Habitat-Driven Variation in Sexual Dimorphism of Amphipods

Sexual dimorphism in morphological traits is widespread across animals and can result from differing life-history strategies, sex-specific competition, and ecological interactions influenced by habitat structure. For epifaunal organisms such as amphipods, habitat complexity mediates access to food, mate encounters, and refuge. This study investigates sex-related variation in body size and gnathopod 2 ratio (gnathopod 2 length/body length) in two amphipod species, Ampithoe ramondi and Caprella acanthifera, across four benthic habitats: rhodolith beds, macroalgae-dominated reefs, seagrass meadows, and black coral forests. A. ramondi occurred in all habitats except black coral forests, with males larger than females only in macroalgae-dominated reefs and exhibiting higher gnathopod ratios, increasing across macroalgae-dominated reefs, seagrass meadows and rhodolith beds. C. acanthifera was found in macroalgae-dominated reefs and black coral forests, with males larger on average but no significant habitat-related variation in dimorphism. These results indicate that sexual dimorphism patterns are species-specific, shaped by habitat-specific ecological pressures and life-history strategies. Expanding such analyses to more taxa and with balanced sampling across habitats and environmental gradients will offer deeper insight into how natural and sexual selection interact and inform how these dynamics may shift under changing climate regimes.