Bonnethead
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| iBonnethead shark | ||||||||||||||||
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| Sphyrna tiburo (Linnaeus, 1758) |
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 Range of bonnethead shark
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The bonnethead shark or shovelhead, Sphyrna tiburo, is a member of the hammerhead shark family Sphyrna. The Greek word 'Sphyrna' translates as 'Hammer', referring to the shape of this shark's head.
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[edit] Appearance
Characterized by a broad, smooth, spade-like head, they have the smallest cephalofoil (hammerhead) of all Sphyrna.
Gray-brown above and lighter on the underside, it is a timid and a harmless shark.
[edit] Size
On average, bonnetheads are about 1 m (3.4 ft) long, being the smallest hammerhead shark.
[edit] Habitat
This species lives in the Western Hemisphere where the water is usually warmer than 70 degrees Fahrenheit. 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 is found closer to the equator, where the water is warmer
[edit] Behavior
The bonnethead is an active tropical shark that swims in small groups of 5 to 15 individuals, but sometimes migrating schools of hundreds and even thousands of these sharks have been reported. Bonnetheads swim continuously, traveling long distances every day, following the changes in water temperature. This species must stay in motion at all times in order to force its gills open to receive oxygen. They will sink if they do not keep moving since hammerhead sharks are among the most negatively buoyant fish. The bonnethead shark uses a special body fluid, called "cerebrospinal fluid" or "Cl-excess", to let others know it is in the area. Only one attack on humans has ever been recorded.
[edit] 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 their bonnet 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, bonnethead uses its teeth to grind its carapace and then uses suction in order to swallow the crab whole. To accommodate the many animals that it feeds on, 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).
[edit] Reproduction
The bonnethead 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.
[edit] Sexual dimorphism
Bonnetheads are the only known sharks to exhibit sexual dimorphism, i.e. male and female adult bonnetheads look different from each other. As adults, female bonnethead sharks are characterized by a broadly rounded head morphology 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.
[edit] Purpose of the hammer
The reasons for cephalofoil has caused scientific debate for more than a decade. What has been found however is that a wing shaped cephalofoil allows hammerheads to swim on a horizontal plane, as well as giving them ability to execute sharp turns. It is also responsible for better electroreception (using ampullae of Lorenzini) and hightened olfactory acuity.
[edit] 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 hammerheads 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, bonnetheads have larger and more developed pectoral fins and are the only species of hammerhead to actively use pectoral fins for swimming.
[edit] Evolution
Using data from mtDNA analysis, scientist found that evolution of hammerhead sharks has probably begun with one phylum that had a highly pronounced cephalofoil (most likely that similar to 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 the 25 million evolutionary process.
[edit] References
- Cortés (2000). Sphyrna tiburo. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 12 May 2006. Database entry includes justification for why this species is of least concern
- Sphyrna tiburo (TSN 160502). Integrated Taxonomic Information System. Accessed on 23 January 2006.
- "Sphyrna tiburo". FishBase. Ed. Ranier Froese and Daniel Pauly. 10 2005 version. N.p.: FishBase, 2005.
