Sphyrna lewini

Geographic Range

Scalloped hammerheads are pelagic sharks that are found worldwide in tropical warm water regions, including the Atlantic, Indian, and Pacific oceans, between 40°N to 36°S lattitude and 80°W to 80°E longitude.

• Biogeographic Regions
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• cosmopolitan cosmopolitan

Habitat

Although they primarily inhabit open marine waters, scalloped hammerheads can also be found near continental and island shelves and often enter bays and estuaries as well.

Scalloped hammerheads utilize different habitats daily. At dawn, they move from their offshore hunting grounds to island shelves, seamounts, and enclosed bays and estuaries. During the day they migrate to drop-off zones and reefs where females form schools for the purpose of social interaction. At dusk the sharks return to offshore pelagic areas and actively search for food.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• pelagic pelagic
• benthic benthic
• reef reef
• coastal coastal

• Other Habitat Features
• estuarine estuarine

Physical Description

Scalloped hammerheads have a small but prominent notch in the center of their characteristically-shaped head, which also bears pronounced grooves along its anterior margin, giving this species its common name. The side “wings” of the head are relatively narrow and are swept backwards. The first dorsal fin is larger than the second and is sickle-shaped, with a rounded tip, while the second dorsal fin has a concave rear edge. Both the pectoral and dorsal fins are slightly rounded. The tips of the pectoral fins are either a dark grey or black. The caudal fin is heterocercal and forked. Body coloration in this species displays typical pelagic countershading, with a grey dorsum and a white ventral surface.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger

Development

Fertilized eggs develop into embryos that are nourished by a yolk-sac placenta attached to either of the female's two uteruses. Scalloped hammerheads give birth to live young that are miniature versions of their parents.

In the Gulf of California, female hammerheads were discovered at a depth of 50 m, whereas males of a similar size were caught at 25 m. It has been suggested that females may grow more rapidly and mature at a larger size than do males due to differences in food availability between these habitats.

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

Mating behavior in scalloped hammerheads is not fully understood, but it has been observed that during mating season, sexually mature males migrate to deeper waters in search of females. Since schools are formed primarily of females, male sharks will enter and swim in an S shape, signaling the desire to mate. Larger and more sexually mature females tend to be located in the center of the schools, pushing smaller females to the outside. When a male locates a receptive female, he bites her pectoral fin and secures himself. During mating, he curls his tail around her body to align their genitalia and inserts one of his claspers (modified anal fins) into her urogenital opening to deposit sperm. Because scalloped hammerheads are negatively buoyant, the mating pair sinks while copulation occurs.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Reproductive maturity of female sharks can sometimes be determined through morphological measurements. Depending on the location of the shark (Northern or Southern hemisphere), some female sharks can begin reproduction anywhere from 15 to 17 years old, which corresponds to a total length of over 2 meters. Male sharks reach sexual maturity at 6 years old, and around 1.5 meters in length. In Northern Australia, the breeding season occurs from February to March, although this is likely to vary according to geographic region. After a gestation period of 9 to 10 months, litters of 12 to 38 young measuring 38 to 45 cm are born live. There is no published information regarding the birth mass of pups, which is due to the fact that pregnant females are rarely caught. Immediately after birth, pups must find their own food, but they usually live in large schools until adulthood.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• viviparous viviparous

As in other viviparous species, female scalloped hammerheads provide nutrition and protection to their internally developing young. As in other sharks, there is no parental care after birth.

• Parental Investment
• female parental care female parental care
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

The oldest known scalloped hammerhead was 31.5 years old, and was caught in the Atlantic Ocean, off the coast of Brazil. The authors of that study estimated the maximum lifespan of this species to be 55 years. In a separate study conducted off the coast of Sinoloa, Mexico, the oldest individual sampled was approximately 10.5 years old. There is no published information available for the expected lifespan of this species in captivity.

Behavior

Both male and female adults may be found alone, in pairs, or in schools. However, most females are found in sexually segregated schools with larger, more mature females in the center. Young scalloped hammerheads live in large schools until they reach maturity. Adult males almost exclusively enter schools when in search of a mate.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• nocturnal nocturnal
• crepuscular crepuscular
• motile motile
• migratory migratory
• solitary solitary
• social social

Home Range

Scalloped hammerheads appear to regularly move through a reasonably well-defined area as a result of their diel movement patterns. However, estimates for the total area encompassed by these movements are currently unavailable. It is also thought that this species undertakes longer seasonal migrations, so classification of their daily movement areas as true home ranges may be incorrect.

Communication and Perception

Like all sharks, scalloped hammerheads use their eyes for visual perception, and nares for detection of chemical cues. Like other elasmobranchs, they have special sensory organs called Ampullae of Lorenzini; these special electroreceptors are located on the underside of the snout and "hammer", and aid in the detection of buried prey, as well as navigation (by sensing changes in the Earth's magnetic field). Pacific scalloped hammerheads are able to follow geological fault lines along the ocean floor. Recently cooled magma forms rocks that have a strong magnetic signal, which the animals follow to the locations where they form congregations to feed, school, and mate.

Sharks are able to visually display aggression, submission, and dominance to conspecifics. When provoked or threatened, an individual will point its pectoral fins downward and arch its back. Sharks indicate submission by rapidly shaking their head side to side. Dominant sharks exhibit a “corkscrewing”, or upward swimming pattern, and will ram submissive sharks with their snout.

• Communication Channels
• visual visual

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• vibrations vibrations
• chemical chemical
• electric electric
• magnetic magnetic

Food Habits

Younger individuals tend to feed in coastal waters, on benthic and neritic fish. Adults live in deeper oceanic waters, feeding on neritic and epipelagic fishes and cephalopods (squid, octopus and cuttlefishes), lobsters, shrimps, and crabs, as well as various smaller sharks and rays. Hammerheads located in the Indo-West Pacific have been observed to prey on sea snakes. Scalloped hammerheads actively forage for food during the night, using a “smash and grab” feeding technique. They accelerate towards prey and either swallow it whole, or disable prey by biting it. Great hammerheads ( Sphyrna mokarran ) have been observed using their hammer-like head to pin stingrays to the ocean floor before consuming them, and it is thought that scalloped hammerheads may use the same technique.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • eats non-insect arthropods
  • molluscivore molluscivore

• Animal Foods
• reptiles
• fish
• mollusks
• aquatic crustaceans
• other marine invertebrates

Predation

While scalloped hammerheads are one of the largest reef fishes and a top predator, they are occasionally preyed on by larger tiger sharks ( Galeocerdo cuvier ), great white sharks ( Carcharodon carcharias ), and killer whales ( Orcinus orca ). Humans also harvest hammerheads for their fins, which are sold in the lucrative global shark fin trade.

Scalloped hammerheads form schools that consist mainly of females, with males rarely entering these aggregations. This behavior lowers an individual’s chance of being attacked by predators. Scalloped hammerheads also exhibit countershading as a form of camouflage in open water, with a grey to bronze colored dorsum and a pale white/grey underside.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Scalloped hammerheads are among the largest of reef sharks, making them top predators of the animals on which they prey.

Scalloped hammerheads have mutualistic symbioses with several species of cleaner fish, allowing wrasses and blennies to remove external parasites from their skin, gills, and mouths.

This species is a host to several types of ectoparasitic invertebrates. At least one leech and over half a dozen copepod species have also been found on the underside of scalloped hammerhead snouts. In addition, this species plays host to at least two different types of platyhelminth endoparasites.

Economic Importance for Humans: Positive

Scalloped hammerheads serve as an important resource for traditional fisheries in Mexico. They are fished both commercially and recreationally, and highly valued for their fins. While their meat has a high concentration of urea and is not typically consumed, the skin is used for leather and oil is obtained from the liver. Their jaws and teeth are also sold as marine curios.

• Positive Impacts
• food food
• body parts are source of valuable material
• source of medicine or drug source of medicine or drug
• research and education

Economic Importance for Humans: Negative

Large hammerhead sharks are considered to pose a potential danger to humans. To date, however, there have been no confirmed attacks on humans by this species.

• Negative Impacts
• injures humans
  • bites or stings

Conservation Status

Scalloped hammerheads are listed as an endangered species by the IUCN Red List, since they are vulnerable to illegal fishing and bycatch during all stages of their lives, making them susceptible to extinction in the future. In 2007, surveys in the northwest Atlantic documented a 98 percent decline from historical estimates, while a 90 percent decline in population abundance was noted in the southwest Atlantic.

As of 2012, hammerheads made up approximately 6% of the identified fins entering the Hong Kong market, translating to an estimated 1.3 million to 2.7 million hammerheads being exploited annually for the global fin trade. This level of exploitation is unlikely to be sustainable and can have substantial negative impacts on marine ecosystems worldwide.

The International Commission for the Conservation of Atlantic Tunas (ICCAT) has banned the retention of hammerhead sharks caught while tuna fishing, but this measure does not address the impact of the lucrative shark fin trade. The U.S.A. passed the Shark Conservation Act of 2010 to prevent finning with US waters.

Additional Links

Encyclopedia of Life

Contributors

Erick Ruiz (author), San Diego Mesa College, Sierra Trujillo (author), San Diego Mesa College, Paul Detwiler (editor), San Diego Mesa College, Jeremy Wright (editor), University of Michigan-Ann Arbor.

References

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