pelvic fin
US /ˈpel.vɪk fɪn/
UK /ˈpel.vɪk fɪn/
1.
either of a pair of fins on the underside of a fish's body, typically behind the pectoral fins, that help control direction and stability
:
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The fish used its pelvic fins to maintain balance in the current.
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Sharks have two pelvic fins located near their cloaca.