Ontogenetic allometry of the first gonopod in the land crab Johngarthia lagostoma (H. Milne Edwards, 1837) (Crustacea: Brachyura: Gecarcinidae): delayed development of the corneous unguis suggests morphofunctional specialization

In Brachyura, reproduction involves a range of behavioral, morphological, and physiological mechanisms that contribute to reproductive success. The land crab Johngarthia lagostoma , endemic to four oceanic islands in the South Atlantic, shows no clear pre- or post-copulatory guarding behavior, and the mechanisms involved in sexual selection remain uncertain. This study investigated the ontogenetic development of the first gonopod using relative growth analyses and high-resolution imaging techniques, with special emphasis on the corneous unguis, a distal non-calcified structure of unknown function. Morphometric analyses were performed using carapace width as the independent variable and measurements from different gonopod regions, including the corneous unguis, as dependent variables. Relative growth was evaluated using power function, and growth shifts were estimated from the intersection between juvenile and adult growth phases. Nano-computed tomography and X-ray analyses were used to describe the overall morphology of the gonopod and provided detailed visualization of its distal region, including the horny claw and associated setae field. Most biometric relationships showed growth shifts at similar sizes (50–55 mm CW ). In contrast, the corneous unguis showed a delayed ontogenetic transition (64.9 mm CW ), indicating a developmental trajectory distinct from that of the other gonopod regions. The coordinated maturation of most gonopod regions, together with the delayed development of the corneous unguis, indicates differential developmental timing among components of the copulatory apparatus. Although the functional role of the corneous unguis remains uncertain, its distinct ontogenetic pattern supports the hypothesis that it may be involved in post-copulatory reproductive processes associated with sperm transfer and/or ejaculate processing.