Chondrichthyes (Sharks, Rays, Skates and Chimaeras)

Chondrichthyes are cartilaginous fish within the vertebrate lineage that are divided in two sub clusters: Elasmobranchs including sharks, rays and skates and Holocephali including the chimaeras. Chondrichthyes have survived and avoid mass extinction for over 400 million years, being one of the most successful and oldest group of vertebrates in terms of historical durability. This lasting success has largely depended on their diverse reproductive adaptations developed during their long evolutionary history. Why? Because the primary requirement for successful survival of any species is their ability to reproduce and give birth to fit newborns that will contribute to future generations. Understanding the process of reproduction requires knowledge of the chondrichthyan species’ reproductive adaptations where the most effective adaptation is one that produces as many fit progeny as necessary to ensure species survival in any giving aquatic environment. Chondrichthyans developed different reproductive adaptations, and although generalizations of these reproductive strategies are difficult as many species uncover unique adaptations, shared grounds can be distinguished. The reproductive strategies are expressed through a combination of a wide range of reproductive modes and reproductive cycles. These reproductive modes are classified based on embryo development sites and embryo nourishment. According to embryo development sites, two categories can be distinguished: oviparity, where females lay eggs that typically develop and hatch outside the maternal body; and viviparity, where embryo development occurs inside the maternal body and females give birth to fully developed newborns. According to embryo nourishment throughout its development, these two reproductive categories are further divided as lecithotrophy, where the nutrients are supported solely by a yolk-sac with no maternal input; and matrotrophy, where at least part of the embryo nourishment are supplied by maternal input of nutrients. The reproductive cycles denote the beginning to the end of each reproductive phase, combining: (1) the length of follicle development within the ovary that will be fertilized to form the embryos, (2) mating/fertilization, (3) the subsequent egg deposition/ pregnancy period, and (4) a resting period preceding a new follicle development cycle (not all species have resting periods). Finally, although out of the scope of this chapter, it is important to mention that the brain-pituitary-gonadal axis is a cascade system that triggers and regulates the entire reproductive process, promoting follicle production, ovulation, mating, fertilization, embryo development, and parturition.