Shark

Overview
Sharks are fish with cartilaginous skeletons, meaning the body structure is not supported by bones like other fish, but rather the same pliable substance that the human nose is constructed from. They have been swimming in the world’s oceans for more than 400 million years. Sharks breathe oxygen in the water with the help of five to seven gill slits on either side of their body. Sharks play a vital role in maintaining the health of marine ecosystems, and their stocks are in serious trouble. More than 100 million sharks are killed by commercial fisheries every year. They are intentionally harvested for their fins, meat, or liver oil, and they are incidentally caught as bycatch in fisheries targeting other species.

Over 350 species of sharks of all shapes and sizes swim in waters the world over, ranging in size from the roughly 9-inch pygmy shark to the ocean’s largest fish – the whale shark, which can grow to over 50 feet long. The great white shark has a finely engineered body shape, long and sleek, that allows it to swim through the water with ease. Hammerheads, as their name suggests, have a wide-shaped head with eyes located far on either side, helping them to see better in the water.

Diet
Shark diets vary just as greatly as their sizes. The whale shark, for instance, sucks in algae and small prey expelling the water through its gills. The great white, on the other hand, feeds on many types of fish and marine mammals.

Shark diets can be determined by the animal’s hunting habits. Some sharks, such as blue sharks, mako sharks, great white, and hammerheads, are fast moving hunters. These sleek predators will eat a variety of prey including: squid, fish, sea lions, seals, and small whales.

Other sharks are slower moving bottom feeders that skim the sandy ocean floor. Their favorite foods are crustaceans, like crabs, and clams. Zebra horn sharks, angel sharks, and the wobbegong, a group of carpet sharks, are all examples of bottom feeders. Filter feeders, like the whale shark, and also basking sharks and megamouth sharks, sieve tiny animals and plankton bits as they slowly swim by with their large, open mouths.

Sharks do not drink water like bony marine fish. Their bodies have a high salt concentration, even saltier than the sea itself. They absorb water through osmosis – that procedure biology teachers lecture about in which water flows across a semi-permeable membrane from an area of a higher concentration of water to a lower one. This process equalizes the two concentrations, so in the case of sharks, water flows in to the even saltier gills of a shark. Sharks can secret extra salt through their rectal gland.

Reproduction
The gestation period for some sharks, like the spiny dogfish for example, takes up to two years. Unlike bony marine fish, shark reproduction is internal. Further distinctions from bony fish are that some sharks can store sperm and some give birth to live young. Sharks are born in one of three ways: from an egg externally, from an egg inside the mother’s womb, or through live birth. All sharks born from inside the mother, live or from an egg, initially feed on a yolk sac. Those born from eggs also feed on smaller siblings and unfertilized eggs until they are born. Sharks born live from their mothers feed on the placenta, similar to mammals.

Threats
Sharks are slow-growing and often reproduce later in life, making them particularly vulnerable to exploitation. Add to that the stereotypical perspective that all sharks are man-eating monsters, and it’s easy to see why many species of sharks are in great danger.

Sharks are highly migratory and regularly cross national boundaries. However, outside of Antarctica, no international limits on shark harvest exist. With increasing demand for shark fin soup and little protection, sharks worldwide are at great risk. To acquire the so-called delicacy shark fin soup, fishing fleets sever the sharks’ fins from their bodies before casting the rest of the crippled animals back into the water to die.

Currently the U.S. Shark Finning Prohibition Act is on the books to protect sharks, though unfortunately, it’s another law whose name over-promises. The law carries a loophole that makes enforcement difficult. Sharks are allowed to be landed after their fins have been cut off. To close the loophole and fully protect sharks, fishermen need to be required to land sharks whole. This will prove that only the legal number and species are being harvested, providing an accurate count of sharks caught for their fins.

The accidental catch of sharks, known as bycatch, is another threat to these creatures. Over-fishing of sharks for their meat, liver oil and leather also puts them at risk.