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Sharks have a reputation for being ancient or primitive animals. However, their reproductive strategies have evolved to be more advanced than those of bony fish and in some cases their strategies are as highly developed as mammals.
Bony fish have a reproductive strategy that involves producing large amounts of small eggs. The female fish release millions of eggs into the water and the male fish simply cast their sperm near the eggs. Fertilisation is left to chance and only a tiny percentage of eggs will develop into fish. Many of these fish will be eaten before they reach maturity but a few will survive long enough to reproduce. Scientists know this strategy as ‘r-selection’.
Sharks, however are a k-selected species as they expend a large amount of effort in producing few well developed offspring, that therefore have a relatively higher survival rate compared to bony fish. Sharks produce a small number of larger young that have a much higher survival rate compared to bony fish. Reproduction varies between sharks but all involve internal fertilisation. There are three types of shark reproduction; ovoparity, ovoviviparity (aplacental yolk sac viviparity) and viviparity, although there are still variations within each method.
When a young shark is born, they are miniature versions of the adults and have to fend for themselves straightaway. To help their young survive, some sharks give birth or lay their eggs in nursery areas. The water in nursery areas is usually warm, shallow and has lots of fish for the young shark to feed on. There are no adult sharks or other major predators in these areas, so the young are able to grow, in comparative safety, to a bigger size before facing the real underwater world.
Sharks are considered one of the most threatened species in the world. They are vulnerable to the impacts of man for a number of reasons. They produce only a few young, the number of offspring varies from two for the Bigeye Thresher to 135 for the Blue Shark a year. The length of pregnancy in sharks is relatively long, averaging between 9-12 months, with the longest pregnancy at 22 months for the Spiny Dogfish. In addition, not all shark species reproduce every year; many have a resting stage of 1-2 years before reproducing again. Young sharks are also slow to grow and reach sexual maturity late. For example, Lemon Sharks take 15 years to mature, and Spiny Dogfish take 20 years. Many sharks are killed before maturity by other marine predators or man and so haven’t yet reproduced. And of the ones that survive, again these only produce a few young that may take another 15-20 years to mature.
Tens of millions of sharks are killed every year by humans and so shark populations continue to decline at an alarming rate. Sharks are vital to the marine ecosystem - just like other top predators in terrestrial environments such as wolves and lions - they help to maintain the balance of their ecosystem by keeping other populations in check.
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