Pilotfish

They are often confused because both travel with large marine animals, but they do it in opposite ways. A remora has a suction disc on top of its head, derived from a modified dorsal fin, and physically attaches itself to the host's body, riding along without swimming. A pilotfish has no such disc; it is a strong, continuous swimmer that stays near a shark, ray, or turtle by matching its speed and tucking into the reduced-drag pressure wave just ahead of or beside the host, the same trick a cyclist uses drafting behind another rider. If a pilotfish's host swims away quickly, the pilotfish must actively keep up or be left behind, whereas a remora simply stays attached.

Several likely benefits combine. Swimming in a large shark's slipstream reduces the energy the smaller fish needs to spend to keep moving, since it is riding currents set up by the host's body. Staying close to a shark also affords protection, because most predators that might eat a pilotfish are themselves wary of the shark. And the arrangement provides food, both scraps and disabled prey left behind after the shark feeds and, in a mildly beneficial exchange for the shark, small parasites and dead skin that the pilotfish picks from the shark's body. Young pilotfish get similar shelter and easy pickings from jellyfish and floating debris before they take up with larger hosts.

For thousands of years, sailors and naturalists noticed pilotfish swimming just ahead of or beside sharks and concluded that the little fish was actively leading the shark to food, or even guiding lost or blinded sharks safely to harbour, an idea recorded as far back as ancient Greek writers including Aristotle, who thought a similar fish piloted tuna. The name "pilotfish" preserves that belief. Careful modern observation has found no evidence the fish leads the shark anywhere; it is simply staying in the most energetically favourable position relative to a moving host, and the shark shows no sign of depending on it for direction or food-finding.

Occasionally, but it is rare, and the relationship is stable enough that it has persisted across many shark and pilotfish encounters and probably many millions of years. Sharks generally seem to tolerate pilotfish closely shadowing them and do not treat the smaller fish as routine prey, possibly because the pilotfish is fast enough to avoid a strike, because it provides a mild cleaning benefit, or simply because it is not worth the shark's effort to chase such a small, agile target when larger prey is available. Isolated reports of sharks eating pilotfish do exist, so the association is not risk-free for the smaller fish, but it is generally categorised as commensalism, with the pilotfish benefiting and the shark largely unaffected.