Two subspecies of ayu, Plecoglossus altivelis altivelis (nominotypical subspecies) and P. 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.

中文翻译：

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南方亚种Plecoglossus altivelis ryukyuensis ayu分生特征的地理连续性和不连续性

香鱼的两个亚种 Plecoglossus altivelis altivelis（命名亚种）和 P. a. ryukyuensis (Ryukyu-ayu)，在形态、行为和遗传上都不同。对来自包括两个亚种的 9 个种群的标本进行了调查，以检查它们的分生特征是否遵循被广泛观察到的地理趋势，即约旦规则。遵循乔丹的规则，11 个特征中的 7 个特征的频率随着纬度的增加而增加。然而，与约旦统治的预期相反，琉球 ayu 的臀鳍鳍条比温带的典型亚种多，这表明这两个亚种之间的这一特征存在遗传差异。这两个亚热带种群在鳞片和背鳍条的数量上表现出地理倾斜的逆转，表明这些不共享基因库。目前的研究，没有使用遗传指标，在进一步考虑琉球香鱼的遗传状态方面仍然存在争议。