The morphological basis of labriform rowing in the deep-sea Bigscale Scopelogadus beanii (Percomorpha: Beryciformes)

A recent shipboard video reveals that the deep-sea melamphaeid fish Scopelogadus beanii is capable of rowing labriform propulsion. These data indicate that S. beanii engages in powerful and highly maneuverable drag-based locomotion, an energetically expensive swimming mode in which the pectoral fin produces thrust from a fore-aft rowing stroke. The pectoral-girdle musculoskeletal morphology, including muscle insertions and origins is described herein, and estimations of isometric force produced by each division are reported based on calculations of physiological cross-sectional area. The study species possesses similar adductor muscle configuration but has one fewer abductor muscle as compared to previously studied rowers. In addition, girdle muscle masses are compared with a previously published morphospace of labriform swimmers. From this analysis, it is inferred that muscles occupying a portion of morphospace not yet described for rowing fishes, power the dynamic sculling of S. beanii. Placing these results in an ecomorphological context, the resource utilization of S. beanii is considered, specifically that it is a consumer of gelatinous zooplankton, a consistently distributed food resource. Although S. beanii resides in the dimensionless bathypelagic zone of the open ocean, a swimming mode that increases maneuverability is perhaps an adaptation to feed among dense aggregations of its prey.