By Hannah Roddy
Sharks are fascinating creatures, especially when it comes to discussing their reproduction and development. Sharks are divided into two groups: those that lay eggs (oviparous), and those that give live birth (viviparous).
Oviparous species lay eggs, known as mermaid’s purses, that mature and hatch outside the mother’s body. As with any embryo in an egg, the sharks are nourished by a yolk sac until they hatch.
Viviparous sharks give live birth after the eggs hatch internally, and can be divided into two further categories. Placental species have a umbilical cord adjoining mother and embryonic tissue, providing constant nutrients to the fetus. Aplacental embryos, lacking the connection of a placenta, only receive nutrients from the egg yolk.
About 60 percent of shark species give live birth, and why could this be? While egg laying requires less energy from the mother, it does expose the eggs to the hostile open ocean and predators. Live-birth ensures the mother protects her eggs through the embryo’s development and gives each newborn shark pup a greater chance of survival.
In fact, research has demonstrated an evolutionary transition from oviparity to viviparity, bringing rise to questions about the parental care and involvement of mother sharks with their pups.
Dulvy, N. K., and J. D. Reynolds. “Evolutionary Transitions among Egg-laying, Live-bearing and Maternal Inputs in Sharks and Rays.” Proceedings of the Royal Society B: Biological Sciences 264.1386 (1997): 1309-315. Web.
H. Greven. “Viviparous Sharks.” Viviparous Sharks. Shark Information, 06 Dec. 2009. Web.
Hamlett, William C. Reproductive Biology and Phylogeny of Chondrichthyes: Sharks, Batoids and Chimaeras. Enfield, NH: Science, 2005. Print.