A remora (Echeneis naucrates) and its host, a zebra shark (Stegostoma fasciatum).
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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 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 some controversy over whether a remora's diet is primarily leftover fragments, or actually the feces of the host. In some species (Echeneis naucrates and E. neucratoides) 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. Many sources also suggest that for some remora/host pairings the relationship is closer to mutualism, with the remora cleaning bacteria and other parasites from the host.
There are eight species in four genera:
Similar reports have also come from Japan and from the Americas. In fact, some of the first records of the "fishing fish" in the Western literature come from the accounts of the second voyage of Christopher Columbus. However, Leo Wiener considers the Columbus accounts to be apocryphal: what was taken for accounts of the Americas may have in fact been notes that Columbus derived from accounts of the East Indies, his desired destination.
Because of the shape of the jaws, appearance of the sucker, and coloration of the remora, it sometimes appears to be swimming upside-down. This probably led to the older common name reversus, although this might also derive from the fact that the remora frequently attaches itself to the tops of manta rays or other fish, so that the remora in fact is upside-down while attached.