Egg case (Chondrichthyes)
Living chondricthyans that produce egg cases include some sharks, skates and chimaeras.
[6] Shortly after the egg case finishes developing, it is deposited outside the body; common locations include kelp forests and rocky seafloors.
After a period of development, typically a week or two, small slits open on each side of the egg case to aid water flow.
[2] It is thought that viviparity is the ancestral condition for sharks, and that it evolved through the elongation of retention time of retained oviparity.
[8][5] Hatchlings are considered large for sharks, reaching over 14 cm in length by the time they leave the egg case.
[5] As a member of the order Heterodontiformes, the whitespotted bullhead sharks is thought to be oviparous, but egg cases have never been observed.
The egg cases of catsharks are purse-shaped with long tendrils at the corners that serve to anchor them to structures on the sea floor.
[1][10] Studies have been done where egg cases were removed from gravid females to ensure proper identification in regard to skate species.
[1] Egg cases have distinguishable characteristic traits that are unique to that species, thus making it a great tool for identifying a skate.
A keel runs laterally along both sides of the outer edge of the egg case; it is a flexible structure.
The flattened collagen tissue joins on the anterior end of the egg capsule to form a tail.
[14] The egg case genera Palaeoxyris and Fayolia, which are thought to have been produced by hybodonts and xenacanths respectively, two groups of extinct shark-like cartilaginous fish more closely related to modern sharks and rays than to chimaeras, resemble those of bulldog sharks in having a spiral collarettes running around them.
[15] Predation on egg cases is thought to be a major source of mortality for developing oviparous sharks, skates and chimaeras.
[16] In general, predation is the leading cause of mortality for marine fish eggs, due to their abundance and high nutritional value.
[16] Parental care ends when the egg case is released from the body, so the embryo relies on its tough, leathery exterior as its only source of protection.
