Backward swimming in elongated-bodied abyssal demersal fishes: Synaphobranchidae, Macrouridae, and Ophidiidae.

The deep-sea demersal fish fauna is characterized by a prevalence of elongated-body forms with long tapering tails. Using baited camera landers at depths of 4500-6300 m in the Pacific Ocean, we observed multiple instances of backward swimming using reverse undulation of the slender body in four species: the cutthroat eel Ilyophis robinsae, abyssal grenadier Coryphaenoides yaquinae, and cusk-eels Bassozetus sp. and Barathrites iris. Backward swimming was used as an escape or repositioning maneuver, reversing for up to seven tail beats before resuming forward swimming in a new direction. The eel I. robinsae reversed with a swimming wave frequency of 0.51-0.95 Hz, wavelength 0.6-0.75 of the body length (L), and large amplitude movements of the head from side to side. C. yaquinae reversed relatively slowly at 0.21-0.52 Hz and wavelength 0.5-0.7 L aided by propulsive movements of the pectoral fins and minimal lateral movement of the head. The ophidiids also used reversed propulsive body waves augmented by paddling with the pectoral fins but with some lateral movement of the head. Pectoral-fin movements in all species were in synchrony with the body movements. The elongated-body form enables backward swimming by reversal of the anguilliform propulsive wave and has the advantage that the fish automatically returns to safety along the path recently traveled. This maneuverability conferred by an elongated body may be a significant factor in selection for body shape in deep-sea fishes.