Ecomorphology of the rectal gland of three batoids (Elasmobranchii: Myliobatiformes)

Abstract

The rectal gland (RG) is a specialized organ that performs a major role in salt excretion of marine elasmobranch species. We aimed to describe the rectal gland ecomorphology in three stingray species (Hypanus marianae, Hypanus guttatus, and Aetobatus narinari) from benthic and pelagic habitats. The gland is lobulated and has two distinct forms: small compact S-shaped for H. marianae (mean width: RG length ± SD = 1.06 ± 0.23 cm), elongated S-shaped for H. guttatus (2.15 ± 0.06 cm), and pod-shaped for A. narinari (3.14 ± 0.57 cm). Rectal gland lengths were different between species (Kruskal–Wallis; X2 = 39.27; df = 2; P < 0.05). The capsule's dense connective tissue was three times thicker in A. narinari than in the Hypanus species (K–W; df = 2, p = 0.014). The secretory parenchyma of H. marianae was simpler, differing from other species that showed a well-defined lobule arrangement around the central duct. Indeed, the central duct of A. narinari and H. guttatus exhibited branches that invaginated into lobes and originated the lobular ducts, while the central duct lacks branches and opened directly into the secretory tubule in H. marianae. Furthermore, central duct cells in the stratified epithelium of A. narinari showed a higher number (average 7.4 ± 1.51/cells) and cell layer thickness (0.09 mm) than the other species analyzed (ANOVA; F = 6.90, p = 0.01). The morphological differences between the rectal gland of Myliobatiformes analyzed showed that their phylogenetic history could be a better explanation to the changes between basal (H. marianae) and derived (H. guttatus and A. narinari) species.