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Sharks invest a lot of energy into producing only a few well developed young that have a better chance of survival. Reproduction takes many forms in sharks and rays, with the three main methods being oviparity, viviparity and ovoviviparity - although there are still variations within each.
Skate, chimaera and some species of shark (including catsharks and horn sharks) produce eggs that are encased in a tough, ‘leathery’ eggcase (or mermaid’s purse) that is laid on the seabed. A female shark may spend a long time laying her eggcases to ensure they’re securely fixed in a safe place, as it can take between 6-9 months before they’re ready to hatch. Depending on the species, features such as curled tendrils, horns and sticky mucus filaments attach the eggcase to a substrate (e.g. seabed, reef, seaweed) while the young shark or skate develops.
This protective capsule acts as a life-support machine, with everything that is needed enclosed within. The embryo absorbs nutrients from a yolk sac before hatching as a miniature version of the adult.
Instead of laying eggs, the female will carry them inside her body, providing extra safety from potential predators. The embryos develop within an eggcase which has a thin membrane-like covering rather than the hard mermaid’s purses. Once the food supply from the yolk has finished and the embryo has fully developed and hatched inside the female, she will then give birth to the young. In some species, the pups are not born immediately after hatching out of the eggcases, instead they stay in the uterus where they are provided with more food in the form of unfertilised eggs that are released to feed the growing embryos. This is known as oophagy.GRUESOME FACT – the pups of Sandtiger Sharks not only eat unfertilised eggs but their un-hatched siblings too! This is called intrauterine cannibalism.
VIVPARITY (live birth)
Viviparity is the most advanced method of reproduction. The shark develops inside the mother’s body, receiving nutrients and oxygen from the mother through an umbilical cord. This is the same method used by mammals, however unlike mammals, when the pups are born they are immediately independent and must fend for themselves.FASCINATING FACT – some female sharks (such as Bonnethead, Blacktip and Zebra Sharks) are able to reproduce without having a male fertilise the eggs - this is known as parthenogenesis (or ‘virgin births’).
To improve their pups’ chance of survival, some sharks give birth or lay their eggs in nursery areas. Here the water is usually warm and shallow, there is a good food supply and they are less accessible to potential predators. Once they’ve reached a good size they venture out into the underwater world alone.
WHY ARE SHARKS UNDER THREAT?
Some species of sharks are amongst the most threatened species in the world. Their biological traits make them particularly vulnerable to overfishing, some of these include:
- Slow growing - the Greenland Shark is estimated to live ~400 years!
- Late to mature - the Greenland Shark reaches sexual maturity ~150 years! Many young sharks are killed before they’ve reached maturity and been able to reproduce.
- Long pregnancies - averaging between 9-12 months. The Greeneye Dogfish has the longest recorded pregnancy at 31 months!
- Give birth to only a few young - varying from two pups for the Bigeye Thresher to 135 for the Blue Shark.
- May not reproduce every year - some species have a resting phase of 1-2 years
Tens of millions of sharks are killed each year and many populations continue to decline at an alarming rate. Without sharks, marine ecosystems face an uncertain future but by getting involved with shark conservation, you can help keep our seas healthy - visit our Get Involved page to find out more.
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