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Changes in day-length entrain the endogenous clock of organisms leading to complex responses to photoperiod. In long-lived organisms experiencing several seasons this response of the clock to photoperiod is phenotypically plastic. However, short-lived organisms often experience a single season without pronounced changes in day-length. For those, a plastic response of the clock to different seasons would not necessarily be adaptive. In aquatic ecosystems, zooplankton species like  Daphnia  live only for some weeks, i.e. one week up to ca. two months. However, they often show a succession of clones that are seasonally adapted to environmental changes. Here, we found that 16  Daphnia  clones per each of three seasons ( = 48 clones) from the same pond and year differed in clock gene expression with a homogenous gene expression pattern in ephippia-hatched spring clones and a bimodal expression pattern in summer and autumn populations indicating an ongoing adaptation process. We clearly demonstrate that spring clones were adapted to a short, and summer clones to a long photoperiod. Furthermore, we found that gene expression of the melatonin-synthesis enzyme AANAT was always lowest in summer clones. In the Anthropocene,  Daphnia ’s clock might be disturbed by light-pollution and global warming. Since  Daphnia  is a key-organism in trophic carbon transfer, a disruption of its clock rhythm would be devastating for the stability of freshwater ecosystems. Our results are an important step in understanding the adaptation of  Daphnia ’s clock to environmental changes.

Photoperiodic adaptation of aanat and clock gene expression in seasonal populations of Daphnia pulex