Scalloped Hammerhead

Photo: Kris Mikael Krister · CC BY

Scalloped hammerheads are the defining pelagic encounter at Ras Kisimani between June and September, when cooler, nutrient-rich currents from the south concentrate prey along the point and sharks aggregate at the meeting of current systems in numbers rarely seen elsewhere on the East African coast. Unlike the solitary great hammerhead, scalloped hammerheads form large schools during the day — sometimes dozens of individuals spiralling in the blue water column off the point's edge — that are thought to represent social gatherings related to courtship and dominance rather than cooperative feeding. Their unusual head shape houses a greatly expanded array of electrosensory ampullae of Lorenzini that allows detection of prey buried in sand at distances of half a metre, a capability demonstrated in laboratory studies but also observed at Ras Kisimani where sharks repeatedly quarter sandy patches between coral heads.