Great Hammerhead Shark

The Great Hammerhead Shark (Sphyrna mokarran) is the largest and arguably one of the most distinctive members of the hammerhead family, Sphyrnidae. This magnificent apex predator is a highly sought-after sighting for divers, known for its impressive size and unique cephalofoil. However, its populations face severe threats, leading to its classification as Critically Endangered by the IUCN Red List since 2018.

Found in warm temperate and tropical waters worldwide, the Great Hammerhead inhabits both coastal and pelagic environments, frequenting continental shelves, island terraces, coral reefs, and lagoons. While it can be observed at depths ranging from 1 m to 300 m, it is most commonly encountered in shallower waters, typically to 100 m, preferring temperatures of 20°C and higher. This solitary and nomadic species is also migratory, with some populations moving poleward during warmer months. As an opportunistic carnivore, its diet primarily consists of stingrays, which it is known to pin to the seabed with its broad head, along with bony fish, other sharks, cephalopods, and crustaceans. Sightings, though rare, have been recorded at dive sites such as Maracaibo Deep.

Identification

Identifying the Great Hammerhead Shark is relatively straightforward due to several key physical characteristics:

• Cephalofoil (Head): Its most distinguishing feature is the exceptionally wide, hammer-shaped head, or cephalofoil. Unlike other hammerhead species, the Great Hammerhead's anterior margin is nearly straight, with a prominent indentation at the midline. The eyes and nostrils are positioned at the ends of these flattened lobes.
• Size and Body Shape: As the largest hammerhead, individuals can reach a maximum length of 6.1 m (20 ft), though mature adults are more commonly observed between 3.5-4.6 m (11-15 ft). The body is stout and streamlined, typically dusky brown to light gray or olive dorsally, fading to a cream or white underside, providing effective countershading.
• Dorsal Fins: The first dorsal fin is notably tall, strongly falcate (sickle-shaped), and recurved, originating over or slightly behind the pectoral fin axil. The second dorsal fin is also relatively high with a strongly concave rear margin.
• Pelvic Fins: The pelvic fins are large and sickle-shaped with distinctly concave rear margins, a feature that helps differentiate it from the scalloped hammerhead.
• Teeth: Its teeth are triangular and strongly serrated.
• Other Features: The Great Hammerhead lacks an interdorsal ridge.

The Great Hammerhead Shark (Sphyrna mokarran) is a truly magnificent and highly migratory species, inhabiting tropical and warm temperate waters across the globe. Divers seeking an encounter with this impressive apex predator can find it in a variety of marine environments, showcasing its adaptability and wide-ranging movements.

Geographic Distribution:

This species boasts a circumglobal distribution, typically found between latitudes 40°N and 37°S. Its extensive range includes:

• Atlantic Ocean: From North Carolina, USA, south to Uruguay, encompassing the Gulf of Mexico, the Caribbean Sea, and extending from Morocco to Senegal, including the Mediterranean Sea.
• Indian Ocean: Found throughout the entire Indian Ocean basin.
• Pacific Ocean: Ranges from the Ryukyu Islands of Japan, across to Australia, New Caledonia, and French Polynesia, and in the Eastern Pacific from southern Baja California, Mexico, down to Peru.

Habitat and Depth:

Great Hammerheads are versatile, occupying both coastal-pelagic and semi-oceanic habitats. They are often observed in areas rich with prey, making them a thrilling sight for divers. Their preferred environments include:

• Coastal Waters: Commonly found over continental shelves, island terraces, and within shallow lagoons and bays.
• Reef Systems: They frequently associate with coral reefs, reef walls, and drop-offs, utilizing these structures for hunting.
• Open Ocean: While often near land, they are also found in more offshore, pelagic zones, including epipelagic and mesopelagic waters.

Regarding depth, Great Hammerheads exhibit a broad range. They can be found in remarkably shallow inshore waters, sometimes less than 1 m deep, particularly when hunting. However, they are also known to venture into deeper offshore waters, with observations extending to depths of 300 m. Most commonly, divers will encounter them in depths ranging from 1–80 m.

Ecosystem Associations and Substrate:

As an apex predator, the Great Hammerhead Shark plays a crucial role as an upper-trophic level consumer and a mobile link species, connecting various ecosystems through its extensive movements. They are particularly known for their diet, which heavily features stingrays, and have been observed using their distinctive cephalofoil (hammer-shaped head) to pin prey, such as stingrays, to the seabed. This behavior indicates a preference for sandy or soft substrates where such prey might be found, in addition to their association with rocky and coral reef environments.

Water Temperature:

Great Hammerheads thrive in warm waters, generally preferring temperatures of 20°C and higher. Research indicates their peak performance for activities like swimming and hunting occurs around 29.3°C. While they tend to stay within their preferred temperature zones, they demonstrate a remarkable thermal flexibility, capable of performing well in waters ranging from 15.8°C to 30.3°C. This adaptability allows them to migrate seasonally, moving towards cooler waters in summer months in some populations.

The Great Hammerhead Shark (Sphyrna mokarran) is a truly magnificent and critically endangered apex predator, instantly recognizable by its distinctive, wide cephalofoil, or "hammer-shaped" head. Reaching an impressive maximum size of 6.1 meters (20 ft), this powerful shark is a highly sought-after, albeit rare, sighting for divers in tropical and subtropical waters worldwide.

Underwater Encounter Behavior

Great Hammerheads are generally described as solitary, nomadic, and migratory sharks. While potentially dangerous due to their size and predatory nature, they rarely attack humans and can sometimes exhibit inquisitive behavior towards divers. Divers have reported that Great Hammerheads tend to be shy or non-reactive, often avoiding encounters. However, some reports suggest they may approach divers closely or even charge when people first enter the water. When confronted, they may display agonistic behavior, dropping their pectoral fins and swimming stiffly. Responsible diving practices are crucial, emphasizing maintaining a respectful distance and avoiding sudden movements to prevent stress or disturbance to these majestic creatures. Dive sites like Maracaibo Deep offer rare opportunities, while locations such as Bimini in the Bahamas and Fuvahmulah Atoll in the Maldives are known hotspots for encounters.

Feeding Habits

The Great Hammerhead is an active and opportunistic predator with a diverse diet. Their preferred prey includes stingrays and other batoids, which they hunt using their highly specialized cephalofoil. The cephalofoil is equipped with numerous electroreceptors (Ampullae of Lorenzini) that detect the electrical signatures of stingrays buried in the sand. Once a stingray is located, the shark uses its head to pin the ray to the seafloor, immobilizing it before taking bites from its pectoral fins. Despite the stingray's defensive barbs, Great Hammerheads are thought to be immune to their venom, and stingray barbs are often found embedded in the sharks' jaws and heads. Beyond rays, their diet also includes a wide variety of bony fish (such as groupers, sea catfishes, tarpon, and sardines), crustaceans (crabs, lobsters), and cephalopods (squid, octopus). They are also known to prey on smaller sharks and can be cannibalistic, especially when food is scarce. Feeding primarily occurs at dawn or dusk.

Social Structure

Great Hammerhead Sharks are predominantly solitary and nomadic. Unlike their scalloped hammerhead cousins, which are known to form large schools, adult Great Hammerheads are rarely seen in aggregations. While typically solitary hunters, they may occasionally come together in small groups or aggregations, particularly during migrations or around abundant food sources. There is limited detailed information on their social interactions, but they are generally considered to be independent predators.

Notable Seasonal Patterns Relevant to Divers

Great Hammerhead Sharks are highly migratory, undertaking long-distance movements in response to water temperature, prey availability, and breeding cycles. They are found in warm temperate and tropical seas, generally preferring waters of 20°C and higher. During warmer months (summer), they tend to move poleward, and in colder seasons (winter), they migrate back towards the equator in search of warmer waters. For instance, in Bimini, Bahamas, and Jupiter, Florida, Great Hammerheads show philopatric behavior, routinely returning and staying in these areas during specific seasons (October-April in Bimini, October-March in Jupiter). Some individuals have been tracked making long-distance movements from the Bahamas to as far north as South Carolina and Virginia. Breeding occurs every two years, with a gestation period of 10-11 months, and births typically take place in late spring and summer in the Northern Hemisphere, and from October to January in the Southern Hemisphere. Mating has been observed near the surface, a unique behavior among sharks. Divers seeking to encounter these impressive sharks may find seasonal patterns at specific dive sites to be a useful guide.