The bonnethead (''Sphyrna tiburo''), also called a bonnet shark or shovelhead, is a small member of the

hammerhead shark The hammerhead sharks are a group of sharks that form the family Sphyrnidae, so named for the unusual and distinctive structure of their heads, which are flattened and laterally extended into a "hammer" shape called a cephalofoil. Most hammerhe ...

genus ''

Sphyrna ''Sphyrna'' (from the Greek word σφῦρα, "hammer") is a genus of hammerhead sharks with a cosmopolitan distribution in the world's oceans. Members of ''Sphyrna'' have a tendency to inhabit coastal waters along the intertidal zone rather th ...

'', and part of the family Sphyrnidae. It is an abundant species in the

littoral zone The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal a ...

of the North Atlantic and the

Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin and a marginal sea of the Atlantic Ocean, largely surrounded by the North American continent. It is bounded on the northeast, north and northwest by the Gulf Coast of the United ...

, is the only shark species known to display

sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most an ...

in the

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines * Morphology (archaeology), study of the shapes or forms of artifacts * Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies ...

of the head, and is the only shark species known to be

omnivorous An omnivore () is an animal that has the ability to eat and survive on both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize the nut ...

Description

The shark is characterized by a broad, smooth, spade-like head: it has the smallest cephalofoil (hammerhead) of all ''Sphyrna'' species. The body is grey-brown above and lighter on the underside. Typically, bonnethead sharks are about long, with a maximum size of about . The generic name ''Sphyrna'' derives from the Greek word for "hammer", referring to the shape of this shark's head; the specific name ''tiburo'' derives from the Spanish word "tiburón", meaning "shark". Sphyrna tiburo head.jpg, Head, underside Sphyrna tiburo head2.jpg, Head, upper side Sphyrna tiburo upper teeth.jpg, Upper teeth Sphyrna tiburo upper teeth posterior.jpg, Upper teeth, posterior Sphyrna tiburo lower teeth.jpg, Lower teeth Sphyrna tiburo lower teeth posterior.jpg, Lower teeth, posterior

Morphology

Sexual dimorphism

Bonnethead sharks are the only sharks known to exhibit sexual dimorphism in the morphology of the head. 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 In biology, a clasper is a male anatomical structure found in some groups of animals, used in mating. Male cartilaginous fish have claspers formed from the posterior portion of their pelvic fin which serve to channel semen into the female's c ...

cartilages.

Pectoral fins and swimming

The

pectoral fins Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as see ...

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, 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

mtDNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial DNA ...

analysis, a scientist has found that the evolution of hammerhead sharks probably began with a taxon that had a highly pronounced cephalofoil (most likely that similar to the winghead shark, ''Eusphyra blochii''), and was later modified through selective pressures. Thus, judging by their smaller cephalofoil, bonnethead sharks are the more recent developments of a 25-million-year evolutionary process.

Distribution and habitat

This species occurs on both sides of the American coast, in regions where the water is usually warmer than . In the

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...

, it ranges from

New England New England is a region comprising six states in the Northeastern United States: Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont. It is bordered by the state of New York (state), New York to the west and by the Can ...

, where it is rare, to the

and

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

, and in the

Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the contine ...

it ranges from southern

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

to northern

Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = National seal , national_motto = "Firm and Happy f ...

. During the summer, it is common in the inshore waters of

the Carolinas The Carolinas are the U.S. states of North Carolina and South Carolina, considered collectively. They are bordered by Virginia to the north, Tennessee to the west, and Georgia to the southwest. The Atlantic Ocean is to the east. Combining Nor ...

and

Georgia Georgia most commonly refers to: * Georgia (country), a country in the Caucasus region of Eurasia * Georgia (U.S. state), a state in the Southeast United States Georgia may also refer to: Places Historical states and entities * Related to the ...

; in spring, summer, and fall, it is found off

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

and in the Gulf of Mexico. In the winter, the bonnethead shark is found closer to the equator, where the water is warmer. While it is still abundant in the North Atlantic Ocean and Gulf of Mexico, it has become significantly less common in the

Caribbean Sea The Caribbean Sea ( es, Mar Caribe; french: Mer des Caraïbes; ht, Lanmè Karayib; jam, Kiaribiyan Sii; nl, Caraïbische Zee; pap, Laman Karibe) is a sea of the Atlantic Ocean in the tropics of the Western Hemisphere. It is bounded by Mexico ...

, and has been nearly

extirpated Local extinction, also known as extirpation, refers to a species (or other taxon) of plant or animal that ceases to exist in a chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinct ...

from most of its

South Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...

and Pacific range. It frequents shallow estuaries and bays over seagrass, mud, and sandy bottoms.

Ecology

Behavior

The bonnethead shark is an active

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

shark that swims in small groups of five to 15 individuals, although schools of hundreds or even thousands have been reported. They 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 Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...

of marine

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

Diet

The shark feeds primarily on

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...

s, consisting mostly of

blue crab Blue crab may refer to: * Blue Crab 11, an American sailboat design * ''Callinectes sapidus'' – Chesapeake or Atlantic blue crab of the West Atlantic, introduced elsewhere * '' Cardisoma guanhumi'' – blue land crab of the West Atlantic * '' Dis ...

s, but also shrimp,

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s, 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 In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions o ...

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. 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). Bonnetheads also ingest large amounts of

seagrass Seagrasses are the only flowering plants which grow in marine environments. There are about 60 species of fully marine seagrasses which belong to four families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and Cymodoceaceae), all in the or ...

, 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. The shark may perform this activity to protect its stomach against the spiny carapaces of the

which it feeds on. A 2018 study with a carbon isotope-labelled seagrass diet found that they could digest seagrass with at least moderate efficiency, with 50±2% digestibility of seagrass organic matter, and had

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell w ...

-component-degrading enzyme activity in their hindgut.

Reproduction

The bonnethead shark is

viviparous Among animals, viviparity is development of the embryo inside the body of the parent. This is opposed to oviparity which is a reproductive mode in which females lay developing eggs that complete their development and hatch externally from the ...

. Females reach sexual maturity around , while males reach maturity around . Four to 12 pups are born in late summer and early fall, measuring . Bonnetheads have one of the shortest gestation periods among sharks, lasting only 4.5–5.0 months. A bonnethead female produced a pup by

parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...

. The birth took place at the

Henry Doorly Zoo Omaha's Henry Doorly Zoo and Aquarium is a zoo in Omaha, Nebraska. It is accredited by the Association of Zoos and Aquariums and a member of the World Association of Zoos and Aquariums. Its mission is conservation, research, recreation, and educ ...

in Nebraska; DNA analysis showed a perfect match between mother and pup.

Conservation

The bonnethead was formerly classified as a

least-concern species A least-concern species is a species that has been categorized by the International Union for Conservation of Nature (IUCN) as evaluated as not being a focus of species conservation because the specific species is still plentiful in the wild. T ...

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, but is still reasonably abundant there as well as in the Atlantic Coasts of

the Bahamas The Bahamas (), officially the Commonwealth of The Bahamas, is an island country within the Lucayan Archipelago of the West Indies in the North Atlantic. It takes up 97% of the Lucayan Archipelago's land area and is home to 88% of the ar ...

and

Mexico Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...

. However, significant declines have been reported in the

and Atlantic

Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...

and massive declines along with widespread disappearance on the Atlantic Coast of

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...

as well as most of the shark's

range, leading it to be uplifted to '

Endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and in ...

" in 2020. Since October 2021, ''S. tiburo'' has been classified as Largely Depleted by the IUCN.

References

External links

*
Species Description of Sphyrna tiburo at www.shark-references.com
* {{Taxonbar, from=Q338891 Sphyrna Viviparous fish Fish of the Americas Fish of the Dominican Republic Fish of the Caribbean Fish of the Western Atlantic Western American coastal fauna Fish described in 1758 Taxa named by Carl Linnaeus