Ecomorphological dimorphism of juvenile Trachurus japonicus in Wakasa Bay, Japan

Morphometric analyses of marine pelagic fishes have generally been conducted for stock identification rather than for ecomorphological understanding. Many papers on stock identification of Trachurus japonicus reported polymorphisms in the Japanese Islands waters. However, none of them has found polymorphism in fish less than 100 mm standard length (SL), despite the environmental differences experienced by juvenile cohorts. The objective of this study was to detect ecomorphological polymorphism of juvenile T. japonicus (< 100 mm SL) in Wakasa Bay, Japan, where multiple juvenile cohorts appear. From analyses of size frequency distributions and otolith microstructure, five cohorts were recognized in the bay from September 2003 to August 2004. We then compared 17 morphometric characters on body, fin, and otolith morphology, and found cohort-specific and roughly dimorphic pattern (a streamlined morph and a compressed morph). The dimorphism was markedly observed in 50-70 mm SL, and was regarded as specific to the juvenile stage by comparison with the senior dimorphisms (>= 100 mm SL). Referring to the literatures on functional morphology, the streamlined morph and the compressed morph were considered to be suitable to body and caudal fin (BCF) periodic propulsion and BCF transient propulsion, respectively. The juvenile dimorphism was interpreted as adaptive in its developmental environments (i.e., ecomorphological dimorphism) by relating the functional differences to the inferred ecological differences: the streamlined morph is adaptive to feed on larval Engraulis japonicus in coastal waters, whereas the compressed morph is adaptive to associate with jellyfishes in offshore waters.