A large hammerhead that schools by day at cleaning stations on offshore seamounts.
Cocos and the northern Galapagos are dived for this animal more than for any other, and the two islands offer it differently. At Cocos the schools come to fixed cleaning stations on the seamounts and can be watched from the rock at 25–35 m. In the Galapagos they are met on current-swept points at Darwin and Wolf, usually in passing rather than at a station.
The schools are predominantly female. They form during the day and break up at night, when individuals disperse into deep water to hunt — the reverse of what a diver might assume from seeing them massed in daylight.
Approaching a school directly pushes it away. Bubbles do the same, which is why these are among the few dives where rebreathers make a visible difference to what is seen. Divers who settle on the bottom below the station and wait generally see more than divers who swim toward the animals.
Difficulty is given as 1 at Cocos and 2 in the Galapagos, but the variance is high at both. A week at either can produce one dive with several hundred animals and five with none, and the difference is usually current rather than season.
Assessed by the IUCN as Critically Endangered. Give the assessment year wherever this is cited, since the category has been revised more than once.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.