Skin denticle diversity in the giant manta and the lesser devil ray: A three-dimensional analysis

Manta and devil rays (family Mobulidae, order Myliobatiformes) are among the most distinctive elasmobranchs. They are fully pelagic, ram filter-feeders, and possess unique cephalic lobes that funnel water into the mouth cavity. Despite their charismatic appearance, mobulid rays are also one of the most threatened elasmobranch families. Due to their protected status, large body size, and oceanic distribution, studying their ecology and taxonomy remains challenging. In other chondrichthyan species, the study of dermal denticles (tooth-like structures embedded in the skin) using museum specimens has proven valuable for understanding anatomy, species differentiation, ecological adaptations, and habitat associations. In this study, we investigate the diversity of dermal denticle morphology and size in two key mobulid species: Mobula birostris (the giant manta ray) and Mobula hypostoma (the lesser devil ray). Using micro-computed tomography on skin samples of museum specimens, we characterized denticle morphology, volume, surface area, and density across different body regions. Our results reveal pronounced inter- and intraspecific variations. Mobula birostris is entirely covered by large, stellate denticles that vary significantly across the body. Highly stellated denticles (with 8-15 projections) were observed on the pelvic fins and tail, which were considerably larger than those on the head (which had between 3 and 8 projections). Smaller denticles occurred on the cephalic lobes and gill regions. This diversity of morphotypes challenges the previous assumption that each manta species possesses a single diagnostic denticle morphotype. For instance, denticle morphologies previously considered characteristic of M. alfredi and M. yarae were identified in distinct body regions of the same M. birostris individuals. These findings indicate that denticle morphology in manta rays is regionally variable and cannot reliably be used alone for species identification. In contrast, Mobula hypostoma exhibits a single denticle morphotype: small, spiny denticles that are unevenly distributed, with several body areas completely devoid of denticles. This suggests that specific denticle features in mobulids are shaped by diverse evolutionary pressures and functional demands. Overall, this study provides the first detailed characterization of denticles across body regions of two mobulid species, highlighting their potential ecological significance and cautioning against the use of denticle morphology alone as a taxonomic character in manta rays.