Which fish has the largest mouth relative to its body size?

The fish with the largest mouth relative to its body size is the pelican eel.

The pelican eel (Eurypharynx pelecanoides) is a deep-sea eel named for its enormous, pelican-like mouth. Its loosely hinged jaws can unfold and inflate, creating a wide scoop for collecting prey in the dark, food-poor deep ocean. The jaws alone may occupy about a quarter of the eel’s total length.

Despite its frightening appearance, the pelican eel is not primarily a fish-eater. Its tiny teeth and stomach contents suggest a diet dominated by small crustaceans such as shrimp. The huge mouth likely helps it gather scattered prey efficiently and take advantage of rare feeding opportunities.

A common mix-up is the name “gulper eel,” which can refer to related eels in the genus Saccopharynx as well as the pelican eel. The pelican eel is the only known species in its genus, Eurypharynx, and usually grows to around 75 centimetres long.