Swimming out across the reef and down a vertical wall, we were immediately aware of scalloped hammerheads scything through the water beneath us, heads sweeping from side to side. Continuing our descent and staying close to the wall, we soon found more hammerheads passing above us and around us.
There seemed to be one enormous shoal of hundreds of hammerheads cruising up and down the wall. As the dive continued, the number of hammerheads just got more and more. It often took several minutes for the shoal to pass by. Towards the end of the dive we just held position at the edge of the reef and the top of the wall and watched them all pass one way and then back the other way.
Small groups would regularly break off from the shoal and swim up and into the shallows to a cleaning station. When ready for cleaning they would stop swimming, turn on one side and hold still while angel fish and wrasse pecked away at their skin. They were much more cautious of divers while being cleaned. Whereas the hammerheads would swim quite close to divers on the reef wall, at the cleaning station they would swim away before I could get close enough for good pictures.
One of the reasons that scalloped hammerhead sharks form these enormous shoals is to do with mating. The females manoeuvre for position in the middle of the shoal, with the biggest occupying pride of place in the centre. By being in the dominant position the females make themselves more attractive to the males. At night, the shoals disperse while the individual sharks hunt for food, with males and females pairing off for mating. When mating, the male wraps his body about the female and holds on with his teeth. Many of the large females in the shoal had very obvious scars on their sides as a consequence. As the day breaks, the sharks return to re-form the shoal and the whole dance is repeated.
Hammerheads are one of the most highly evolved types of shark. The wide separation of the eyes gives superior vision. The front of the head between a sharks eyes is spanned by sense organs which detect changes in pressure and electrical field. In the case of hammerhead sharks, the width of the head makes these senses more acute than those of other sharks. The head also acts as a wing, improving manoeuvrability. Another attribute of higher evolution is that young hammerhead sharks are nurtured through a placenta inside the mother before birth. Other `less' evolved sharks lay eggs.