Chondrichthyes (Sharks, Rays, Skates and Chimaeras).

AWRUCH C. A.

Encyclopedia of Reproduction

Elsevier

Año: 2018;

Chondrichthyes are cartilaginous fish within the vertebrate lineage thatare divided in two sub clusters: Elasmobranchs including sharks, rays andskates and Holocephali including the chimaeras. Chondrichthyes have survivedand avoid mass extinction for over 400 million years, being one of the mostsuccessful and oldest group of vertebrates in terms of historical durability. Thislasting success has largely depended on their diverse reproductive adaptationsdeveloped during their long evolutionary history. Why? Because the primaryrequirement for successful survival of any species is their ability toreproduce and give birth to fit newborns that will contribute to futuregenerations. Understanding the process of reproduction requires knowledge ofthe chondrichthyan species? reproductive adaptations where the most effective adaptationis one that produces as many fit progeny as necessary to ensure speciessurvival in any giving aquatic environment. Chondrichthyans developed differentreproductive adaptations, and although generalizations of these reproductivestrategies are difficult as many species uncover unique adaptations, sharedgrounds can be distinguished. The reproductive strategies are expressed througha combination of a wide range of reproductive modes and reproductive cycles. Thesereproductive 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 thematernal body; and viviparity, where embryo development occurs inside thematernal body and females give birth to fully developed newborns. According toembryo nourishment throughout its development, these two reproductive categoriesare further divided as lecithotrophy, where the nutrients are supported solelyby a yolk-sac with no maternal input; and matrotrophy, where at least part ofthe embryo nourishment are supplied by maternal input of nutrients.Thereproductive cycles denote the beginning to the end of each reproductive phase,combining: 1) the length of follicle development within the ovary that will be fertilizedto form the embryos, 2) mating and 3) the subsequent egg deposition/pregnancyperiod.