Chondrichthyes (cartilaginous fish, including sharks, rays and chaemeras) >

Reproduction in cartilaginous fish

Reproduction in cartilaginous fishes is complex, and starts with courtship and copulation. The claspers are filled with sperm from the urogenital papilla of the male and inject the seminal fluid into the female. Fertilization is internal in the female: sperm is stored in the oviducts and fertilizes the eggs entering them from the ovaries. In some sharks the shell or nidamental glands of the oviducts are specialized to store sperm, and eggs are fertilized as the eggcase is secreted around the descending egg.

Cartilaginous fishes typically produce few, large, yolky eggs and have a low rate of reproduction. Young are born or hatched at an advanced stage of development after an extended gestation or incubation period that may extend from a month in some egglaying sharks to over two years in some livebearing sharks. There is no free-swimming larval stages in cartilaginous fishes, unlike many bony fishes, and newborn or newly hatched individuals usually resemble miniature adults. Some egg-laying sharks and skates have elongated tails when hatched, but these soon assume more adult proportions as the hatchling grows.

Approximately 43% of cartilaginous fish species lay eggs, including all of the bullhead sharks (Order Heterodontiformes), some carpet sharks (order Orectolobiformes), many ground sharks (order Carcharhiniformes, including most of the catsharks, family Scyliorhinidae), all known skates (order Rajiformes), and all known chimaeras. These deposit eggs in purselike, conical, or spindle-shaped egg cases on the sea bottom. In egg-laying or oviparous species the egg cases are often deposited on rocks or in algae or corals, and have horns, tendrils, lateral fins, or spiral flanges that help to wedge or otherwise anchor them to the substrate. Some species have nesting sites that a number of females repeatedly use. Females of at least one species of shark are known to pick up their egg cases after laying them, and carefully place them in an appropriate nesting site. The eggs hatch after incubating for one to 15 months, and the young begin to feed on their own. As far as is known, no cartilaginous fishes care for their eggs or young beyond placing them in an appropriate site, and a few species of sharks may even feed on them. Certain egg-laying sharks are known to suck out the contents of egg-cases in captivity and may do so when free-ranging.

Livebearing sharks and rays typically have long gestation periods, from a few months to over two years. Litter size varies from one or two to 136 young, but most species have less than 20 young in a litter. The unborn young of many livebearing sharks and rays are nourished by the yolk stored in their yolk sacs, but in some sharks and especially the stingrays and their relatives (Myliobatoidei) the uterine walls secrete a milky nutrient fluid that is ingested by the fetus. The fetuses of many of the more advanced ground sharks (Order Carcharhiniformes) exhaust their yolk supply early in their development. Their yolk sacs then become connected to the maternal uterine walls and form a yolk sac placenta, analogous to the placenta of mammals, to transfer nutrients from the maternal circulatory system to the fetus.

The unborn young of the mackerel shark group (order Lamniformes) have a bizarre and unique mode of nutrition: uterine cannibalism. The fetuses deplete their yolk when very small but unlike other fetal sharks develop functional teeth and begin their predatory trade well before birth. They swallow eggs coming down the oviducts and in at least one species, the Spotted raggedtooth shark, the first fetus in an oviduct kills and eats all younger fetuses before switching to eggs for its nutrition. In several members of the group two or more fetuses survive in an oviduct, and somehow avoid preying on one another. These cannibal fetuses can grow up to one m long or more before birth in larger species such as the Spotted raggedtooth, Common thresher, and Great white sharks.

Text by Leonard J.V. Compagno, David A. Ebert and Malcolm J. Smale