Great hammerhead sharks (Sphyrna mokarran), the largest of the hammerhead species, are formidable predators found worldwide in tropical seas. Reaching impressive lengths of up to 6 meters, these sharks are easily identifiable by their unique hammer-shaped head, or cephalofoil, and their disproportionately large first dorsal fin. While they are known to have a broad diet, great hammerheads exhibit a particular preference for stingrays, employing specialized hunting techniques to capture their prey.
Habitat and Distribution
Great hammerheads are typically found in coastal reefs and lagoons but are also known to seasonally migrate thousands of kilometers into the open ocean. Their habitat ranges worldwide in tropical seas. In the western Atlantic, they can be found from the coasts of North Carolina down to Uruguay, the Gulf of Mexico, and the Caribbean.
Physical Characteristics
Great hammerheads are the largest hammerhead species, reaching lengths comparable to white sharks - up to 6 m. Mature sharks weigh in excess of 500 pounds and can weigh more than 900 pounds. Their backs are olive green or brownish-gray, fading to white at the belly. Great hammerheads are clearly identifiable by their hammer-shaped head (or cephalofoil) that has a single notch at the centre, and their disproportionally large first dorsal fin. Their fins also appear more sickle-shaped than other hammerhead sharks. They are pale grey in colour.
Dietary Preferences and Hunting Strategies
Great hammerheads have a relatively broad diet, including stingrays, other sharks, bony fish, and crustaceans, but they do tend to have a particular preference for stingrays. They are specialized ray hunters and have been observed using their hammer to pin down and immobilize rays they have found on the seafloor. Their hammer (or cephalofoil) helps them find rays buried in the sand through electroreception and is used to pin them down so they can’t escape. The lateral expansion of the head allows hammerheads to perform agile turning movements when hunting. When going after stingrays, hammerheads use their heads like actual hammers. They slam the stingray into the ocean floor, pinning it down until the stingray grows tired and they can successfully eat it. Hammerheads also have highly sensitive electroreceptive organs that enable them to detect small electrical impulses of their prospective prey.
The Role of the Cephalofoil in Hunting
The peculiar shapes of sphyrnid cephalofoils appear to serve several purposes. First, the flattened and expanded head acts as a hydrodynamic bow plane that allows the shark to raise and turn its head quickly and sharply. This feature gives the shark enhanced maneuverability to snap up its prey. Second, the nostril is much expanded compared with other groups of sharks and may provide hammerhead sharks with a keener ability to locate prey and follow scents to their sources. Third, the extended spacing of the eyes may provide a wider field of view; it may also widen the lateral field of view and increase anterior depth perception. Fourth, the wide underside of the head-with its expanded surface area-houses more electroreceptive organs (see sensory reception), which are important for detecting the electrical impulses of prey. These organs may even detect the electrical impulses of prey buried in sediment. Lastly, many larger hammerhead species hunt stingrays and other benthic (that is, bottom-dwelling) fish; they have been observed using the wide margins of their heads to ram and pin prey to the substrate before biting them.
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Cannibalistic Behavior
Great hammerheads can even be cannibalistic, eating their own peers when food is hard to come by.
Energetic Benefits of Predatory Choices
New research published in the journal Oecologia reveals great hammerheads may gain significant energetic advantages by preying on large targets like large fish, rays and other sharks, according to lead author and FIU alumna Erin Spencer. The models predicted that a 250-pound great hammerhead would only need to consume one 55-pound blacktip shark roughly every three weeks to meet its energy demands. This single large meal could sustain the hammerhead for up to two months during periods when blacktip shark populations are scarce. In stark contrast, if the hammerhead were to rely on smaller reef-associated fish, it would need to catch one or two 2.2-pound fish every single day to sustain itself.
Hunting Other Sharks
While many sharks hunt for small, abundant prey when looking for a meal, great hammerheads have a supersized strategy with some big payoffs - eat other sharks. For a long time, scientists have observed great hammerheads hunting other sharks, including blacktips during their seasonal migrations off Florida. Great hammerheads are known to hunt blacktip sharks (Carcharhinus limbatus) as they collect off the coast of southern Florida on their seasonal migrations. “Animals must make careful decisions regarding how and what to hunt,” said Yannis Papastamatiou, study author and shark behavior expert, in a statement. “Do you go after large prey that have more energy but are harder to catch? Or should you just go after any potential prey you encounter? Here we apply these ideas to an incredible predator, the great hammerhead, and provide insight into why they might target large prey like other sharks or rays.”
Hunting Habits and Energetic Efficiency
The hunting habits of sharks are more than just an interesting factoid, they could help scientists set up more effective conservation measures. Models suggest that on a diet of small fish, great hammerheads need to eat 0.9% of their body weight per day, on average, to survive. On a diet of large sharks, they need only 0.7% of their body weight per day. All things being equal, great hammerheads have figured out that they’re better off letting the little guys escape and saving their energy for big fish.
Sensory Adaptations for Hunting
Hammerheads also have highly sensitive electroreceptive organs that enable them to detect small electrical impulses of their prospective prey. The extended spacing of the eyes may provide a wider field of view; it may also widen the lateral field of view and increase anterior depth perception.
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Reproduction and Life Cycle
Great hammerheads are viviparous, meaning they have a placenta and give birth to live young. After a gestation period of approximately 11 months, a female may produce up to 55 pups that each measure 0.5-0.7 m in length. Great hammerheads grow faster than many other sharks of their size and tend to mature earlier at 5-9 years of age. Litter size: 13 to 42 pups. Life span: 30 to 40 years.
Conservation Status and Threats
Great hammerheads are Critically Engendered due to their severe declines as a result of overfishing. They are caught globally both as target and bycatch in longline, purse seine and gillnet fisheries. They are also considered valuable trophy catches in recreational fisheries and are often caught in beach protection programmes that target large sharks. Great hammerheads have been valuable to fisheries globally for their meat and oil, but given their size (particularly of their impressive first dorsal) their fins are also highly sought after for display. They are similarly popular with recreational fishermen. Unfortunately, great hammerheads are fragile and have exceptionally high at-vessel mortality, meaning that even catch and release fisheries pose a significant threat to them. Overall declines of more than 70% mean this species is now listed on Appendix II of CITES, which restricts their international trade. Great hammerheads are classified as critically endangered globally by the International Union for Conservation of Nature, facing threats from overfishing and habitat loss.
Conservation Implications
Knowing their specific dietary needs and preferred hunting grounds is vital for developing effective conservation strategies that ensure their long-term survival, Papastamatiou said. In order to help protect critically endangered species like great hammerheads, we need to understand their underlying biology and how they interact with their environment,” Spencer said. “It’s important that we continue to study these big questions, especially in the face of a changing ocean.”
General Information
Hammerhead sharks have smaller mouths than sharks of comparable size. They are also unable to open their mouth as widely as other sharks. However, other adaptations make up for any deficiencies. Although they may have smaller mouths, they have 17 rows of teeth. Sharper and more serrated teeth are at the front, with flatter, larger teeth toward the back. This helps them to grab, crush, and grind their prey. Hammerhead sharks can dive as deep as 984 feet, but they usually remain in coastal waters at depths closer to 328 feet. Great hammerhead sharks are solitary hunters and generally search for prey at dusk. Hammerheads experience competition from other sharks and apex predators such as great white sharks and killer whales. There is little information on the amount of food great hammerheads require. However, in the wild, they are known as opportunistic feeders and will eat as much as they can catch. Since great hammerhead sharks are apex predators, they have no marine predators once they reach adulthood.
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