Geographical continuity and discontinuity in the meristic characteristics of ayus of the southern subspeciesPlecoglossus altivelis ryukyuensis

Two subspecies of ayu,Plecoglossus altivelis altivelis(nominotypical subspecies) andP. a. ryukyuensis(Ryukyu-ayu), differ morphologically, behaviorally, and genetically. Specimens from nine populations comprising both subspecies were investigated to examine whether their meristic characteristics follow a widely observed geographical tendency known as Jordan's rule. Seven out of the eleven characteristics increased in frequency with latitude, following Jordan's rule. Contrarily to the expectation from Jordan's rule, however, Ryukyu-ayu had more anal fin rays than nominotypical subspecies in the temperate zone, suggesting genetic divergence underlying this characteristic between these two subspecies. The two subtropical populations showed a reversal of the geographic cline in the number of scales and dorsal fin rays, indicating these do not share a gene pool. The present study, without using genetic indicator, remains disputable to consider genetic status within Ryukyu-ayu further.

Automated summary

Two subspecies of ayu, Plecoglossus altivelis altivelis and P. a. ryukyuensis, exhibit morphological differences and genetic divergence, with Ryukyu-ayu showing unique characteristics compared to the nominotypical subspecies. The meristic characteristics of these two subspecies follow a geographical tendency known as Jordan's rule, except for some traits. This study, without using genetic indicators, suggests the presence of genetic divergence within Ryukyu-ayu.