Grow to 30–90 centimetres long (1–3 ft), and their distinctive first dorsal fin takes the form of a modified oval sucker-like organ with slat-like structures that open and close to create suction and take a firm hold against the skin of larger marine animals. By sliding backward, the remora can increase the suction, or it can release itself by swimming forward. Remoras sometimes attach to small boats. They swim well on their own, with a sinuous, or curved, motion.
Remoras are primarily tropical open-ocean dwellers, occasionally found in temperate or coastal waters if they have attached to large fish that have wandered into these areas. In the mid-Atlantic, spawning usually takes place in June and July; in the Mediterranean, in August and September. The sucking disc begins to show when the young fish are about 1 centimetre long. When the remora reaches about 3 centimetres, the disc is fully formed and the remora is then able to hitch a ride. The remora’s lower jaw projects beyond the upper, and there is no swim bladder.
Some remoras associate primarily with specific host species. Remoras are commonly found attached to sharks, manta rays, whales, turtles and dugong (hence the common names sharksucker and whalesucker). Smaller remoras also fasten onto fish like tuna and swordfish, and some small remoras travel in the mouths or gills of large manta rays, ocean sunfish, swordfish and sailfish.
The relationship between remoras and their perfect hosts is most often taken to be one of commensalism, specifically phoresy. The host they attach to for transport gains nothing from the relationship, but also loses little. The remora benefits by using the host as transport and protection and also feeds on materials dropped by the host. There is controversy whether a remora’s diet is primarily leftover fragments, or the feces of the host. In some species consumption of host feces is strongly indicated in gut dissections. For other species, such as those found in a host’s mouth, scavenging of leftovers is more likely. For some remora and host pairings the relationship is closer to mutualism, with the remora cleaning bacteria and other parasites from the host.