Pilot-fish (Naucrates ductor), a well-known fish which accompanies sharks and follows ships. It usually keeps to the open sea, and is very widely distributed in tropical and temperate regions. From the Mediterranean it sometimes follows ships to
British coasts. In length it is about a foot, in shape like a mackerel, in colour variable, though generally grayish blue with five transverse dark blue bands. The first dorsal fin is represented by a few spines. Its zoological position is beside the horse-mackerels in the family Carangidae.
Many wonderful stories are told about the pilot-fish, which seems to be the Pompilus of the ancients. It is said to guide the shark to its prey—nay, more, to show sailors their desired course. It certainly is a very frequent companion of the shark, especially if that fish be swimming alone, but the precise nature of the association is doubtful. The pilot-fish probably follows the shark as it follows a ship for the sake of scraps of food, and perhaps eats the parasitic crustaceans, &c., with which the skin of the shark is often infested. Moreover, companionship with the shark probably protects the pilot-fish from its enemies.