The frilled shark, notable for its long gestation period of up to three years, is a viviparous species, showcasing a unique mode of live-bearing that is prevalent among cartilaginous fishes such as sharks, skates, and rays. Viviparity is a reproductive method seen across various vertebrates with over 150 occurrences, yet mechanisms in cartilaginous fishes remain largely unexplored due to their elusive nature. Researchers led by Shigehiro Kuraku aimed to bridge this knowledge gap, focusing on egg yolk proteins that persist in viviparous sharks and rays but were lost in mammals during the transition to live birth. Their study highlighted that while all cartilaginous fish possess copies of vitellogenin (VTG), which plays a critical role in egg-laying, those in viviparous species also express VTG in the uterus, indicating a potential dual functionality in nutrient provision to embryos. Specifically, the frilled shark, which provides no maternal nutrients, contrasts with the spotless smooth-hound that utilizes a placenta. This ongoing research, supported by the Squalomix consortium, seeks to further understand reproductive adaptations in these fascinating marine species despite challenges in obtaining biological samples.
