Hatchetfishes Hold Horizontal Attitudes During Diagonal Descents

Abstract
Based on analysis of videotape records, species of the genusArgyropelecusare capable of swimming diagonally downwards at considerable speeds without altering their body postures. Morphological evidence strongly suggests that they are able to swim diagonally upwards in a similar way. Their distinctive hatchetlike profile presents a streamlined airfoil in both diagonal and horizontal movements, even though the body is not tilted. Modifications of the caudal musculature probably enable all hatchetfishes (including members of the other two genera,PolyipnusandSternoptyx) to use the tail for propulsion for both diagonal and horizontal movements. The dorsal fin probably aids in diagonally downward movements and the pelvic and anal fins in diagonally upward movements, whilst the pectoral fins serve as stabilizers for these movements, and as an axis of rotation for striking at prey. The seemingly ungainly shape of the hatchetfish is the result of achieving a streamlined, fish-like shape for horizontal and diagonal movements without tilting of the body, presumably in order to maintain an effective camouflage in midwater.

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