Embryonic behavior and skeletogenesis in developing skate Okamejei kenojei

Cartilaginous fishes (sharks, skates and chimaeras) exhibit diverse behavioral patterns and unique endoskeleton, which provide insights into their ecological adaptations and evolution. However, research on the development of cartilaginous fish is still limited. To evaluate the relationship between embryonic behavior and cartilage development in cartilaginous fishes, the developing Okamejei kenojei was analyzed through behavioral, anatomical, and histological approaches, with an atlas of embryonic behavior and skeletal morphology. The result shows that the behavior of skate embryos evolves from early rhythmic movements to vigilance behavior to external stimuli. Data from Alcian blue and Alizarin red staining and histology sections showed that the vertebrae are the earliest regions to mineralize, with the mineralization process starting at the neural arch area and expanding along the body axis. In the anterior area, mineralized structures spread along the synarcual and neurocranium towards the pectoral fins and fin rays. Interestingly, a novel branching pattern of fin rays was observed in the pectoral fins of embryonic O. kenojei, characterized by the inward growth of the perichondrium into the cartilage element, potentially linked to the morphogenesis of the skate's pectoral fins. Additionally, this study provides a set of open-source morphological data for O. kenojei, which will serve as a valuable reference for marine animal conservation and evolutionary developmental biology.