Daphnia, keystone herbivores in lakes, routinely produce immediately hatching eggs; additionally, they also produce resting eggs enveloped by an ephippial case, a thickened carapace that allows population survival under harsh environmental conditions. To examine differences in ephippial morphology between Daphnia species in different subgenera, we conducted microscopic observations and genetic analyses based on the mitochondrial 12S rRNA gene in ephippia from surface sediment in Lake Biwa, Japan. The lengths and heights of ephippia identified as Daphnia galeata Sars (Hyalodaphnia) were less than 0.82 and 0.50 mm, respectively, whereas those of Daphnia pulicaria Forbes (Daphnia) were greater than 0.87 and 0.53 mm, respectively, with the ephippial lengths of the two species differing significantly. The results indicate that D. galeata and D. pulicaria inhabiting Lake Biwa can be distinguished based on ephippium size, with a boundary ephippium length of approximately 0.86 mm. In concordance with this inference, historical data indicated that the length of ephippia recovered from sediment cores did not exceed 0.86 mm prior to the 1980s when D. galeata was the predominant species; however, it exceeded the threshold after 2000, coinciding with the coexistence of D. galeata and D. pulicaria.

水蚤，湖泊中的基石食草动物，通常会立即产卵。此外，它们还产生被卵囊包裹的静止卵，卵囊是增厚的甲壳，可以使种群在恶劣的环境条件下生存。为了研究不同亚属中水蚤种类之间的表皮形态差异，我们基于日本琵琶湖表层沉积物中的线粒体中线粒体12S rRNA基因进行了显微镜观察和遗传分析。被鉴定为Daphnia galeata Sars（Hyalodaphnia）的以弗皮亚的长度和高度分别小于0.82和0.50 mm，而Daphnia pulicaria Forbes（Daphnia））分别大于0.87和0.53 mm，这两个物种的短暂性长度明显不同。结果表明，居住在琵琶湖上的D. galeata和D. pulicaria可以根据的大小进行区分，边界a的长度约为0.86 mm。根据这一推论，历史数据表明，从沉积物芯中回收的ephippia的长度在1980年代之前（D. galeata是主要物种）不超过0.86 mm 。但是，它在2000年之后超过了阈值，与D. galeata和D. pulicaria并存。