Fish faeces as the primary source of chemical cues inducing fish avoidance diapause in Daphnia magna Miroslaw  Slusarczyk* & Ewa Rygielska Department of Hydrobiology of University of Warsaw, Banacha 2, 02-097 Warszawa, Poland (*Author for correspondence: E-mail: m.s.slusarczyk@uw.edu.pl) Key words: kairomone, alarm substance, induced response, diapause, anti-predator defence Abstract In a laboratory batch culture experiment, females of Daphnia magna were exposed to five different experimental media containing either: (1) water from an aquarium with fish, (2) extract of fish faeces, (3) mixture of both media, (4) extract of homogenised conspecific Daphnia, or (5) control water without the addition of extra cues. The experiment was planned to test potential pathways of excretion of the chemical cues that induce resting-egg formation in D. magna and to find an effective way of collecting these chemical cues. The results indicate that fish faeces are the prevailing source of the chemical cues that induce resting- egg production in D. magna. The ease of collection and the possibility of storing it in a frozen state make it a convenient cue for inducing diapause response in Daphnia. The results of the experiment imply that in natural conditions Daphnia may face high concentration of the inductive signals once migrating to the bottom zone where fish faeces commonly accumulate. Introduction The production of resting stages, which are typi- cally more resistant then active individuals to various unfavourable conditions, is a common way of genome protection in seasonally deterio- rating environments. The diapause response is most widely associated with harsh abiotic (over- freezing, desiccation) and biotic (food depletion, overcrowding) factors (Danks, 1987). It seems to be also used by some freshwater crustaceans to avoid periods of high predation risk from fish (Threlkeld, 1979; Hairston, 1987). The mechanism of fish avoidance diapause may be triggered by indirect cues like photoperiod and temperature when the token cues are correlated with a sea- sonally occurring elevated risk of fish predation as indicated by studies on summer diapause of cala- noid copepod Diaptomus sanguineus (Hairston & Kearns, 1995). It may be also cued by direct sig- nals – fish kairomones, chemical compounds re- leased into environment by fish predators as indicated by experimental studies on cladoceran Daphnia magna (  Slusarczyk, 1995; Pijanowska & Stolpe, 1996). Not only chemical composition but also the ways inductive substances are released into the environment remain unclear. At least three potential sources of inductive cues could be considered. They could be released directly from injured tissues of an attacked prey, excreted by fish through the surface of the body, or freed from fish faeces. Fish kairomones, which trigger behavioural defence reactions of D. magna were found so far in the fish faeces, mucus and intestinal tissues (Loose et al., 1993). Furthermore, the diapause response of D. magna to fish predation seems to be triggered Hydrobiologia 526: 231–234, 2004. J. Pijanowska, P. Dawidowicz, A. Jachner & K. Szeroczyn ´ska (eds), Cladocera. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 231