Sphyrna tiburoBonnet shark

Geographic Range

The bonnethead is confined to the warm waters of the western hemisphere. It ranges from New England, where it is rare, to the Gulf of Mexico and Brazil and from southern California to Ecuador. It is common in the inshore waters of the Carolinas and Georgia in summer, and off Florida and in the Gulf of Mexico in spring, summer, and fall (Castro, 1987).

Habitat

S. tiburo is believed to migrate southward in winter or to deeper offshore waters in Florida and the Gulf of Mexico, but little is known about its movements. It often travels in schools of five to fifteen individuals. Migrating schools of hundreds and even thousands of these sharks have been reported (Parsons1993).

Physical Description

The characteristic shovel- or bonnet-shaped head makes this hammerhead the easiest to identify. Body moderately compact; head flattened, spade-shaped front margin of head not lobed without nasal grooves, the anterior margin of the head is evenly rounded between the eyes; mouth arched; mouth corners posterior to oculonarial expansion. The frontal teeth have erect, smooth-edged cusps, while subsequent teeth have oblique cusps; the outermost teeth of the lower jaw are modified into flat crushers. First dorsal fin high, originating slightly posterior to base of pectoral fin; second dorsal with rear lobe not well developed, higher and shorter than anal one; eyes separated from nasal grooves by a distance of 1.5 times diameter of eye. Some are dark brown in the lateral dorsal region, lighter in ventral region while others are gray or greensih gray above and paler below. Average size is 70-100 cm (28-39 in.) Maximum size is about 110 cm (43 in.) These are the smallest of family Sphyrnidae (Cortes and Parsons,1996).

  • Range mass
    3 to 4 kg
    6.61 to 8.81 lb
  • Average mass
    0.0035 kg
    0.01 lb

Reproduction

S. tiburo reproduces sexually and is viviparous. Maturity is reached at about 75 cm (30 in). The pups are born in late summer and early fall and measure 30-32 cm (12-13 in) at birth and approximately 172 g. Usually eight to twelve pups are produced in each litter.

Survivorship for young individuals, especially newborn pups, may be affected by size-selective predation.

Geographic variation does seem to have an effect on the survival of the pups, as well as the weight and size. In Florida Bay and Tampa Bay there was a study of two populations of bonnethead sharks. Size at maturation, age at maturation, time of fertilization, rate of embryonic development, size at birth, the energetic investment in producing offspring, gestation period, and the incidence of fertility were found to differ between these population. Food limitations and seasonal differences may play a role in these differences (Castro, 1987).

Behavior

A special body fluid, cerebrospinal fluid, is used for personal communication in S. tiburo. It is also known as the Cl- excess. This chemical allows S. tiburo to let others know it is in the area. Not much is known about the cerebrospinal fluid of these sharks because not enough studies have been done on this species (Rall,1967).

There are few data on how the bonnethead shark moves as well as few data about the mating behavior.

This species must remain in motion at all times in order to survive. Sharks breathe through their gills and in order to receive oxygen they must force their gills open by swimming. Also, they sink if they do not keep moving (Rall 1967).

Food Habits

The diet of S. tiburo is dominated by crustaceans, consisting mostly of blue crabs, but also it feeds upon shrimp, mollusks, and small fishes. Seagrasses were also found in stomach contents.

It was found that stomach content weight of females was higher than that of males, probably because females, due to reproduction, have a higher energy budget. They need to feed more in order to store energy for when they reproduce (Cortes , Manire, and Hueter,1996).

Economic Importance for Humans: Positive

This species is used commercially as dried meat similar in quality to second-class cod; the meat is dark and frequently is consumed fresh. However, even though marketed, it is of little or no economic importance to man.

Economic Importance for Humans: Negative

Only one attack on humans by this species has been recorded, and it is generally considered to be harmless.

Conservation Status

Contributors

Shauna K. Hessing (author), University of Michigan-Ann Arbor.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

coastal

the nearshore aquatic habitats near a coast, or shoreline.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

References

Castro, J. 1987. The Sharks of North American Waters. Texas A&M University Press, USA.

Cortes, E. and Glenn R. Parsons. 1996. Canadian Journal of Fisheries and Aquatic Sciences. National Research Council of Canada, Canada.

Cortes, E., Manire, C. and Robert E. Hueter. 1996. Bulletin of Marine Science, 58(2): 353-367. Roesential School of Marine and Atmospheric Science, Mississippi.

Kobayaski, D. 1994. Fisheries Research, Amsterdam, The Netherlands.

Parsons, G.R. 1990. Marine Biology, 104(3): 363-367. Springer International, Mississippi.

Parsons, G.R. 1993. Marine Biology, 117: 23-31. Springer International, Mississippi.

Parsons, G.R. 1993. Environmental Biology of Fishes, 38: 25-35. Kluwer Academic, Printed in the Netherlands.

Rall, G. 1967. Sharks, Skates, and Rays. Johns Hopkins Press, Baltimore, Maryland.