Unveiling gynandromorphy in the freshwater crab Vela carli (Roux, 1931) (Decapoda: Brachyura: Gecarcinucidae): the first reported case in the family

Abstract Many brachyuran crab species exhibit sexual dimorphism, a defining characteristic marked by morphological differences between males and females, particularly in reproductive structures such as the gonopores, pleon and pleopods. While gynandromorphy — individuals simultaneously exhibiting male and female traits — has been documented in several marine and freshwater crab families, it remains unreported in the family Gecarcinucidae. This study documents the first instance of gynandromorphy in Vela carli (Roux, 1931), a narrowly endemic gecarcinucid species of the Central Western Ghats, India. Crabs of the species were collected from the Silent Valley National Park, Kerala State. Morphological analyses focused on pleon shape, pleopod structure, and presence of female gonopores (vulvae) to characterise sexual dimorphism and gynandromorphic traits. Three subadult gynandromorphs exhibited a unique mosaic of male and female traits. Their pleopod 1 was consistently male-like (uniramous, modified as first gonopod), while the pleopod 2 varied from absent to rudimentary female-like endopods or fully developed second gonopods. Their pleopod 3 was modified as a uniramous gonopod typical of the second gonopod in the largest gynandromorph, whereas pleopods 4 and 5 were absent in all individuals, indicating an incomplete male pleopod set with female-like morphology on pleonal somite 3. Female gonopores were present in all three gynandromorphs but reduced in size relative to adult females. These features suggest early developmental mosaicism as the likely cause as the crab’s habitat was probably free from environmental pollutants. Additionally, adult female sexual characters of V. carli were described in detail for the first time. This discovery fills a critical gap in the understanding of crustacean sexual dimorphism and developmental anomalies, particularly within Gecarcinucidae. The observed phenotypic mosaics highlight complex sexual differentiation mechanisms in freshwater crabs. Given the species’ narrow endemic range and protected habitat, further research on reproductive biology and the ecological consequences of gynandromorphism is essential for conservation.