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EveryFish

Anal fin

The single fin on the underside behind the vent, which steadies a fish from below.

The anal fin sits on the belly midline behind the vent, the opening for waste and eggs. Together with the dorsal fin it keeps a fish from rolling. In knifefish, including the electric eel, a very long anal fin runs along most of the body and ripples to drive the fish forwards or backwards while the body stays straight. In male guppies, mollies and swordtails part of the anal fin is shaped into the gonopodium, a rod that passes sperm to the female.

See also

Gonopodium Dorsal fin Pelvic fin

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