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A pilot fish is a small fish which is known for being in the company of larger predators. It has a type of symbiotic relationship with a larger animal known as "mutualism," where unrelated species have a relationship that isn't based on predator/prey. The pilot fish eats parasites off the skin of their host animal, and in return, they receive protection from other species.
The fish is considered carnivorous, in that it eats small scraps of flesh and ectoparasites. Ectoparasites are tiny creatures which live on the outside of another animal's body, feeding off them. By using the parasites as a source of food, the pilot fish can form mutually-beneficial relationships with larger animals, such as sea turtles, or predators, such as sharks. These animals often suffer from having parasites on their skin, and by eating them, the fish offers them relief.
Pilot fish also serve their hosts as "dentists." They will swim into their host's mouth and eat all of the food particles that are stuck between its teeth. When their host is a shark, it can be an impressive sight to see the much smaller fish inside such a dangerously toothy mouth. The pilot fish is not in danger of being eaten, since it provides benefits to the shark. This has led to some surfers putting pilot fish stickers on the bottom of their surfboards so as to discourage sharks from taking a bite out of them.
These fish can actually be used as a food fish, but they are so beneficial as cleaners that they are seldom eaten by any animal. The unusual relationship that pilot fish have with other creatures has led to many romantic myths. Before the nature of the mutual relationship was fully understood, it was thought that pilot fish led their hosts to food, rather than feeding off of parasites on their host. Since pilot fish also follow their hosts to feed off of scraps from their kills, they would sometimes follow sailing ships for the waste thrown overboard and were considered a good omen that the ship would safely reach port.
Pilot fish are also often used as a metaphor in literature. Herman Melville, author of Moby Dick, used the pilot fish to represent living in a dangerous place in his poem "The Maldive Shark." In The Christmas Invasion, a 2005 episode of the British science fiction television programme Doctor Who, the Doctor refers to some dangerous aliens as "pilot fish." He is referencing the myth of the fish leading their hosts to prey, since the arrival of the aliens heralds the arrival of a far more dangerous race, the Sycorrax.