findings highlight the close relationships between obligate coral-dwelling fishes and their threatened host habitats, demonstrating species-specific associations between coral structure, fish morphology, and social organisation that appear to be facilitated by the evolution of body size and shape.