Elasmobranches Reproductive Potential

Abstract

Chondrichthyes species have a wide variety of reproductive strategies that can be broadly divided into two major categories: oviparous and viviparous species, the latter being further subdivided into aplacental (categorised in aplacental yolk sac, oophagy, adelphophagy, trophonemata) and placental species. More recently, viviparity has been divided as lecithotrophic or matrotrophic, according to the existence of maternal nutrients to the embryo development.

To properly identify the reproductive strategy, the study of both the different components of the reproductive trait (mainly the ovary, oviductal gland and uterus in the females and the testes and genital ducts in the males) and the reproductive cycle is fundamental.

Reproductive cycles are complex and poorly understood. The reproductive cycle includes the ovarian cycle (how often a female develops a batch of vitellogenic oocytes and ovulates a batch of eggs) and the gestation period (length of time between fertilisation and parturition). Parturition and ovulation may be annual, biennial or longer. Some species present a synchrony in the ovarian and gestation periods while others show asynchronous cycles.

The meaning of the term maturity ranges from defining the onset of maturation to the period of time when a female undergoes parturition and produces a litter of pups. Facing such a long period that may last for some years and the presence of several events in the reproductive trait that occur during this period it is not surprising that in the literature some arbitrarily exists in the assignment of maturity criteria: the presence of selected hormones in the blood, the onset of vitellogenesis, first mating, first sperm storage, and first ovulation. Therefore a common criterion should be implemented to harmonise maturity definition as the estimation of reproductive parameters will depend on the criterion adopted.

Reproductive parameters include the estimation of: a) sex ratio at birth, b) the fecundity (both ovarian and uterine) related with maternal age or size, c) the maturity ogive and d) the maternity ogive. The latter is more difficult to be obtained since it needs information on the periodicity of the ovarian cycle and gestation period.

In this chapter will characterise the different modes of reproduction present in elasmobranches species through the interpretation of the different events that occur in the reproductive trait of both females and males. A standardise maturity scale will also be implemented based on easily identifiable macroscopic and microscopic characteristics of the reproductive trait. Then, the ovarian cycle and gestation period as well as the mating period will be characterised using both direct and indirect methods (indices) that will help to define the timing of reproductive cycle in both females and males. Finally, the methods to estimate the main reproductive parameters will be discussed in oviparous and viviparous species.