Some Sharks & Rays have a Type of Placenta
Rays commonly demonstrate a method of nutrition delivery to their embryos by creating a placental analogue, something that acts like placenta. A part of the uterine lining secretes a nutritive substance called embryotrophe, which is ingested by the embryo.
Placental viviparity refers to the method of nourishment that involves the yolk in the yolk sac and occurs only in the Carcharhiniforme order of sharks. The embryo ingests the yolk for the first few weeks and as it runs out, the sac gets long and thin, one side becomes vascularised with blood vessels and grows together with the uterine wall, creating a yolk sac placenta. All the nutrients in the mom’s blood are then shared through the placenta, a nearly inexhaustible supply of energy. As long as mum’s healthy, baby shark will be, too. This method of nourishment has evolved independently 11-20 different times within the Elasmobranch group, resulting in the diversity of the structure.
Sharks either lay eggs or give live birth.
Sharks deposit eggs through oviparity or give birth to live young through vivparity. Their fecundity, potential reproductive ability, ranges from 1-2 a year to a possible 300 produced by whale sharks.
The oviparous form of reproduction, egg-laying, is considered a primitive form and common for Benthic, Littoral, and Bathyal Sharks, about 40% of shark species. Tough egg cases are laid on the substrate or attached to structures on the sea floor. The embryos within the cases are nourished by the yolk sac but those nutrients are limited so the young emerges small. Incubation in the case can last from a few months to more than a year. Oxygenation and ventilation occurs through the slits in each side of the egg case as the embryo constantly fans its tail, increasing water flow. All eggs are laid in pairs and there is some evidence that the substrate upon which the eggs are laid is somewhat chosen by the mother. Bullhead sharks have been observed picking up egg cases and wedging them into secure positions in the substrate.
Viviparity is the mode of reproduction when the embryos remain in the uterus for all development. The embryo’s nourishment can come from the yolk or the yolk can be supplemented by a connection to the mum. When there is no placental connection between mum and embryo but the embryo remains within the uterus for development, it is called aplacental viviparity, also referred to as ovoviviparity. There are three forms dependent on how the embryo is nourished. The embryo can be dependent on yolk only, nourished by other eggs in the uterus, or nourished through placental analogues.