Scalloped hammerhead

The scalloped hammerhead was first named Zygaena lewini and then renamed Sphyrna lewini by Edward Griffith and Hamilton Smith in 1834. It has also been named Cestracion leeuwenii by Day in 1865, Zygaena erythraea by Klunzinger in 1871, Cestracion oceanica by Garman in 1913, and Sphyrna diplana by Springer in 1941. Sphyrna comes from the Greek and translates to hammer. It is a sister species to Sphyrna gilberti, differing by the number of vertebrae. Though once considered a distinct species, McEachran and Serret synonymized Sphyrna couardi with Sphyrna lewini in 1986. These sharks are classified as ground sharks in the order Carcharhiniformes. ==Description==

The scalloped hammerhead is easily distinguished from other hammerhead sharks by the central indentation on the anterior margin of the head. The maximum length of the scalloped hammerhead is and the maximum weight is , per FishBase. A female caught off Miami was found to have measured and reportedly weighed , though was in a gravid state then. These sharks have a very high metabolic rate, which governs behavior in acquiring food. They occupy tertiary trophic levels. The scalloped hammerhead shark, like many other species, uses the shore as a breeding ground. The dentition of these sharks consists of small, narrow, triangular teeth that have smooth edges, except in larger fish, which may have somewhat serrated teeth. The teeth in the front of the mouth have erect cusps, and the teeth in the bottom jaw have cusps that are more erect than those of the upper jaw. File:Sphyrna lewini upper teeth.jpg|Upper teeth File:Sphyrna lewini lower teeth.jpg|Lower teeth File:Sphyrna lewini juvenile.jpg|Juvenile File:Sphyrna lewini young.jpg|Juvenile ==Distribution and habitat==

The scalloped hammerhead is a coastal pelagic species; it occurs over continental and insular shelves and in nearby deeper water. It is found in warm temperate and tropical waters, worldwide from 46°N to 36°S. It can be found down to depths over , but is most often found above . During the day, they are more often found close to shore, and at night, they hunt further offshore. Adults are found alone, in pairs, or in small schools, while young sharks occur in larger schools. Here, female sharks give birth to live young; juveniles remain in the shallow root system of the mangroves for around three years. After this time they leave Golfo Dulce and migrate back to Cocos Island, to feed in pelagic waters. ==Behavior==

Schooling These sharks are often seen during the night, day, and morning in big schools, sometimes numbering hundreds, most likely because large groups can obtain food more easily than singles or small groups, especially larger and trickier prey, as commonly seen. The younger the sharks, the closer to the surface they tend to be, while the adults are found much deeper in the ocean. They are not considered dangerous and are normally not aggressive towards humans. Sexual dimorphism The female scalloped hammerheads migrate offshore at a smaller size than males because the larger classes of the hammerhead, such as those from long, travel deeper down. Unlike females, males reach sexual maturity at a much smaller size. The male-to-female ratio of the scalloped hammerhead is 1:1.29. They move in the night and use the environment as a map, similar to a human reading a topographical map. Although they have high metabolic rates, they have a tendency to be sedentary and allow currents to carry them as they swim. As a result, this causes scalloped hammerheads to be selective where they swim and the depth at which they tend to stay. They also make use of Earth's magnetic field. Predation The scalloped hammerhead has several advantages to capture its prey. The shape of its head allows it to burrow into the seafloor and pin stingrays down, while the wide head and special sensory cells allow the scalloped hammerhead to successfully detect fish. ==Reproduction==

The gestation period is reported to be around 12 months. Scalloped hammerheads give live birth. Compared to other species, the scalloped hammerhead produces large litters (12–41 pups), Nursery grounds for this species are predictable and repeated over the years, and they are faithful to their natal sites. ==Diet==

Scalloped hammerheads are generalists and opportunistic predators, eating whatever is available and abundant in their area. There is no difference in what male and female sharks eat, but larger sharks are noted to eat larger prey than smaller, juvenile hammerheads. Juveniles typically feed on inshore fish, while adults feed on inshore fish as well as larger organisms that live in deeper waters. Specifically, this shark feeds primarily on bony fish such as sardines, mackerel, and herring, and occasionally they feed on cephalopods such as squid, octopus, and cuttlefish, and crustaceans such as lobsters, shrimps, and crabs. Larger specimens may also feed on smaller species of shark, such as the blacktip reef shark, Carcharhinus melanopterus, and rays. Hammerheads located in the Indo-West Pacific have been observed to prey on sea snakes. ==Conservation==

The scalloped hammerhead was the first shark species to be protected under the U.S. Endangered Species Act. As of 2019, the scalloped hammerhead has been categorized as Critically Endangered by the IUCN Red List. The IUCN cites overfishing as the main cause for the drop in population numbers. Scalloped hammerheads are over harvested for their large sizes and fins that have 'high fin needle content;' their fins are sold for US $100–120 per kilogram. Many conservation efforts have been taken to protect this species, such as using genetic information from fins purchased from a Hong Kong market to pinpoint where scalloped hammerheads are being caught and track their exploitation levels. Fishing bans have been placed on some of these areas, such as in the Western Yucatan Peninsula, during breeding seasons to protect the young and juvenile scalloped hammerheads. In many areas, officials have implemented management regulations on fishing vessels. They have made a regulation that prohibits taking the sharks onboard to transship, sell, or store for future selling. In parts of the Atlantic Ocean, their populations had declined by over 95% in the past 30 years. Among the reasons for this drop are overfishing and the rise in demand for shark fins. Researchers attribute this growth in demand to the increase in shark fins as an expensive delicacy (such as in shark fin soup) and in 2008 called for a ban on shark finning, a practice in which the shark's fins are cut off and the rest of the animal is thrown back in the water to die. Hammerheads are among the most commonly caught sharks for finning. "This species tendency to aggregate in large groups making capture in large numbers on long lines, bottom nets and trawls even easier." Hammerhead sharks are overfished all around the world for their fins and liver oil. an estimated 1.3 to 2.7 million fins are collected each year from smooth and scalloped hammerhead sharks for the shark-fin trade. DNA barcoding can assist in the identification of scalloped hammerhead remains to aid conservation efforts. According to a January 2021 study in Nature which studied 31 species of sharks and rays, the number of these species found in open oceans had dropped by 71 per cent in around 50 years. The scalloped hammerhead was included in the study. ==See also==