A scale-eating triacanthodid, Macrorhamphosodes uradoi: prey fishes and mouth "handedness" (Tetraodontiformes, Triacanthoidei)

Abstract

Gut content analyses of deep-sea triacanthodid (Macrorhamphosodes uradoi) specimens from Tosa Bay and the Kyushu-Palau Ridge, southern Japan, revealed that the species primarily feeds on fish scales. A total of 55 042 scales from 48 specimens were grouped into 19 types, 14 being identifiable between family and species levels. Scale composition differed significantly between the two localities, with only 6 types occurring at both. Glossanodon semifasciatus (Argentinidae) and Emmelichthys struhsakeri (Emmelichthyidae) were the dominant prey in Tosa Bay and on the Kyushu-Palau Ridge, respectively. The scale morphology indicated that M. uradoi steals scales from the caudal fin and base of prey. The long tubular snout of M. uradoi was recognized as an adaptation for obtaining scales while keeping the body in a relatively posterior position, thereby minimizing the likelihood of being noticed by the prey. Furthermore, the mouth was twisted to various degrees, either to the left or the right, the angle of twisting not being correlated with specimen size. The variable nature of twist angles was hypothesized as providing a variety of strike tactics, each angle defining an optimum, resulting in lessened opportunities for learned responses by prey fishes.