How a population adapts to environmental changes is a central topic in ecology, but long-term changes in the phenotype of an organism have rarely been studied in aquatic systems. In this study, we examined the morphological changes in Daphnia pulex from the initial establishment stage, using paleolimnological analysis, in Lake Fukami-ike (Nagano Prefecture, Japan). We measured morphological traits of ephippia and subfossils of D. pulex and compared the results with the population dynamics assessed in an earlier study. D. pulex established a stable population in the lake in the early 2000s. The length of the ephippia significantly increased in the mid-2000s and then decreased more recently, whereas the length of the post-abdominal claw as a proxy of body size did not show significant changes. This suggests that their adult body size was larger, and the time to maturity was delayed or the shift to the dormant phase got later in the mid-2000s, although the mean body size did not change. Subfossils of Chaoborus and the historical records suggest that planktivorous fish were less abundant and Chaoborus larvae were more abundant in the mid-2000s than in more recent periods. These results suggest that D. pulex adaptively changed phenotypic traits in response to changes in the predator community in the studied lake.

种群如何适应环境变化是生态学的中心课题，但是在水生系统中很少研究生物体表型的长期变化。在这项研究中，我们使用古湖泊学方法研究了深水湖（日本长野县）从初始建立阶段起的水蚤的形态变化。我们测量了D. pulex的ephippia和亚化石的形态特征，并将结果与早期研究中评估的种群动态进行了比较。普雷克斯在2000年代初期在湖中建立了稳定的种群。ephippia的长度在2000年代中期显着增加，然后在最近逐渐减少，而腹部后爪作为体型的长度没有显示出明显的变化。这表明它们的成年个体较大，尽管平均体型没有变化，但它们的成熟时间推迟了，或者到了潜伏期才在2000年代中期推迟。潮虫属的化石和历史记录表明，与最近时期相比，2000年代中期浮游鱼类的数量较少，潮虫幼虫的数量较多。这些结果表明D. pulex 适应性变化的表型性状，以响应所研究湖泊中捕食者群落的变化。