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The bonnethead shark, also known as the shovelhead (Sphyrna tiburo) is an abundant species of hammerhead shark found along the coasts of the temperate and tropical waters of the Americas which are generally warmer than 70 °F (21 °C). They are notable for being not only the only species of shark to display sexual dimorphism, but they are also the currently only known omnivorous shark.
Bonnetheads are small in overall length, being between only 2-3 feet (0.61–0.91 m) on average, with a maximum size of roughly 5ft (150 cm). They are characterized by a broad, smooth, spade-like head (hence their alternate moniker of "shovelhead"). Its cephalofoil is considered to be the smallest of all known hammerhead species. They exhibit countershading, and are gray-brown dorsally with a lighter shading on their ventral side.
Pectoral fins and swimming
Because of their cephalofoil, most hammerheads cannot achieve yaw (side-to-side motion) and roll, but control pitch (up-and-down motion) via their hammerhead. However as the bonnethead's foil is smaller than those of other hammerheads, it is not as successful at using strictly the foil for this process and thus instead combines it with their pectoral fins for much of their mobility. Their pectorals are larger and more developed when compared to their cousins, allowing them to swim more like the average fish instead of like their kin.
Of all known shark species on the planet Earth, bonnetheads are the one and only one that appears to exhibit sexual dimorphism, with it being displayed both in body length (females tend to be larger than males) as well as in the morphology of the head. Females have a broadly rounded head, whereas males instead possess a distinct bulge along its cephalofoil's anterior margin. 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.
Bonnetheads are active tropical sharks that cluster together in small shivers of 5-15 individuals; with this in mind, schools of hundreds and even thousands of members have also been reported. They must move to respire and as such must continue to move. Should the cease to move, they sink, as hammerheads are among the most negatively buoyant of marine vertebrates. They move to follow changes in water temperatures, as well.
The shark feeds primarily on crustaceans, consisting mostly of blue crabs, but also shrimp, mollusks, and small fish. Its feeding behavior involves swimming across the seafloor, moving its head in arc patterns like a metal detector, looking for minute electromagnetic disturbances produced by crabs and other creatures hiding in the sediment. Upon discovery, it sharply turns around and bites 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 on which it feeds, 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).
Bonnetheads also ingest large amounts of seagrass, which has been found to make up around 62.1% of gut content mass. The species appear to be omnivorous, the only known case of plant feeding in sharks. They can digest seagrass with at least moderate efficiency, with 50±2% digestibility of seagrass organic matter, with known cellulose-component-degrading enzyme activity in their hindgut.
The bonnethead shark is viviparous, and are capable of producing pups via parthenogenesis. Females reach sexual maturity at about 32", while males reach maturity around 24". 4-12 pups are born in late summer and early fall, measuring 12 to 13 inches (300 to 330 mm).
Bonnetheads have one of the shortest gestation periods among sharks, lasting only 4½–5 months.