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Fish locomotion
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=== Mechanism === {{Further|Fish fin}} [[File:Lampanyctodes hectoris (fins).png|thumb|right|upright=1.2|Fins used for locomotion: (1) pectoral fins (paired), (2) [[pelvic fin]]s (paired), (3) [[dorsal fin]], (4) adipose fin, (5) anal fin, (6) [[caudal fin|caudal (tail) fin]] ]] Fish swim by exerting force against the surrounding water. There are exceptions, but this is normally achieved by the fish contracting [[muscle]]s on either side of its body in order to generate waves of [[flexion]] that travel the length of the body from nose to tail, generally getting larger as they go along. The [[Euclidean vector|vector]] [[force]]s exerted on the water by such motion cancel out laterally, but generate a net force backwards which in turn pushes the fish forward through the water. Most fishes generate thrust using lateral movements of their body and [[caudal fin]], but many other species move mainly using their median and paired fins. The latter group swim slowly, but can turn rapidly, as is needed when living in coral reefs for example. But they can not swim as fast as fish using their bodies and caudal fins.<ref name=Breder/><ref name=Sfakiotakis/> [[File:Skeletal anatomy of tilapia.png|thumb|left|upright=1.5|Skeletal anatomy of ''[[Tilapia]]''<ref name=GeSCI2021>[https://oer-studentresources.gesci.org/wp-content/courses/Biology/Bio-F4-Support-and-Movement-Plants-and-Animals/locomotion_in_finned_fish.html Locomotion in Finned Fish], ''[[Global e-Schools and Communities Initiative]]'' (GeSCI) United Nations. Retrieved 7 Sep 2021. [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>]] Consider the [[tilapia]] shown in the diagram. Like most fish, the tilapia has a streamlined body shape reducing water resistance to movement and enabling the tilapia to cut easily through water. Its head is inflexible, which helps it maintain forward thrust.<ref name=GeSCI2021 /> Its [[Fish scale|scales]] overlap and point backwards, allowing water to pass over the fish without unnecessary obstruction. Water friction is further reduced by mucus which tilapia secrete over their body.<ref name=GeSCI2021 /> [[File:6DOF en.jpg|thumb|right|Like a plane or submarine, a fish has [[six degrees of freedom]].]] The backbone is flexible, allowing muscles to contract and relax rhythmically and bring about undulating movement.<ref name=GeSCI2021 /> A [[swim bladder]] provides buoyancy which helps the fish adjust its vertical position in the [[water column]]. A [[lateral line]] system allows it to detect vibrations and pressure changes in water, helping the fish to respond appropriately to external events.<ref name=GeSCI2021 /> Well developed fins are used for maintaining balance, braking and changing direction. The pectoral fins act as pivots around which the fish can turn rapidly and steer itself. The paired pectoral and pelvic fins control [[Pitch (aviation)|pitching]], while the unpaired dorsal and anal fins reduce [[Yaw (rotation)|yawing]] and [[Roll (flight)|rolling]]. The caudal fin provides raw power for propelling the fish forward.<ref name=GeSCI2021 />
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