Bonnethead

Bonnethead shark

The **bonnethead shark** or **shovelhead** (*Sphyrna tiburo*) is the smallest member of the hammerhead shark genus *Sphyrna*, and part of the family Sphyrnidae. It is an abundant species on the American littoral (intertidal zone near shore), and is the only shark species known to display sexual dimorphism in the morphology of the head.

## Description

Characterized by a broad, smooth, spade-like head, the bonnethead has the smallest cephalofoil (hammerhead) of all *Sphyrna* species. The body is grey-brown above and lighter on the underside. On average, bonnethead sharks are about 2–3 ft (0.61–0.91 m) long, with a maximum size of about 5 ft (150 cm). Females tend to be larger on average than males.

The Greek word *sphyrna* translates as "hammer", referring to the shape of this shark's head; *tiburo* is the Taino word for shark.

### Morphology

#### Sexual Dimorphism

Bonnethead sharks are the only sharks known to exhibit sexual dimorphism in the morphology of the head. Adult female bonnethead sharks 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.

#### Pectoral fins and Swimming capabilities

The pectoral fins on most fish control pitching (up-and-down motion of the body), yawing (the side-to-side motion), and rolling. Most hammerhead sharks do not yaw or roll and achieve pitch using their cephalofoils. The smaller cephalofoil of a bonnethead shark is not as successful at controlling pitch, so they have to rely on the combination of cephalofoils and their large pectoral fins for most of their motility. Compared to other hammerheads, bonnethead sharks have larger and more developed pectoral fins and are the only species of hammerhead to actively use pectoral fins for swimming.

## Evolution

Using data from mitochondrial DNA analysis, scientist have found that the evolution of hammerhead sharks has probably begun with a taxon that had a highly pronounced cephalofoil (most likely that similar to the winghead shark, *Eusphyra blochii*), and has later been modified through selective pressures. Thus, judging by their smaller cephalofoils, bonnethead sharks are the most recent developments of a 25-million-year evolutionary process.

## Distribution and Habitat

Bonnetheads occur off the American coast, in regions where the water is usually warmer than 70 °F (21 °C). It ranges from the area around 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. The bonnethead frequents shallow estuaries and bays over grass, mud and sandy bottoms, a primary and plentiful hunting ground.

## Ecology

### Behavior

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 following 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.

### Diet

The bonnethead 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. Buried fish are extracted and swallowed whole. Bonnetheads have also been found to ingest large amounts of seagrass, which may make up around 50% of their stomach contents. This may be related to incidental ingestion and learning how to hunt, as the phenomenon is mostly observed in pups and juveniles. However, the species may potentially be omnivorous, as they appear to be able to digest about 56% of the matter in seagrass. This would constitute the only known case of plant feeding in sharks.

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).

### Reproduction

The bonnethead shark is viviparous. Females reach sexual maturity at about 32 in, while males reach maturity at around 24 in. Four to 12 pups are born in late summer and early fall, measuring 12 to 13 inches (330 mm).

Bonnetheads have one of the shortest gestation periods among all known sharks, lasting only 4.5–5 months.

A bonnethead female produced a pup by parthenogenesis. The birth took place at the Henry Doorly Zoo in Nebraska and subsequent DNA analysis has shown a perfect match between mother and pup.

## Conservation

The bonnethead is an abundant species and is currently classified as a least-concern species by the IUCN. It is heavily targeted by commercial and recreational fisheries, and constitutes up to 50% of all small shark landings in the Eastern US.

## In Popular Culture

Bonnetheads are well known to the public, as they are frequently kept in public aquariums. Their small size and adaptable nature make them suitable for captivity. Experienced Private aquarists also may occasionally keep them. Bonnethead sharks also appear in many documentaries on sharks, but their most notable appearance was during the Shark Week episode Hammerheads: Nomads of the Sea, in which a female bonnethead was filmed giving birth to a litter of four pups.