Bonnethead
| Bonnethead shark | |
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| Species: | S. tiburo
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| Sphyrna tiburo (Linnaeus, 1758)
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- This article refers to the species of shark. For the Harley Davidson motorcylce engine, see Shovelhead engine.
The bonnethead shark or shovelhead, Sphyrna tiburo, is a member of the hammerhead shark genus Sphyrna. The Greek word sphyrna translates as hammer, referring to the shape of this shark's head - tiburo is the Taino (Native American) word for shark.
Appearance
Characterized by a broad, smooth, spade-like head, they have the smallest cephalofoil (hammerhead) of all Sphyrna.
Grey-brown above and lighter on the underside, it is a timid and a harmless shark.
Size
On average, bonnethead sharks are about 1 m (3.4 ft) long, being the smallest hammerhead shark.
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.
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. It also utilizes an electric-field sensing system similar to radar. 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.
Diet
It feeds primarily on crustaceans, consisting mostly of blue crabs, but also shrimp, mollusks, and small fishes. 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 in order to swallow the crab whole.
To accommodate the many types of animals that it feeds on, the bonnethead shark has small, sharp teeth in the front of the mouth (for grabbing soft prey) and flat, broad molars in the back (for crushing hard-shelled prey).
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. The pups are born in late summer and early fall, measuring 12 to 13 inches.
Researchers from Queen's University Belfast and the Southeastern University (Florida), 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.[1]
Sexual dimorphism
Bonnethead sharks are the only sharks known to exhibit sexual dimorphism; that is, male and female adults look different from one another. In morphology, adult female bonnethead sharks have a broadly rounded head, whereas males possess a distinct bulge along the anterior margin of the cephalofoil. This bulge is formed by the elongation of the rostral cartilages of the males at the onset of sexual maturity and corresponds temporally with the elongation of the clasper cartilages.
Purpose of the hammer
The reasons for cephalofoil has caused scientific debate for more than a decade. Whatever the ultimate purpose, a wing shaped cephalofoil allows hammerhead sharks to swim on a horizontal plane and gives them the ability to execute sharp turns. It is also responsible for better electroreception (using ampullae of Lorenzini) and heightened olfactory acuity.
Pectoral fins and swimming
The pectoral fins on most fish control pitching (up-and-down motion of the body), yawing (the side-to-side motion) and rolling. Most hammerhead sharks do not yaw or roll and achieve pitch by using their cephalofoil. The smaller cephalofoil of a bonnethead shark is not as successful and they therefore have to rely on the combination of cephalofoil and their large pectoral fins for most of their motility. Compared to other hammerheads, bonnethead sharks have larger and more developed pectoral fins and are the only species of hammerhead to actively use pectoral fins for swimming.
Evolution
Using data from mtDNA analysis, scientist have found that evolution of hammerhead sharks has probably begun with one phylum that had a highly pronounced cephalofoil (most likely that similar to the winghead shark, Eusphyra blochii, 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.
References
- Template:IUCN2006 Database entry includes justification for why this species is of least concern
- "Sphyrna tiburo". Integrated Taxonomic Information System. 23 January.
{{cite web}}: Check date values in:|date=and|year=/|date=mismatch (help) - Froese, Rainer; Pauly, Daniel (eds.) (2005). "Sphyrna tiburo" in FishBase. 10 2005 version.