Hydrobiologia 491: 221–239, 2003. E. van Donk, M. Boersma & P. Spaak (eds), Recent Developments in Fundamental and Applied Plankton Research. © 2003 Kluwer Academic Publishers. Printed in the Netherlands. 221 Chemically induced anti-predator defences in plankton: a review Sandra Lass & Piet Spaak Department of Limnology, EAWAG/ETH, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland Tel: +41 1 823 5187. Fax: +41 1 823 5315. E-mail lass@eawag.ch Received 5 July 2001; in revised form 29 January 2002; accepted 8 February 2002 Key words: inducible defence, Daphnia, ciliate, rotifer, algae, kairomone, adaptation, cost, identification Abstract Planktonic organisms exhibit diverse morphological, behavioural and life-history responses to the chemical presence of potential predators. Prey organisms have been found to sense such predators via predator-derived kairomones. The induced reactions are assumed to reduce predation risk and thus to be adaptive. Numerous studies have investigated various aspects of inducible defences in different crustaceans, in rotifers, planktonic ciliates and algae. As a first step, we summarise recent work on chemically induced anti-predator defences in morphology, life history and behaviour. Morphological defences have been found in a wide range of different plankton organisms and recent studies on predator-induced morphologies mainly addressed the question of costs for these changes. Life-history responses were mainly studied in cladocerans and several studies have recently addressed some novel topics, such as diapause induction and the influence of predator kairomones on hatching of resting stages. Behavioural anti-predator defences also have been found for several plankton species and are characterised by relatively fast induction times. We further identified four research directions in which substantial progress has been made recently: (I) The effects of simultaneous exposure to infochemicals from different predators and the consequences of a complex chemical environment. Some environmental contaminants, such as synthetic chemicals or heavy metals, have been found to potentially disturb natural chemical communication in aquatic predator-prey systems. (II) The influence of genetic variation on the reaction to infochemicals and its implications. Clonal differences have not only been found for the presence or absence of a certain trait but also with respect to the type of response. (III) The degree to which different types of responses to a specific kairomone are coupled. Recent studies underline the uncoupling of different anti-predator responses of which some have been considered to be coupled. (IV) Studies on the chemical properties and on the metabolic origin of predator kairomones. Substantial progress has been made recently, especially with respect to the identification of predator kairomones that are important for planktonic ciliates. The identification and isolation of kairomones are an important step towards studies addressing the consequences of predator-induced defences on the level of populations, communities and ecosystems. So far most studies have considered effects and consequences on the level of individual prey organisms and studies taking the consequences at higher ecological levels into account are rare. Introduction Predator-induced responses in plankton have elicited increasing research interest within the last two dec- ades. Since Larsson & Dodson (1993) reviewed the state-of-the-art of the research on chemical commu- nication in planktonic animals, further studies on this phenomenon have been carried out and several new research lines on anti-predator defences in plankton have developed. Chemical communication is a well-known eco- logical phenomenon mediating interactions between organisms via infochemicals (Dicke & Sabelis, 1992). Kairomones constitute the class of infochemicals evoking a behavioural or physiological reaction in the receiver that is adaptively favourable for the receiver but not for the sender. Predator-derived kairomones play an important role in ecological and evolutionary processes that enable the prey to survive predation pressure. The wide variety of responses to predator