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, named for the unusual and distinctive form of their heads, which are flattened and laterally extended into a Hammerhead shark#Cephalofoil, cephalofoil (a T-shape or " ...

genus ''

Sphyrna ''Sphyrna'' 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 than the open ocean, as their prey such as inv ...

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

littoral zone The littoral zone, also called litoral 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 flood ...

of the

North Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...

and the

Gulf of Mexico The Gulf of Mexico () is an oceanic basin and a marginal sea of the Atlantic Ocean, mostly surrounded by the North American continent. It is bounded on the northeast, north, and northwest by the Gulf Coast of the United States; on the southw ...

, is the only shark species known to display

sexual dimorphism Sexual dimorphism is the condition where sexes of the same species exhibit different Morphology (biology), morphological characteristics, including characteristics not directly involved in reproduction. The condition occurs in most dioecy, di ...

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 regularly consumes significant quantities of both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize ...

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'' probably derives from a misspelling of '' sphyra '', the Greek word for "hammer"; the specific name ''tiburo'' derives from the Spanish word , 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 Rostral may refer to: Anatomy * Rostral (anatomical term), situated toward the oral or nasal region * Rostral bone, in ceratopsian dinosaurs * Rostral organ, of certain fish * Rostral scale The rostral scale, or rostral, in snakes and other sca ...

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 ...

cartilages.

Pectoral fins and swimming

The

pectoral fins Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only b ...

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 and mDNA) is the DNA located in the mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in ...

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 oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...

, it ranges from

New England New England is a region consisting of 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 ...

, where it is rare, to the

and

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

, 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 the definition, to Antarctica in the south, and is bounded by the cont ...

it ranges from southern

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

to northern

Peru Peru, officially the Republic of Peru, is a country in western South America. It is bordered in the north by Ecuador and Colombia, in the east by Brazil, in the southeast by Bolivia, in the south by Chile, and in the south and west by the Pac ...

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

the Carolinas The Carolinas, also known simply as Carolina, 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 ...

and

Georgia Georgia most commonly refers to: * Georgia (country), a country in the South Caucasus * Georgia (U.S. state), a state in the southeastern United States Georgia may also refer to: People and fictional characters * Georgia (name), a list of pe ...

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

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

and in the Gulf of Mexico. In the winter, the bonnethead shark is found closer to the

equator The equator is the circle of latitude that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern Hemispheres of Earth, hemispheres. It is an imaginary line located at 0 degrees latitude, about in circumferen ...

, 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 is a sea of the Atlantic Ocean, North Atlantic Ocean in the tropics of the Western Hemisphere, located south of the Gulf of Mexico and southwest of the Sargasso Sea. It is bounded by the Greater Antilles to the north from Cuba ...

, and has been nearly

extirpated Local extinction, also extirpation, is the termination of a species (or other taxon) in a chosen geographic area of study, though it still exists elsewhere. Local extinctions are contrasted with global extinctions. Local extinctions mark a chan ...

from most of its

South Atlantic The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for ...

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, where the sun may shine directly overhead. This contrasts with the temperate or polar regions of Earth, where the Sun can never be directly overhead. This is because of Earth's ax ...

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 the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

of marine

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

Diet

The shark feeds primarily on

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

s, consisting mostly of

blue crab Blue crab may refer to: * Blue Crab 11, an American sailboat design * ''Callinectes sapidus ''Callinectes sapidus'' (from the Ancient Greek language, Ancient Greek ,"beautiful" + , "swimmer", and Latin , "savory"), the blue crab, Atlantic blue ...

s, but also

shrimp A shrimp (: shrimp (American English, US) or shrimps (British English, UK)) is a crustacean with an elongated body and a primarily Aquatic locomotion, swimming mode of locomotion – typically Decapods belonging to the Caridea or Dendrobranchi ...

mollusk Mollusca is a phylum of protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant species of molluscs are recognized, making it the second-largest animal phylum after Arthropoda. The ...

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 via electromagnetic fields. The electromagnetic force is one of the four fundamental forces of nature. It is the dominant force in the interacti ...

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 A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

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 (ocean), marine environments. There are about 60 species of fully marine seagrasses which belong to four Family (biology), families (Posidoniaceae, Zosteraceae, Hydrocharitaceae and ...

, 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 chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...

-component-degrading enzyme activity in their hindgut.

Reproduction

The bonnethead shark is

viviparous In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juve ...

. Females reach sexual maturity around , while males reach maturity around . Four to twelve 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 + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

. 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. In August 2014, TripAdvisor rated it the "world's best zoo" ...

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 evaluated and categorized by the International Union for Conservation of Nature (IUCN) as not being a focus of wildlife conservation because the specific species is still plentiful in the wil ...

by the

IUCN The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the status ...

. 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 Archipelagic state, archipelagic and island country within the Lucayan Archipelago of the Atlantic Ocean. It contains 97 per cent of the archipelago's land area and 88 per cent of ...

and

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

. However, significant declines have been reported in the

and Atlantic

Central America Central America is a subregion of North America. Its political boundaries are defined as bordering Mexico to the north, Colombia to the southeast, the Caribbean to the east, and the Pacific Ocean to the southwest. Central America is usually ...

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 considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

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, inv ...

" 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