Bonnethead shark

Appearance

Characterized by a broad, smooth, spade-like head, they have the smallest cephalofoil of all ''Sphyrna''. Grey-brown above and lighter on the underside, it is a timid and a harmless shark.

Behavior

The bonnethead shark is an active tropical shark that swims in small groups of 5 to 15 individuals. Curiously however, schools of hundreds or even thousands have been reported. Bonnethead sharks move constantly following changes in water temperature and to maintain respiration. The bonnethead shark will sink if it does not keep moving since hammerhead sharks are among the most negatively buoyant of marine vertebrates. The bonnethead shark uses a special body fluid, called "cerebrospinal fluid" or "Cl-excess", to let others know it is nearby. Like other sharks it is capable of electroreception to detect its preys. This system allows the bonnethead shark to position itself for biting prey within a few feet where its eyes are least able to assist. This shark is not dangerous to humans. It can be seen in aquariums.

Habitat

This species lives in the Western Hemisphere where the water is usually warmer than 70 °F . It ranges from New England, where it is rare, to the Gulf of Mexico and Brazil, and from southern California to Ecuador. During the summer it is common in the inshore waters of the Carolinas and Georgia; in spring, summer, and fall, it is found off Florida and in the Gulf of Mexico. In the winter, the bonnethead shark is found closer to the equator, where the water is warmer.

Reproduction

The bonnethead shark is viviparous, which means females produce eggs that are retained and nourished in the reproductive system until the young are mature enough to be released to the outside. They reach sexual maturity at about 30 inches. Four to twelve pups are born in late summer and early fall, measuring 12 to 13 inches .

Researchers from Queen's University Belfast and the Southeastern University , have recently shown that a bonnethead female produced a pup by parthenogenesis in 2001. The birth took place at the Henry Doorly Zoo in Nebraska and subsequent DNA analysis has shown a perfect match between mother and pup....hieroglyph snipped...

Food

It feeds primarily on crustaceans, consisting mostly of blue crabs, but also shrimp, mollusks and small fish. Seagrasses have been found in its stomach contents. Their feeding behavior involves swimming across the seafloor, moving its head in arc patterns like a metal detector, looking for minute electro-magnetic disturbances produced by crabs and other creatures hiding in the sediment. Upon discovery, they sharply turn around and bite into the sediment where the disturbance was detected. If a crab is caught, the bonnethead shark uses its teeth to grind its carapace and then uses suction to swallow.

To accommodate the many types of animals that it feeds on, the bonnethead shark has small, sharp teeth in the front of the mouth and flat, broad molars in the back .

Evolution

Using data from mtDNA analysis, scientist have found that evolution of hammerhead sharks has probably begun with a taxon that had a highly pronounced cephalofoil , and has later been modified through selective pressures. It is thus assumed today that, judging by their smaller cephalofoil, bonnethead sharks are the more recent developments of a 25 million year evolutionary process.