Longest fish fins (relative to body size)

The fish with longest fins relative to body length is the tripodfish (Bathypterois grallator) a benthic species distributed widely in the Atlantic, Pacific and Indian Oceans. It measures approximately 30 cm (1 ft) long in total body length, but each of its two pelvic fins and its lower caudal fin possesses an extremely elongate ray that can measure over 1 m (3 ft 3 in) long, i.e. more than three times this fish's total body length. When held vertically downwards, these rays become stiff (possibly due to fluid being pumped into them), enabling the fish to stand motionless high above the ocean bottom, as if resting upon a tripod, in order to sense approaching prey without warning them of its own presence, and using its antenna-like pectoral fins as tactile organs. When swimming, however, its tripod rays become much more flexible, trailing horizontally behind it. The tripodfish is the only member of its taxonomic family (Ipnopidae) to possess such proportionately lengthy fins.

The tripodfish is a marine species, but there is a totally unrelated freshwater fish that can behave in a similar manner. The West African butterfly fish (Pantodon buchholzi) has large pectoral fins, which it generally holds out horizontally like a pair of butterfly wings, hence its name, and which it can use to launch itself out of the water. In addition, its much smaller pelvic fins each contain four very long thin rays that point downwards, and if present in very shallow water it will utilize these extra-long rays to "stand" on substrata beneath it.