Interspecific variation in ephippial size between Daphnia galeata and D. pulicaria in Lake Biwa, Japan

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 .