Fish
Great Hammerhead Shark
Sphyrna mokarran · also called great hammerhead
The great hammerhead is the biggest of the nine hammerhead species and the strangest-looking large shark in the sea. Its head is drawn out into a broad, straight-edged blade, and everything about how it senses and catches prey depends on that shape.
Overview
Great hammerheads live in warm seas worldwide, moving between shallow coastal flats, coral reefs, continental shelves, and the open ocean. Some populations migrate toward the poles in summer and back toward the equator in winter. They are usually seen alone — unlike the scalloped hammerhead, which forms large daytime schools.
The cephalofoil
The hammer, properly called a cephalofoil, is the defining feature of the family and most extreme in this species, where its front edge is nearly straight. Spreading the sensory organs across it brings several advantages:
- Smell. Widely spaced nostrils sample water over a broad path and help the shark work out which direction a scent is coming from.
- Electroreception. The underside is studded with jelly-filled pores (ampullae of Lorenzini) that pick up the tiny electrical fields given off by living animals, including prey hidden under sand.
- Vision. Eyes on the tips of the cephalofoil give a wide field of view.
- Manoeuvrability. The head works like a wing, and great hammerheads can roll and turn more sharply than sharks of similar size. They often swim rolled onto their side to save energy.
Diet and hunting
Great hammerheads eat a wide range of fish and invertebrates, but they have a particular appetite for stingrays, along with other rays, skates, and smaller sharks — including other hammerheads. Cruising just above the bottom, the shark detects a buried ray electrically, pins it with a downward strike of the head, and bites off its pectoral “wings” to stop it escaping. It appears immune to the venom of the stingray spines that frequently lodge in its jaw.
Reproduction
Great hammerheads are viviparous — the embryos are nourished through a placental connection and born live. After a pregnancy of about 11 months a female gives birth to a litter that can range from a handful of pups to more than fifty, every second year. This slow output leaves the species little capacity to recover from heavy fishing.
Threats and conservation
The great hammerhead is Critically Endangered. Its fins are among the most prized in the international shark-fin trade, making it a prime target, and it is also caught incidentally on tuna and swordfish longlines and in coastal nets. Because the species is physiologically fragile, post-release survival is poor even where regulations require it. Populations in the Atlantic, Indo-Pacific, and elsewhere are estimated to have fallen by 80% or more.
Protections are increasing — the species is listed on CITES Appendix II, restricting international trade, and is protected in some national waters — but effective recovery depends on cutting fishing mortality across the shark’s wide, boundary-crossing range.
Great Hammerhead Shark: common questions
What is the hammer-shaped head for?
The flattened head, called a cephalofoil, spreads the shark's senses across a wider span. The nostrils are set far apart, helping it track a scent trail and pinpoint its direction, and the underside is packed with electroreceptors that detect the faint electrical fields of animals buried in the sand. The wide head also acts as a hydrofoil, giving the shark exceptional agility and a tighter turning circle than other sharks.
How does a great hammerhead hunt stingrays?
It sweeps its head close to the seabed like a metal detector, picking up the electrical signals of rays hidden under the sand. When it finds one, it uses the broad head to slam the ray down and pin it against the bottom, then bites off the wings to disable it before eating. Great hammerheads are often found with stingray barbs embedded in their mouths, apparently without ill effect.
Are great hammerheads dangerous to people?
They are large, and their size and reputation put them among the sharks considered potentially dangerous, but confirmed attacks on humans are very rare and no unprovoked fatality is firmly attributed to the species. Divers report that great hammerheads are usually shy and keep their distance.
Why is the great hammerhead Critically Endangered?
Its fins are large and highly valued in the shark-fin trade, so the species is heavily targeted and also caught as bycatch on longlines and in nets. Great hammerheads fight hard when hooked and build up so much lactic acid that many die even when released. Combined with slow reproduction, this has driven estimated declines of more than 80% in many regions.