Scalloped Hammerhead Shark

Large daytime aggregations near seamounts and offshore islands are one of this species' most studied and unusual behaviours, since most sharks hunt and travel largely alone. Several explanations have been proposed and are not mutually exclusive — schooling may offer protection from larger predators for smaller or younger individuals, provide social and mating opportunities in a predictable, easily relocated site, and allow individuals to use magnetic and geological features of seamounts as navigational landmarks during longer migrations. At night, sharks that were schooling during the day typically disperse individually to hunt over a much wider area before regrouping the following day.

The clearest identifying feature is the shape of the head itself, called a cephalofoil, which in this species has a distinctly wavy, scalloped front edge with a central notch, unlike the smoother, straighter-edged heads of the great hammerhead or the smaller, more rounded head of the bonnethead. It also tends to be intermediate in size between the very large great hammerhead and the much smaller bonnethead, and while all hammerhead species use their wide heads to sense and pin prey, the scalloped hammerhead is unusual among them for its strong tendency to form large daytime schools rather than hunting alone.

Its large, valuable fins make it a heavily targeted species in the international shark fin trade, and it is also frequently caught as unwanted bycatch in longline and gillnet fisheries targeting other species, since its schooling behaviour concentrates many individuals in predictable, easily fished locations. Like most sharks, it grows slowly, matures late, and produces relatively few young, meaning populations cannot recover quickly once heavily depleted. Combined assessments across its global range estimate declines of 80% or more over three generations, which is why the IUCN classifies it as Critically Endangered despite it still being found in many coastal regions worldwide.

The wide, flattened cephalofoil is packed with electroreceptive organs called ampullae of Lorenzini, spread out much further apart than in a typically shaped shark head, which lets it detect the faint electrical fields produced by prey buried in sand, such as its favoured stingrays, with unusual precision. Once it locates a buried ray, it uses the underside of its head to pin the ray firmly to the seafloor, preventing it from swimming away or using its venomous tail spine effectively, before manoeuvring to take a bite. The wide-set eyes on either side of the head also give it an unusually broad field of vertical vision.