Scalloped Hammerhead Sharks (Sphyrna lewini) are an easily distinguished group of sharks in the hammerhead family. Sharks in this family have an unmistakable double-headed hammer shaped head, giving them the name “hammerhead”.
Description
Like other member of this family, its eyes and nostrils are located at the extreme end of its wide head. There are a few different guesses as to why hammerheads have adapted their distinctive shape:
- Location of eyes could allow them to see more.
- Provides more surface area for the ampullae of Lorenzini (the sensory organ in their head that allows them to detect electric fields given off by other animals) to detect prey.
- Have been seen using their head to pin stingrays and other prey to the ocean floor.
Image courtesy of NOAA Fisheries
Scalloped Hammerheads are distinguished from other members of this group by the ridges found along the front edge of its head that form a “scallop-like” pattern.
- There is one prominent notch in the center of their head with other pronounced grooves along the rest of the slightly arched hammer-like head.
- First dorsal fin is larger than the second and curved
- Anal fin has a pronounced notch
- Coloring is greyish/brownish on top (dorsal side) and white/pale yellow on bottom (ventral side)
- They have an arched mouth with small, smooth and lightly serrated teeth
Image courtesy of DPI.NSW
Scalloped hammerheads usually reach lengths of 10-11ft in females and 7-11 feet in males, with some reports of individuals as large as 14ft.
Behavior
Scalloped Hammerheads are a more reclusive species, making them harder to observe. They do not socialize with people often, even divers, as they are scared off by the bubbles. If you freedive, and are able to dive deep enough you may be lucky enough to get close to them in the wild.
While there have been no fatalities reported in connection with humans and this species, they will display threat postures towards people if they do not like them in their space.
Image courtesy of zubludiving
Most sharks are solitary creatures, but the Scalloped Hammerhead Sharks migrate in large schools of hundreds (or more) during certain seasons. This makes them especially vulnerable to targeted fishing, where their numbers can be depleted in almost no time.
Habitat
Scalloped Hammerhead sharks are a pelagic species found worldwide in tropical waters, including the Atlantic, Indian, and Pacific oceans.
Image courtesy of Research Gate
They are mainly found in open waters, but can also be found in continental shelves and bays/estuaries. They tend to stay in open waters until dawn, when they migrate closer to shore, and then return to pelagic areas in search for food at dusk.
Adults spend the majority of their time offshore, forming separate male and female schools. During summer months they form large schools of mainly females heading North.
Diet
One of the hammerhead’s favorite prey are stingrays. They use their specialized head shape to search for them along the seafloor. Some individuals have been found with 50 or more stingray spines in their mouth and digestive system at one time. They also feed on other sharks, schooling fish, eels, reef fish, squid, octopus, and crustaceans.
Some Scalloped Hammerhead Sharks found in the Indo-West Pacific have also been observed preying on sea snakes.
Reproduction/Lifespan
Scalloped Hammerhead Sharks are viviparous, mating via internal fertilization and giving birth to live young. The gestation period lasts 9-12 months with the pups (baby sharks) being born in the spring and summertime. They can mate every year and generally give birth to 12-41 young. The baby sharks will start their life in shallower reef habitats before moving to deeper waters. Predation rates are high for juveniles, with the majority of predators being other sharks, including their own species.
Scalloped Hammerhead Sharks live to about 30 years of age, and don’t reach sexual maturity until they are 15-17 years old and 2 meters long in females and 6 years of age and 1.5 meters long in males.
Threats
Scalloped Hammerhead Sharks are listed under the IUCN red list as Critically Endangered. One of the main reasons for their declining population is overfishing. Their large fins are of high value, making them subject to heavy fishing pressures by commercial fisheries. Their habit of forming large schools makes them especially vulnerable of overfishing, with hundreds of sharks being captured at one time with longlines, bottom nets, and trawls.
They are also sometimes hunted by larger predatory sharks and killer whales.
Image courtesy of IUCN Red List
Fun Fact
Scalloped hammerhead Sharks for mutualistic relationships with several other marine species. Researchers have found cleaner fish (wrasses and blennies) and shrimp removing external parasites from their skin, gills, and mouths.
Image courtesy of Alamy