The most important step in the life of a shark, and all other living creatures, is reproduction. Having a successful reproduction with a high number of surviving offspring is essential for the survival of the whole species. Sharks are no exception of this rule, and they have adapted a variety of ways to reproduce.

Unlike the bony fishes that rely on external fertilization (the female ejects thousands of small eggs to be fertilized outside the body by males), all sharks and rays accomplish this by internal fertilization. The process of internal fertilization involves highly developed internal and external reproductive organs.

The reproductive organs of the male shark include the claspers and a pair of testis. The claspers are modifications of the pelvic fins and have the function of transferring the semen from the male into the female cloaca. The pelvic fins of the female have no special modifications.


(© J. Lokrantz/Azoteimages)

The reproductive system in female sharks consists of paired ovaries, which produce eggs. In many sharks only one of the two ovaries are functional. The eggs released by the ovary/ovaries passes through the ostium and into the shell gland, where they are fertilized and receive a protective case. The protective case can either be a horny shell as in egg-laying species, or a thin membrane as in the livebearers. The encased and fertilized egg then descend to the uterus. In most sharks the developing embryos are safeguarded inside the female uterus, where, according to the species, they may develop freely, be attached by a placenta, or be sealed in leathery egg cases. A few species deposit the egg case on the bottom or in vegetation.


(© J. Lokrantz/Azoteimages)

Sharks that lay egg cases with developing embryos are called oviparous. The embryo inside the case receives nourishment from the egg yolk. The egg case may very in size and form, but are usually conical or rectangular with long spiral filaments extending from the corners. When first laid the case is soft but it quickly hardens.

Sharks that bear young that are nourished by the mother through a placental attachment are called viviparous. This form of embryo development is the leased common, occurring in hammerheads, blue shark, bull shark, and some of the smooth dogfishes. The connection between the embryo and the mother is formed through modification of the yolk sac.

The most common form of reproduction in sharks is when the young are developed internally but are unattached to the mother. They may be enclosed in a egg case or free swimming in the uterus but when they leave the mother they are fully developed. This is called ovoviviparous. The young sharks of ovoviviparous sharks rely almost entirely on their on egg yolk for nourishment. Some nutrients may be passed on from the mother through the lining of the uterus. The embryo of some sharks can also obtain nourishment by eating their unborn brothers and sisters, both the unfertilized eggs and the unborn embryos. This phenomenon is known as uterine cannibalism or ovophagy and occurs in sharks such as those of the sand tiger and mako. The result of this cannibalism is that often only one embryo in each uteri survives.

The gestation period for different sharks vary from a few month to more than a year. An extreme example is the spiny dogfish, which carries the unborn puppies for 22 to 24 months. This is the longest known gestation period for any vertebrate.