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Gregory R. Mann, Ph.D. {ret.}

Bonnethead Hammerhead Shark

“Sphyrna tiburo”

The Bonnethead Hammerhead Shark is the smallest of the hammerhead sharks and can be distinguished by its smooth, rounded, shovel-shaped head. The eyes are located at the ends of the evenly rounded lobes of the flattened head, rather than a hammer shaped. Also the head lacks a notch at the mid-line. The pectoral fins are short and straight along the rear margin. The Bonnethead Hammerhead Shark reach an average size of 3-5 feet. The maximum recorded weight of a Bonnethead Hammerhead Shark is about 24 pounds. This shark has a remarkable dentition, with small sharp teeth in the front of the jaw for grasping either its mate or a soft-bodied prey and broad molar-like teeth at the back of the jaw for crushing hard-shelled invertebrates. Like all sharks, they have additional rows of teeth that are used as the older teeth become lost or worn. Its body is plain gray to gray-brown and occasionally to a green tint above & shading to a light color underneath. Bonnethead Hammerhead Sharks feed daylight hours and feeds primarily on invertebrates, crabs, shrimp, snails, cephalopods as well as small bony fishes. Upon locating a prey the Bonnethead Hammerhead Shark swims slowly within range, followed by a quick acceleration to attack the prey. The prey is then crushed in the molariform teeth. This differs from the capture event typically of other sharks, where the jaws are initially closed & biting ceases at jaw closure. This allows the Bonnethead Hammerhead Shark to take advantage of prey that is not available to other species of sharks. After the prey is crushed, it is moved by suction to the oesophagus. The Bonnethead Hammerhead Shark has a well-developed sensory & nervous systems that allow them to be efficient predators. Vision & hearing capabilities are exceptional as well as the sensitivity of the lateral line to small vibrations, alerting them to nearby potential prey.

The behavior of the Bonnethead Hammerhead Sharks has been well-studied. Some sharks exhibit specific types of behavior toward other sharks, including patrolling, head-shaking, jaw-snapping, hitting and hunching. This behavior is believed to establish & maintain dominance and other antagonistic relationships. Another example, an individual Bonnethead Hammerhead Shark may swim over top of another and “hit” the shark below it with the edge of its head. The shark that was “hit” accelerates off and often bears a slight contusion in the area where it was struck. A study conducted on a captive colony of Bonnethead Hammerhead Sharks demonstrated that this species forms linear dominance hierarchies, with the size & sex of an individual determining its position in the “pecking order”. As the group swarm about their enclosure, subordinate individuals would “give way” to dominate specimens that is, they would change their course to avoid colliding with the dominant shark. Bonnethead Hammerhead Sharks are found in North Carolina to Brazil in the Western Atlantic Ocean and from southern California to Ecuador in the Eastern Pacific Ocean. This shark occurs in many different habitats within the temperate & tropical waters of its range. It is abundant surf zone, bays & estuaries, on coral or rocky reefs and over sandy or muddy bottoms. It also inhabits waters of the continental shelf to depths of about 260 feet.  The life span of this species has been estimated at 8-12 years.

Bonnethead Hammerhead Sharks are viviparous (giving birth to live young) with a gestation period of 4 to 5 months, which is the shortest gestation period of all other sharks. Females reach sexual maturity when about 2.5 feet long. They retreat to shallow bays and estuaries to give birth, in late summer or early fall. Litter sizes average of 8 to 16 pups and each approximately 14 inches in length. 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. This species must swim continuously so that its gills receive oxygen from the water otherwise it will sink and drown. Bonnethead Hammerhead Sharks swim continuously traveling long distances following changes in water temperature. Considered harmless to humans, this species is shy with only been one recorded unprovoked attack.

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