CLADOCERA Interclonal variation in diel horizontal migration behaviour of the water ﬂea Daphnia magna—searching for a signature of adaptive evolution Helen Michels Æ Susanne Lildal Amsinck Æ Erik Jeppesen Æ Luc De Meester Ó Springer Science+Business Media B.V. 2007 Abstract In shallow temperate lakes, zooplankton populations may exhibit diel horizontal migration (DHM) and move towards macrophytes during the day to avoid ﬁsh. Using a natural Daphnia magna population, we undertook an experimental investigation aimed to describe the genetic variation for DHM and to study whether an adaptive micro-evolutionary response oc- curred to changes in macrophyte coverage and ﬁsh predation pressure through time. Twenty-seven D. magna clones were hatched from ephippia in the sediment of shallow Lake Ring, Denmark. This lake was eutrophied during the 20th century and was subject to restoration measures in the 1970s. The DHM behav- iour of the clones was observed both in the presence and absence of ﬁsh kairomone. Signiﬁcant interclonal variation in DHM behaviour occurred in both treatments. To study the micro-evolutionary response of the Lake Ring D. magna population, two approaches were used. First, we compared the DHM behaviour of clones derived from ephippia collected at different depths. A comparison was conducted between clones resurrected from the period of eutrophication (1960–1980) and from the period of recovery (1986–2000). A signiﬁcant treatment (presence and absence of ﬁsh kairomone) · period interaction effect was identiﬁed, suggesting a signiﬁcant micro-evolutionary response for DHM behaviour. The D. magna clones exhibited a signiﬁcantly stronger horizontal migration response during the period of eutrophication than in the recovery phase. Second, clonal means, representing the inﬂuence of the genotype on the trait, were correlated with environmental condi- tions (macrophyte cover, ﬁsh predation pressure and Secchi depth). The results of this analysis also suggest that a micro-evolutionary response by Daphnia has occurred in reaction to changes in ﬁsh predation pressure. In periods with high ﬁsh predation pressure, Daphnia migrated more strongly towards the plants. Keywords Diel horizontal migration behaviour  Adaptive evolution  Daphnia magna  Paleolimnology  Shallow lake  Resurrection ecology Introduction Recently, there is a rapidly growing interest in the inﬂuence of anthropogenic impact on the genetic Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera H. Michels (&)  L. De Meester Laboratory of Aquatic Ecology, Katholieke Universiteit Leuven, Ch. Debe ´riotstraat 32, Leuven 3000, Belgium e-mail: helenmichels@gmail.com S. L. Amsinck  E. Jeppesen Department of Freshwater Ecology, National Environmental Research Institute, Vejlsøvej 25, Silkeborg 8600, Denmark E. Jeppesen Department of Plant Biology, University of Aarhus, Ole Worms Alle ´, Building 135, Aarhus C 8000, Denmark 123 Hydrobiologia (2007) 594:117–129 DOI 10.1007/s10750-007-9086-1