Hammerhead Sharks: The Schooling Phenomenon

The Hammer and Why It Exists

The hammerhead's distinctive cephalofoil — the broad, flattened head that gives the family its name — is not decorative. It is a sensory instrument. The expanded surface area contains a much higher density of ampullae of Lorenzini (electroreceptors that detect the electrical fields of prey animals) and lateral line organs than a conventionally-shaped shark head would allow. Hammerheads can detect a flounder buried completely under sand from several metres away.

The cephalofoil also improves hydrodynamic lift and manoeuvrability — hammerheads are among the most agile sharks in the ocean, capable of banking turns at high speed that most other shark species cannot match.

The Scalloped Hammerhead (Sphyrna lewini)

Of the nine hammerhead species, the scalloped is the one divers seek. It is the species that schools in extraordinary numbers at specific oceanic seamounts — a behaviour that remains only partially understood. Current hypotheses include:

• Cleaning station behaviour: Larger individuals near the top of the school may have priority access to reef fish that clean parasites
• Social hierarchy formation: The schools appear to have internal structure, with large females at the centre and smaller males and juveniles at the periphery
• Thermoregulatory advantage: The deeper, cooler parts of seamounts allow sharks to temporarily lower body temperature and clear metabolic byproducts
• Navigation reference: Seamounts with measurable magnetic anomalies may serve as waypoints in long-distance migrations

The truth is likely a combination of all of these.

Where Schools Occur

Hammerhead schools are predictable at a handful of sites:

• Wolf and Darwin Islands, Galápagos: The most famous schools in the world; hundreds of scalloped hammerheads at 20–30 metres during the cool season (June–November); accessible only by liveaboard with 15-hour overnight crossing
• Cocos Island, Costa Rica: Schools year-round; the seamounts around Cocos hold more hammerheads per square kilometre than almost anywhere else
• Layang Layang, Malaysia: An atoll in the South China Sea; April to August; schools of 20–50 common, larger aggregations documented
• Monad Shoal, Malapascua (Philippines): Primarily known for thresher sharks but hammerheads appear in certain seasons
• Elphinstone Reef, Egypt (Red Sea): Oceanic hammerheads appear in winter at this open-ocean seamount

Diving with Hammerheads

Hammerheads are notoriously skittish. The key technique is stillness and silence. Bubbles disturb them. Divers who hover motionless and breathe slowly — descending quietly to depth and not approaching — are consistently able to get closer than those who actively pursue the school.

At Galápagos, divers typically descend to a rocky bottom at 20–30 metres and stay low, watching the blue above them where the school circles. The sharks will approach to within 5–10 metres of still, silent divers. Any sudden movement or exhalation of a large breath typically causes the nearest animals to veer away.

Conservation Status

Scalloped hammerheads are listed as Critically Endangered by the IUCN — one of the most threatened shark species globally. Their fins are among the most prized in the shark fin trade (the distinctive scalloped margin makes them identifiable in the market). Populations have declined by over 80% in many regions since the 1980s.