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The several remoras are easily distinguished from all other fishes by the fact that the spiny part of the dorsal fin is modified into a flat oval sucking plate, composed of a double series of cartilaginous crossplates with serrated free edges, and situated on the top of the head and neck. All the remoras, too, are slender of form with the lower jaw projecting well beyond the upper. Their mouths are armed with many small pointed teeth; their soft dorsal and anal fins are about the same in form and size, the one above the other; and their pectoral fins are set high up on the sides. The lower surface of the head is convex, the upper flat (a very conspicuous feature) with the lower surface of the body nearly as deeply colored as the upper so that the back is often mistaken for the belly. The members of this family all attach themselves to other fishes, or to sea turtles, by their sucking disk, usually clinging to the sides of their hosts, but often within the mouth or gill cavities of the larger sharks and of the giant rays.^[17] They are carried about in this way, and they feed on the scraps from the meals of their transporters. All the remoras are tropical; they appear only as strays in boreal seas, usually fast to sharks or to swordfish.

We follow Sumner, Osburn, and Cole^[18] in uniting under one species the shark sucker (naucrates), with more than 21 plates but a sucking disk less than one-fourth as long as the body, and the pilot sucker (naucrateoides), with only 20 or 21 plates but longer, fishes that are otherwise indistinguishable one from the other.

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