Pilotfish

Pilot-fish (Naucratcs ductor), a small fish, allied to the Horse Mackerel, common in tropical and temperate seas. In form it is not unlike a mackerel; its colour is generally bluish-grey, with five dark-blue vertical bands. It constantly follows ships and large fish, especially sharks. The precise nature of the connection between the Pilot-fish and the Shark is unknown; but Giintber endorses Meyen's opinion that the former feeds on the excrements of the latter, and says that it also picks off the parasitic crustaceans with which the shark, like all large fish, is infested.