pelvic fin

US /ˈpel.vɪk fɪn/
UK /ˈpel.vɪk fɪn/
"pelvic fin" picture
1.

腹びれ, 腹鰭

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

:
The fish used its pelvic fins to maintain balance in the current.
魚は流れの中でバランスを保つために腹びれを使った。
Sharks have two pelvic fins located near their cloaca.
サメには総排出腔の近くに2つの腹びれがある。