Comparative Morphology of Egg Cases in Three Elasmobranch Species and the Secretion Process in the Kong Skate <i>Okamejei kenojei</i>

Egg cases in oviparous cartilaginous fishes (sharks, rays, and chimaeras) exhibit diverse morphologies that are closely tied to species-specific reproductive adaptations. However, the diversity and formation mechanisms of these structures remain poorly understood. In this study, we performed a quantitative morphological analysis of egg cases from three species: Okamejei kenojei, Cephaloscyllium sarawakense, and Chiloscyllium plagiosum. The results demonstrated that the egg cases of these species could be distinguished using multiple morphological indices (p < 0.05), supporting species-specificity in egg case morphology. In these species, we observed that egg jelly initially envelops the egg case during early embryonic development and later dissolves, allowing seawater entry-suggesting a conserved reproductive strategy within Elasmobranchii. Furthermore, under artificial breeding conditions, observations of female O. kenojei showed that ovulation occurs before egg case secretion. Specifically, eggs reach the oviduct above the oviducal gland when about half of the egg case has formed. Immunohistochemical staining revealed estrogen and progesterone receptors in the oviductal gland cells. Interestingly, O. kenojei can produce malformed eggs with shark egg case-like features under captive breeding conditions. These findings provide new insights into the species-specificity, timing, and hormonal regulation of egg case formation in cartilaginous fishes, and lay a foundation for future research on their reproductive strategies.