Fins can be modified in striking ways, as illustrated by the (rather cute) Remora, or suckerfish. Its modified, flattened dorsal fin functions as a sucking disc, creating a vacuum that allows the fish to attach itself to its host, usually on the belly or under the gill covering. Remoras are often specialized in particular host types. Thus, whale suckers cling to whales, marlin suckers have a strong preference for marlins, and shark suckers (you guessed it) are usually found on sharks. Often multiple Remoras attach to the same host animal, who can even be a meeting place for mates. Indeed, hosts have been found with a male Remora under one gill covering, a female under the other.
The nature of the relationship between the Remora and its host is somewhat unclear. Ancient authors generally took the suckerfish to be parasitic. For one, it creates drag that may seriously annoy its host. Indeed, the ancients claimed that these fishes regularly slowed down their ships. Moreover, it was said that Remoras consumed the eggs of their hosts. It has since been observed, however, that Remoras eat a mixture of parasites, feces and leftovers from their hosts, thus forming a kind of semi-mutualistic garbage service. The drag problem remains, though, and one can hardly blame sharks for occasionally eating a Remora that tries to attach itself to them.
The Remora’s sucking habits have given it an application as a kind of active bait in fishing (often turtle-fishing). Hook up the Remora to a fishing line, put it in the water, and it will attach itself to the first big animal it encounters. Reel it in, and its host is yours. Hence the Remora’s nickname, “fisher-fish”.