| Shark Biology Primer |
Sharks have skeletons made of cartilageOne of the characteristics that separate sharks and rays from other fish is the fact that sharks and rays of skeletons of cartilage, while other fish have bones. Cartilage is the substance we have in the more flexible part of our nose (as you probably know), and is obviously more flexible than bone. It’s also lighter. Cartilaginous skeletons enable sharks to swim more effectively than their prey, helping them to swim faster and maneuver better than most bony fish.Shark teeth are disposableNew teeth are formed in consecutive rows, and as one falls out, another moves forward and takes its place. It is believed that sharks shed their teeth as often as every eight days, resulting in as many as 30,000 lost teeth in a lifetime… and a very tired Tooth Fairy! Some sharks are 'warm-blooded' (sort of)Several species of the Mackerel Sharks, including the Great White Shark and Mako Sharks, have a circulatory system that retains metabolic heat. While otherwise cold-blooded, this enables them to maintain body heat as much as 25° Fahrenheit (14° Celsius) warmer than the surrounding water. It’s almost like being warm blooded. The advantages are significant, enabling them to extend their range significantly beyond what they would otherwise tolerate. Shark fins are among the most expensive foods in the worldShark fins are primarily used as an ingredient in shark fin soup in Asia. With a cost of one bowl of soup around $100 US, the dried shark fins cost anywhere from $40 to $300 per pound, depending upon the fin and species. A single Whale Shark pectoral fin can sell for up to $15,000. High demand for shark fin soup is largely behind this high cost, and the reason why so many shark populations are under threat of destruction.Sharks can sense electricitySharks have receptors that are sensitive to electric fields. These receptors are called ampullae of Lorenzini. The ampullae are canals filled with a kind of jelly on the shark’s head that are able to receive electric stimuli through the skin of the shark. The shark is able to ‘read’ minute changes in electric current in its environment through the ampullae. This is a sense every bit as developed and important to the shark as sight, hearing, smell, touch, and taste.How does the shark use its electro-reception? When a fish or some other prey moves its muscles, there is a slight change in electric current. A fish that is wounded and struggling will produce a different amount of electricity. Sharks are able to sense this weak change in current through the water even before they might otherwise see or smell them, and before their prey can see the shark. All shark species use this electroreception ability to locate prey. Some species that enjoy bottom-dwelling fish or mollusks that bury themselves in the sand are able to locate these animals with their electro-sense. Sharks also use their electro-sensors to help navigate the oceans. They are able to induce an electric current by their own movement across the earths electromagnetic fields, and this helps them to determine and stay on course. Apparently, the US Navy is impressed with this ability of sharks, and wants to develop similar technology. Waiting for shark pups requires patienceThe next time you hear someone complain about 9 months of pregnancy, consider the gestation of a shark. About 70% of sharks give birth to live young. Gestation period varies by species, averaging around 22 months. But there is quite a range. Some species of Bullhead Shark may have a gestation period as short as 5 months, while some researchers have claimed that the Basking Shark might be as long as 3.5 years—although 24 months is more likely.Breathing makes a shark smileWhile joy or happy amusement may have nothing to do with it, breathing is a joint venture of the mouth and gills. Shark gills are different than those of bony fish. Bony fish have one gill, covered by a hard protective plate that is operated by a system of bones and muscles. The action of the muscles pumps water through the fish’s mouth and out through the gills where the water is filtered for oxygen intake. In contrast, a shark has between 5 and 7 pairs of gill slits, depending on the species. Shark gill slits have very little musculature. In most species, sharks must move forward to enable the water to flow through the open mouth and through the gills in order to breathe. The open mouth gives the appearance of a smile in many sharks.Cannibals in the wombSharks give birth in one of several ways:Ovipararity: the shark lays egg casings that contain the developing embryos. There are two kinds, one where the eggs are laid when formed. And another where eggs develop for some months within the mother’s body before being laid for further development outside of the mother’s body. Ovoviparity: egg cases are retained within the female shark, and the embryos are nourished solely by the yolk, and then born live. Viviparity: the young develop internally, but once the yolk has been consumed, the yolk sack connects with the mother’s uterine wall to form a placenta. Nourishment continues through this placenta until the pups are fully developed and then born live. Sandtiger pup. Illustration by R. Aidan Martin
Over a dozen other shark species are known to undertake a more benign form of intrauterine cannibalism, including most of the Lamnoids, or Mackerel Sharks (Basking, White, Threshers, Mako, and others). In this case, once the yolk has been depleted, unfertilized eggs are released for the continued nourishment of the embryos. The Sandtiger also releases unfertilized eggs for its embryos, but unfortunately, not in time for all of them to enjoy. Comments (4)
malany solonbb said:
Write comment
|
