Scalloped Hammerhead

The scalloped hammerhead is easily distinguished from other hammerhead sharks by the central indentation on the anterior margin of the head. There are also two indentations on either side of the central indentation, which gives the "scalloped" look. They have a very large first dorsal fin that is slightly hooked and a smaller second dorsal fin. Scalloped hammerhead sharks are typically uniform grey, grayish brown, bronze, or olive with a white underside for countershading.

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 500 m (1,600 ft), but is most often found above 25 m (82 ft). 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.

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.

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.

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), and this is most likely due to high infant mortality. Like most sharks, parental care is not seen. Nursery grounds for this species are predictable and repeated over the years, and they are faithful to their natal sites. Their natal sites still cause high infant mortality; a lack of resources prevents all the young from surviving. As a result, only the fittest grow to maturity. Also, should a population get depleted, it recovers through reproduction and not immigration. This species does not seem to attack each other even in periods of starvation. In addition, scalloped hammerheads have migratory behaviors. As a result, deprivation results from migration and young growth.

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. These sharks are slow growing, mature late, and have low fecundity, which makes them very vulnerable to fishing. 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. Another important effort has been protecting Sphyrna lewini nurseries.