There are few teleosts which depart from the ‘classical’ ichthyan body form to such an extent as the seahorse, Hippocampus. In this genus the caudal fin is absent and the tail has become prehensile, the usual function of the caudal fin is taken over by the dorsal and pectoral fins. The dorsal and pectoral fins both have a sculling action, the small anal fin moves ‘bodily’ through the water from side to side.

In the past, interest has generally centred on fast moving fish, which have evolved a limited number of body forms, as few variations are possible without a sacrifice of speed. The evolution of the swimming mechanisms of these fish has favoured an increase in the maximum speed and acceleration. In relinquishing the demands of speed the seahorse has released itself from restrictions which would limit its possible range of form.