John Musick and Julia Ellis have proposed that the ancestral reproductive mode for elasmobranchs was "yolk-sac viviparity

Question

" That is, embryos were retained throughout their development in the oviducts of the female and emerged as miniatures of the adults (i.e., viviparity), but nutrition was provided by yolk that was deposited at the time the egg was formed, not from the mother during development (i.e., lecithotrophy, not matrotrophy). They suggest that oviparity (depositing eggs that develop outside the body of the mother) was associated with the evolution of small body size because it increased the fecundity of small species of elasmobranchs. What is their reasoning? That is, why would oviparity provide greater fecundity than viviparity for small species of elasmobranchs? What other factors might make one mode superior to the other?

Answer 1

An embryo that completes its development in the mother's oviduct occupies far more space than an egg, so an oviparous species can produce more eggs than babies. Musick and Ellis reason that the space constraint would be more severe for small species than for large ones, and that the evolution of small body size would be facilitated by a shift from yolk-sac viviparity to oviparity. They concede that eggs may suffer higher predation than babies, but suggest that this cost was offset by the risk that a female carrying embryos would be killed by a predator, losing her final reproductive effort. Oviparous elasmobranchs do not have parental care, so the death of an oviparous female after her eggs have been released does not affect the survival of the eggs.