Although a type of hammerhead shark, the bonnetheads have a much smaller, shovel-shaped head, but it does have the classic shark-shaped body and distinctly taller first dorsal fin of other hammerheads. They are gray-brown above, fading to white underneath, and grow to around 48 inches long but rarely as much as 59 inches. These warm-water coastal sharks migrate with the seasons, and have excellent senses for hunting and communication with other bonnetheads.
Order – Carcharhiniformes
Family – Sphyrnidae
Genus – Sphyrna
Species – tiburo
English language common names include bonnethead, bonnet hammerhead, bonnet shark, bonnethead shark, bonnetnose shark, and shovelhead. Other common names throughout the world include cabeza de pala (Spanish), cação (Portuguese), cação-chapéu (Portuguese), cachona (Spanish), cambeva-pata (Portuguese), cornuda de corona (Spanish), cornuda tiburo (Spanish), cornudo de corona (Spanish), kaphamerhaai (Dutch), kleiner hammerhai (German), lopatoglów (Polish), martelo (Portuguese), pata (Portuguese), peixe-martelo (Portuguese), pex martillo (Spanish), requin-marteau tiburo (French), rodela (Portuguese), sarda cachona (Spanish), tiburón bonete del Pacífico (Spanish), and uchiwa-shumokuzame (Japanese).
Importance to Humans
Bonnetheads are a common inshore shark and are often taken by small fisheries with shrimp trawls, nets, longlines, and hook-and-line. The flesh is marketed as fresh, fresh frozen, or dried salted for human consumption as well as processed into fishmeal. Although it is marketed, this species if of little economic importance.
Recreationally, bonnetheads can provide great sport on light tackle or fly fishing gear. They are often found on shallow water flats and caught on live and cut bait including crabs.
Danger to Humans
Considered harmless to humans, this species is rather shy. There has been only one recorded unprovoked attack attributed to the bonnethead.
Currently, this species is categorized as a species of “Least Concern” due to its high population numbers by the World Conservation Union (IUCN). The IUCN is a global union of states, governmental agencies, and non-governmental organizations in a partnership that assesses the conservation status of species.
The bonnethead is limited to warm waters of the Northern Hemisphere, ranging in the Atlantic Ocean from New England (U.S.) (rare) south to the Gulf of Mexico and Brazil. It is common throughout the Caribbean Sea including Cuba and Bahamas. This shark is rare in Bermuda. In the Pacific, this shark can be found from southern California to waters off of Ecuador.
Summertime finds the bonnethead commonly residing in the inshore waters off the Carolinas and Georgia (U.S.) while during the spring, summer, and autumn it is found off the coast of Florida and in the Gulf of Mexico. Bonnetheads move closer to the equator, as waters grow colder during the winter months.
Bonnetheads reside on continental and insular shelves, over reefs, estuaries and shallow bays from depths of 32-262 feet (10-80 m). They usually occur in small schools of up to 15 individuals, however during migration events they are seen in groups of hundreds or thousands. As spawning time approaches, bonnetheads tend to group by gender. During pupping season; females predominate in shallow waters where they give birth.
Bonnetheads travel long distances everyday, following changes in the water temperature. This preference for water temperatures over 70°F (21°C) leads to migrations to warmer waters during the winter months. As a result, the bonnethead is found closer to the equator during the winter, moving back to higher latitudes during the summer.
This species must swim continuously so that its gills receive oxygen from the water, otherwise it will sink. Although this shark is not territorial, it appears a hierarchy exists within groups of bonnetheads. Another interesting aspect of this species is a cerebrospinal fluid used in chemical communication among individual bonnetheads, informing others when there is a bonnethead in the area. Further studies are needed to learn more about this communication system.
1. Head is shovel-shaped with evenly rounded between the eyes
The shovel- or bonnet-shaped head is a distinguishing characteristic of this species, making it easy to identify among hammerhead sharks. The eyes are located at the ends of the evenly rounded lobes of the flattened head, increasing the field of vision. When the bonnethead swims, the head rolls from side to side. The arched mouth is located ventrally. The body is moderately compact and lacks a mid-dorsal ridge. The high first dorsal fin originates just behind the base of the pectoral fins. The second dorsal fin is slightly less than ½ as long as the base of the first dorsal fin with a slender free rear corner. The pectoral fins are short and the anal fin has only a slight indentation. The caudal fin has a nearly straight upper margin with a lower lobe about 1/3 as long as the upper lobe with a nearly straight rear edge. Bonnetheads lack air bladders and have strong digestive chemicals in their specialized intestines.
Bonnetheads can be distinguished from other species by the flattened, bonnet-shaped head that is rounded between the eyes rather than hammer-shaped. Also, the head lacks a notch at the midline. It is the smallest of the hammerhead-type (family Sphyrnidae) sharks, reaching an average of 3-4 feet in length in comparison to the scalloped hammerhead which grows to an average of 6 feet and the great hammerhead growing to almost 20 feet in length.
Coloration of the bonnethead ranges from gray to gray-brown, occasionally with a green tint. Dark spots are sometimes seen on the sides of the body. Viewed from the side, the color changes from top to bottom to a lighter gray and then white on the underside. There are no conspicuous markings on the fins.
Dentition of the bonnethead includes small, sharp teeth located in the front of the mouth used for cutting up prey and flat, large molars in the back for grinding hard prey items. The sharp front teeth have short, stout cusps lacking serrations, followed by teeth with oblique cusps and then the flat molars in the back of the mouth. As with all sharks, the bonnethead has additional rows of teeth that are used as the older teeth become lost or worn.
The dermal denticles are larger than those found in the smooth hammerhead (S. zygaena) and vary greatly in arrangement from closely overlapping to loosely spaced. The blades are steeply raised with 5 ridges and 5 sharp marginal teeth.
Size, Age, and Growth
Bonnetheads reach an average size of 36-48 inches (100-120 cm) with a maximum length of approximately 59 inches (150 cm), with females reaching greater lengths than males. The maximum recorded weight of a bonnethead is 24 pounds (10.8 kg). Males mature between 20-30 inches (52-75 cm) and females mature at 33 inches (84 cm) or less in length.
Bonnetheads feed during daylight hours primarily on crustaceans, dominated by blue crabs. They also feed on mantis shrimp (Squilla empusa), pink shrimp (Penaeus durorarum), mollusks, and small fishes. Occasionally bonnetheads will also feed on seagrasses as documented by stomach contents of some individuals. This species has been reported burrowing under coral heads in search of small fishes and invertebrates in the waters of southern Florida. Females tend to feed more often due to the need for increased amount of energy budgeted for reproductive efforts.
Prey items appear to be correlated with seasonality as well as habitat. Although crustaceans are the primarily food source throughout the year, during the autumn diversity of prey items increases with the inclusion of spider crabs (Libinia dubia), purse crabs (Persephona punctata), stone crabs (Menippe mercenaria), and various cephalopods including octopus. Bonnetheads residing inside bays feed on a less diverse array of prey items than those caught off beaches in open waters.
The bonnethead has evolved well-developed sensory and nervous systems that allow them to be efficient predators. Vision and hearing capabilities are exceptional as well as the sensitivity of the lateral line to small vibrations, alerting them to nearby potential prey.
Upon locating a prey item, the bonnethead swims slowly within range followed by a quick acceleration to attack that item. The item is then crushed with the molariform teeth. There are two jaw closing phases, continuing the closure of the jaws. This differs from the capture event typical 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.
In Florida waters, bonnetheads are believed to mate during the spring and autumn or perhaps even year-round. In the waters off the coast of Brazil, mating occurs during the spring. After mating, the females can store sperm for up to four months prior to actually fertilizing the eggs. The control over the fertilization period is believed to be an adaptation to ensure that the pups are born during optimal conditions for their survival.
Bonnetheads are “viviparous”, or live-bearing. Female bonnetheads produce eggs that are maintained and nourished by a yolk-sac during the initial phase of gestation.
The eggs withing the female are tough but elastic with folded ends allowing for growth of the embryos. The embryos released from the eggs absorb the yolk-sac. This sac attaches to the uterine wall of the mother forming a yolk-sac placenta. Blood vessels running through this placenta provide nourishment until birth of the embryo. Also, after hatching, sections of the uterine wall come together to separate each embryo and its placenta in its own uterine compartment. The gestation period, shortest among all sharks, is only four to five months.
Females move to shallow inshore waters during pupping season, giving birth in late summer and early fall. Litter sizes average 4-14 pups, each approximately 14 inches (35 cm) in length and 0.4 pounds (0.2 kg) in weight. 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.
Larger sharks are potential predators of the bonnethead.Parasites
The monogenean Erpocotyle tiburonis has been reported to cause gill lesions on bonnetheads. Other parasites include copepods such as Eudactylina longispina collected from the gill filaments of a bonnethead caught in Tampa Bay, Florida.
Karl Linnaeus first described this species as Sphyrna tiburo in 1758 after initially naming it Squalus tiburo. Synonyms referring to this species in past scientific literature include S. vespertina Springer 1940. The genus name Sphyrna is derived from the Greek word “sphyra” which translates as hammer. Bonnetheads are the smallest species in the family Sphyrnidae.
There are approximately ten related species of hammerheads throughout tropical and temperate regions including the scalloped hammerhead (Sphyrna lewini), great hammerhead (Sphyrna mokarran), and smooth hammerhead (Sphyrna zygaena).
Prepared by: Cathleen Bester