The pilot fish (Naucrates ductor) is a carnivorous commensal fish in the family Carangidae. It is widely distributed and lives in warm or tropical open seas.The pilot fish congregates around sharks, rays, and sea turtles, where it eats ectoparasites on, and leftovers around the host species; younger pilot fish are usually associated with jellyfish and drifting seaweeds. They are also known to follow ships, sometimes for long distances; one was found in County Cork, Ireland, and many pilot fish have been sighted on the shores of England. Their fondness for ships led the ancients to believe that they would navigate a ship to its desired course.
The pilot fish is of a dark blue to blackish-silver colour, with the belly being lighter in colour. The pilot fish is also known to have a temporary variation of colour when excited; its dark-coloured bars disappear, and its body turns a silvery-white colour, with three broad blue patches on its back. It is easily recognisable by its distinctive traverse bands, which are of a much darker colour than the rest of the body, and number between five and seven. The pilot fish can grow up to 60–70 cm in length.
The pilot fish is harmless to human beings and is said to be good eating; however, the fish is difficult to land because of its erratic behaviour when caught.
While pilot fish can be seen with all manner of sharks, they are said to prefer accompanying the oceanic whitetip, Carcharhinus longimanus. The pilot fish's relationship with sharks is a mutualist one; the pilot fish gains protection from predators, while the shark gains freedom from parasites. It was often said by sailors that sharks and pilot fish share something like a "close companionship"; there were even tales of this fish following ships which had captured "their" shark for up to six weeks and showing signs of distress in its absence.
Whatever the veracity of such reports, it is extremely rare that a shark will feed on a pilot fish, and smaller pilot fish are frequently observed swimming into sharks' mouths to clean away fragments of food from between their teeth.