Sharks are an ancient type of predatory fish, which can be found in all marine and some freshwater environments. Over 350 species have been identified. Although all sharks share some basic characteristics, such a cartilaginous skeleton, there are great differences between species, for example some pygmy sharks reach a maximum length of 18cm while the whale shark, the world’s largest fish, may grow to 18 metres. Sharks have adapted to such a wide range of environments that diversity is not unexpected, but what is surprising is that sharks have three different methods of reproduction.

The eggs of all sharks are fertilised internally, but the subsequent development and birth of the young occurs in one of three reproductive modes, depending on the species:

Oviparity - a tough leathery membrane egg case develops around each fertilised egg. The mother shark finds a suitable location, deposits her eggs and then leaves them. The appearance of shark egg cases varies according to the species, some are screw shaped, some have gripping tendrils and some look like pouches (often called mermaid’s purses). The developing embryo obtains nourishment from the egg yolk and slits in the egg case allows water to flow inside to provide the embryo with oxygen, a process it encourages by fanning its tail. Incubation time varies from a few months to in excess of a year. Approximately 40% of sharks reproduce by oviparity; these are more primitive species including species such as carpet sharks and Port Jackson sharks.

Viviparity - eggs hatch inside the mother’s body, the developing embryo is nourished by maternal nutrients through a placenta or uterine milk. After gestation the baby sharks are born fully formed. Live bearing species include bull sharks, hammerheads and reef sharks.

Ovoviviparity - eggs hatch inside their mother’s body, but maternal nutrients are not provided. The embryo obtains nourishment from the surrounding egg yolk, when this is exhausted the embryo may obtain nutrients by consuming unfertilised eggs or by killing and eating other siblings in the uterus. Ovoviviparous species include whale sharks, tiger sharks, great white sharks and grey nurse sharks.

Both methods of livebearing reproduction produce independent, fully formed young who do not receive any parental care.

Another amazing shark reproductive strategy has been observed in captivity in two different species (hammerhead and black tip) when a female shark has produced offspring by herself. This phenomena is called parthenogenesis or virgin birth and occurs when the mother’s chromosomes divide and split, then pair with a copy of itself, producing a baby shark which only has maternal DNA.

Although young sharks are relatively large when born their subsequent growth rate is slow. Many sharks are believed to have long lifespans, the whale shark may live 100 years, they may take decades to reach sexual maturity and their reproductive rates are low – these factors make shark populations vulnerable to overfishing and the cruel and wasteful practice of shark-finning.