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The remora (), sometimes called suckerfish or sharksucker, is any of a family ( Echeneidae) of Actinopterygii in the order Carangiformes. Depending on species, they grow to long. Their distinctive first take 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. The disk is made up of stout, flexible membranes that can be raised and lowered to generate suction. By sliding backward, the remora can increase the suction, or it can release itself by swimming forward. Remoras sometimes attach to small boats, and have been observed attaching to divers as well. They swim well on their own, with a sinuous, or curved, motion.
The earliest remora-like fish in the fossil record is Opisthomyzon from the Early Oligocene of Switzerland. This taxon appears to be closely related to modern remoras and shares several of the iconic traits present in modern remoras, including an adhesive disk (although located posterior to the head rather than directly on it). However, due to its distinctive morphological features from modern remoras, it is placed in its own family, Opisthomyzonidae.
Also known from the Early Oligocene is the earliest true remora in the fossil record, Echeneis carpathica from Poland. From later in the Oligocene, the extinct genus Oligoremora is known from the Chattian of Germany.
Some remoras associate with specific host species. They are commonly found attached to sharks, , whales, turtles, and , hence the common names "sharksucker" and "whalesucker". Smaller remoras also fasten onto fish such as tuna and swordfish, and some of the smallest remoras travel in the mouths or gills of large manta rays, ocean sunfish, swordfish and sailfish.
The relationship between a remora and its host is most often taken to be one of commensalism, specifically Phoresis.
While some of the relationships are mutualistic, it is believed that dolphins with remoras attached do not benefit from the relationship. The attachment of the remora increases the dolphin's drag, which increases the energy needed for swimming. The remora is also thought to irritate the skin of the dolphin.
Remoras, like many other fishes, have two different modes of ventilation. Ram ventilation is the process in which at higher speeds, the remora uses the force of the water moving past it to create movement of fluid in the gills. At lower speeds, the remora will use a form of active ventilation, in which the fish actively moves fluid through its gills. In order to use active ventilation, a fish must actively use energy to move the fluid; however, determining this energy cost is normally complicated due to the fish's movement when using either method. As a result, the remora has proved invaluable in finding this cost difference (since they will stick to a shark or tube, and hence remain stationary despite the movement, or lack thereof, of water). Experimental data from studies on remora found that the associated cost for active ventilation created a 3.7–5.1% increased energy consumption in order to maintain the same quantity of fluid flow the fish obtained by using ram ventilation.
Other research into the remora's physiology came about as a result of studies across multiple taxa, or using the remora as an out-group for certain evolutionary studies. Concerning the latter case, remoras were used as an outgroup when investigating tetrodotoxin resistance in remoras, pufferfish, and related species, finding remoras (specifically Echeneis naucrates) had a resistance of 6.1–5.5 M.
Similar reports come from Japan and from the Americas. 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 : what was taken for accounts of the Americas may have been, in fact, notes Columbus derived from accounts of the East Indies, his desired destination.
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