Naturalist Fact: Bonnethead Shark

Bonnetheads (Sphyrna tiburo) are the smallest species of the hammerhead family reaching an average size of 3-5 feet.  This shark can be distinguished from other hammerheads by its rounded, shovel-shaped head.  They are found in the Western Atlantic and Eastern Pacific oceans.  Unlike most sharks, they have molar-like teeth at the back of the jaw for crushing their prey such as hard-shelled invertebrates, crabs, shrimp, mantis shrimp, snails, cephalopods, as well as small bony fishes.  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 prey hiding in the sediment.  Studies of a colony of captive Bonnetheads revealed that this species has surprisingly complex behavior, body language and social organization.  18 Bonnethead body postures and movement patterns were identified including head shakes, back hunching, puffing out the gill pouches, jaw snapping, hitting other Bonnetheads, circling head-to-tail in lines of up to five, and (in males) clasper flexing.  Half of these appeared to regulate social activities.  A subtle, size-related dominance hierarchy was also noted among the sharks, with submissive individuals giving way to dominants as little as 5% longer than themselves.  This suggests they have a keen awareness of their own size relative to that of others sharing their environment.  Bonnetheads give birth to live young with a gestation period of 4 to 5 months, which is the shortest gestation period of all sharks.  Females reach sexual maturity when about 2 1/2 feet long.  They give birth in late summer or early fall to litter sizes of 8 to 16 pups.  During this time, the females lose their desire for food, which prevents them from feeding on their pups.  Males move to a different location, also an adaptation to avoid feeding upon their own young.  Bonnetheads are the only sharks known to exhibit sexual dimorphism, which is where male and female adults look different from one another.  Adult females have a broadly rounded head, whereas males possess a distinct protuberance at the top of the head.  In 2001 at a zoo in Nebraska, a female Bonnethead produced a pup in a tank containing three other females, but no males.  It was concluded after DNA testing that the reproduction was by parthenogenesis, a form of reproduction in which an unfertilized egg develops into a new individual. This type of asexual reproduction had been seen before in bony fish, but never in cartilaginous fish such as sharks, until this documentation.  The Bonnethead, with its early age at maturity and high litter size and population growth rates are very abundant and therefore considered a species of lesser concern.

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