Sharks have three modes of reproduction and in all three provide no form of parental care. Generally, sharks bear live young however there are some sharks lay eggs. Viviparity, live birth, is the most advanced form. The female provides nourishment for the embryos as they develop inside of her. The nourishment can be delivered as a secretion called uterine milk or through a placenta. Hammerheads are a good example. At birth the young look like miniatures of the adults, sometimes up to two or three feet long and weighing a maximum of 40 pounds.
The most common form of reproduction is ovoviviparity. This is similar to viviparity because the eggs are fertilized, hatch and develop within the female. However the embryos do not receive any direct nourishment from the mother other than the initially provided yolk. In some species, the young receive nutrition by devouring newly ovulated eggs or smaller, less developed embryos or siblings. This is called oophagy.The Portuguese shark and sand tiger are good examples of this type of behavior. In the sand tiger the first embryo to hatch attacks and consumes its siblings before settling down to a diet of eggs. This behavior takes sibling rivalry to its highest level.The final type of development is oviparity. Oviparity refers to sharks that lay eggs in the form of egg cases released into the ocean. These egg cases attach themselves to seaweed or other forms of substrate where possible. Walkers on the beach occasionally find these eggs cases washed up in the surf line.These eggs cases are sometimes called mermaid purses. The whale shark, the largest fish in the sea, can hatch out of the egg at 14 inches but can grow up to 40 feet. Most of the eggs laying species reproduce every year.Bony fishes grow rapidly and mature relatively quickly. Most species of sharks grow slowly and take years to reach maturity. Requiem sharks usually mature in six to 18 years, or longer. Current research indicates that the largest sharks caught are up to 40 years old. It has been pointed out the some dogfish can live to be 70.The number of embryos born to a single mother varies from two (bigeye thresher, sand tiger) to 135 (blue shark). Gestation lasts from three or four months in small sharks to two years in the spiny dogfish and perhaps longer for the frilled shark. A period of nine to 12 months is typical of most large sharks.The young usually emerge tail first, although some species, like the sand tiger, are born headfirst. “Tee-shaped” heads of young hammerheads are soft and pliable at birth to permit easy passage through the birth canal. Sharks give birth in a variety of habitats from the deep ocean floor to coral reef environments, but many seek out shallow, near-shore waters, or estuaries, because of the abundant food supply and absence of most large predators.Any environmental degradation of coastal areas, however, threatens the nursery area of even these nomadic ocean dwellers.
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