Sharks

Sharks have fascinating reproductive systems, with some advanced features for such an ancient group of organisms. Unlike bony fish, sharks have internal fertilization. The male shark uses projections from his pectoral fins, called claspers, to anchor himself to the female. He then transfers packets of sperm into the female's urogenital opening, using pulses of water. The sperm fertilize the eggs inside the female, but what happens next to the developing embryo depends on the species.

Some species of sharks lay eggs with the developing embryo covered by a tough, protective case. This is known as oviparous reproduction. The embryos of these sharks are well supplied with nutritious yolk, unlike the tiny eggs of most bony fish. After some time, the egg hatches and a young shark emerges. Bullhead sharks, whale sharks, and zebra sharks are examples of oviparous species.

Female sharks of most species are ovoviviparous live-bearers, which means they retain their eggs inside the body until the young hatch, which are then born "alive." This method provides the young with protection from predators during their earliest developmental stages. Examples of ovoviviparous sharks are dogfish sharks, angelsharks, and tiger sharks. Some species of sharks have a modification of this type of reproduction. In the white and mako sharks, the embryos hatch inside the mother at age three months, but then stay in the mother for some additional time, obtaining nourishment by eating nutrient-rich, unfertilized eggs the mother produces for them. A further bizarre twist occurs in the sand tiger shark, in which the earliest embryo to hatch in each uterus eats its siblings, so only two offspring are born (one from each uterus).

The most advanced form of shark reproduction occurs in the hammerheads and requiem sharks (except the tiger shark). In these sharks, early in embryonic development a connection (placenta) is created between the embryo and the mother. The embryo obtains nutrients through the placenta for the remainder of its growth, before being born alive. This type of development is called viviparity, and it is similar to the development process of mammals.

Compared to most bony fish, sharks reproduce and grow relatively slowly. Bony fish tend to lay thousands or more tiny eggs, most of which are scattered into the environment and die. Sharks have relatively few (zero to around 100) offspring each year, and the mother invests much energy in each to increase the chance that it will survive. Some female sharks put so much energy into a litter that they must take two years to recover their strength before breeding again. Although young sharks are born relatively large and able to take care of themselves, they grow slowly, sometimes only a few centimeters a year. It may take 15-20 years for an individual to reach sexual maturity. Such low reproductive rates and slow growth combine to make sharks highly vulnerable to overfishing.