The Bonnethead shark (Sphyrna tiburo) is a small member of the Hammerhead shark genus Sphyrna, and part of the family Sphyrnidae. It is an abundant species on the American coastal areas and it is the only shark species known to display sexual dimorphism in the morphology of the head, and also is the only shark species known to be omnivorous.
Common Name– Ground Sharks
Common Name– Hammerhead Sharks
Status: IUCN Red List Least Concern
Average Size and Length: Typically, Bonnethead sharks are about 2–3 feet long, reaching a maximum size of about 5 feet. Females tend to be larger than males.
Average Weight: 13 pounds, with a recorded maximum of 24 pounds.
Teeth and Jaw: The Bonnethead shark has 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.
Head: The Bonnethead shark has a small head and is broadly widened into the shape of a shovel. The front of the head is not notched at the midline. It has the smallest cephalofoil of all Hammerhead sharks.
Demographic, Distribution, Habitat, Environment and Range: The Bonnethead shark can be found off the American coast, in regions 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. Bonnethead sharks are found here in Florida and can live among the mangroves. Check out this clip of a Bonnethead shark swimming among the Florida mangroves.
The Bonnethead shark frequents shallow estuaries and bays over grass, mud, and sandy bottoms. It also inhabits waters of the continental shelf to depths of about 260 feet. They can also be found over coral reefs.
Diet: The Bonnethead shark feeds primarily on crustaceans, consisting mostly of blue crabs, but also shrimp, mollusks, and small fish. 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 feed during the day.
Ram-Suction Index: Combination. Upon locating prey, the Bonnethead 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 and biting ceases at jaw closure. This allows the Bonnethead 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 esophagus.
Aesthetic Identification: Bonnethead sharks are grey-brown on the top with small dark spots, and white counter shading on the bottom. The first dorsal rear tip is in front of the pelvic fin origins. It has a shallowly concave posterior anal fin margin.
Biology and Reproduction: The Bonnethead shark is the only shark species known to display sexual dimorphism in the morphology of the head and is the only shark species known to be omnivorous. Adult females 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.
The Bonnethead shark is viviparous. Females reach sexual maturity at about 2.8 feet, while males reach maturity around 2 feet. 4 to 12 pups are born in late summer and early fall, measuring 12 to 13 inches (300 to 330 mm). They live between 8-12 years.
Bonnethead sharks have one of the shortest gestation periods among sharks, lasting only 4.5–5 months.
Behavioral Traits, Sensing and Intelligence: The Bonnethead shark is an active tropical shark that swims in small groups of five to 15 individuals, although schools of hundreds or even thousands have been reported. Bonnethead sharks move constantly succeeding changes in water temperature and to maintain respiration. The Bonnethead shark sinks if it does not keep moving, since hammerhead sharks are among the most negatively buoyant of marine vertebrates.
The vision and hearing capabilities of Bonnethead sharks are exceptional as well as the sensitivity of the lateral line to small vibrations, alerting them to nearby potential prey.
A study conducted on a captive colony of Bonnetheads demonstrated that this species forms linear dominance hierarchies, with the size and 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.
Some Bonnthead sharks exhibit specific types of behavior toward other sharks, including patrolling, head-shaking, jaw-snapping, hitting, and hunching. Research suggests that the purpose of this behavior is to establish and maintain dominance and other agonistic relationships. Another example is that an individual Bonnethead shark may swim over the top of another and “hit” the shark below with the edge of its head. The shark that was “hit” speeds off.
Speed: Varied from about 29 to 67 cm s−
Bonnethead Shark Recorded Attacks on Humans: 1 recorded, unprovoked attack.