Sex-specific morphological and structural adaptations of the vent crab Xenograpsus testudinatus in a shallow water hydrothermal ecosystem

Abstract

We investigated sex-specific morphometric, structural, and elemental traits of the shallow vent crab Xenograpsus testudinatus from the active shallow-water hydrothermal vents off Kueishan Island, Taiwan, addressing gaps in our understanding of how sex-specific adaptations manifest in extreme vent environments. From 583 collected specimens, 100 adult males and females were analyzed to compare exoskeletal morphology, morphometric indices, and elemental composition. Fifteen morphometric traits revealed pronounced sexual dimorphism, supported by discriminant, regression, and correlation analyses. Carapace width/length-weight relationships exhibited significant allometry pattern (P <= 0.00; except ABL P <= 0.81) that differed between sexes. Scanning electron microscopy (SEM) revealed that distinct sex-specific external and internal characteristics of the carapace, merus, propodus, and pereiopods, while transmission electron microscopy (TEM) showed unidirectional calcite-chitin organization in males versus multidirectional crystalline structures in females. Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) further indicated sex-dependent elemental differentiation, with males exhibiting higher elemental concentrations. Collectively, these findings provide novel insights into the morphological, structural, and elemental bases of sexual dimorphism in X. testudinatus, revealing functional divergence and sex-specific adaptive strategies in shallow hydrothermal vent habitats advancing understanding beyond prior studies, which focused primarily on population and ecological patterns without resolving underlying structural and elemental differences.