Chemical communication in aquatic systems: an introduction

Abstract

Among higher terrestrial organisms we consider it self-evident that interactions are not only influenced byvisual signals, but that also olfactory and auditorysenses are involved in e.g. predator-prey interactionsand food and partner finding. In aquatic systems, how-ever, interactions based on auditory and olfactory com-munications are less obvious and, as a result, empiricalstudies, as well as theory, are generally focused ondirect confrontations between organisms. For example,general food chain theory only considers direct feedinglinks between predator and prey and neglects the possi-bility that some prey may detect and respond be-haviourally or morphologically to the presence of apredator (but see e.g. Abrams 1996). In recent years,however, there has been an increasing interest in under-standing the importance of non-visual signals for com-munication in aquatic systems. Poor light transmissionin water, especially in turbid waters, and high habitatcomplexity reduce the use of vision in aquatic organ-isms. Instead, a number of studies have shown thataquatic organisms from many different taxa and func-tional groups respond to minute concentrations ofchemical substances released by other organisms. Oneof the most well-studied responses to such chemicalsubstances is when a prey organism responds to chemi-cal substances released by a predator (Fig. 1). Numer-ous studies have shown that prey reduce theirvulnerability to predation by changing morphology, lifehistory strategy and:or behaviour when exposed tosubstances emitted by a predator. Further, chemicalcommunication has shown to be important in locatingfood and a partner, as well as in kin recognition.Finally, waterborne chemicals are most probably im-portant for navigation among long-distance migrators,such as salmonids (Fig. 1).In order to assess the ‘‘state of the art’’, stimulatediscussion, and to identify future research areas, weorganised a workshop on chemical communication inaquatic systems at Lund University, Sweden in October1998. We became very enthusiastic when realising, al-ready when announcing this workshop, the enormousinterest for this intriguing research field. It was alsointeresting to note that all the invited scientists immedi-ately accepted our invitation to the workshop, againillustrating that chemical communication in aquaticsystems is indeed a thrilling, albeit still confusing,subject!Here, we will give a short introduction to this re-search field and to the intriguing results presentedduring the workshop. We originally had the intentionto write a review that covered the present state of thefield, but soon realised that although this is a youngand growing area of research there is already enoughmaterial to fill a monograph! Further, there alreadyexists good reviews in several specific sub-areas (whichwe will point to below). Thus, instead of writing acomprehensive review we will settle for a short intro-duction to the field where we highlight those situationswhere chemical communication has proven importantin aquatic systems.When we looked back at who we invited to theworkshop and when we started to outline this Introduc-tion to the Special section of Oikos, it became evidentthat we are biased towards the importance of chemicalcommunication as a means of detecting a consumer(predator or herbivore). This, of course, mirrors ourpersonal interest, but we also think that this is anespecially interesting and dynamic sub-area of chemicalcommunication in aquatic systems where much pro-gress is being made and where much of the resourcesare invested. Consumer-resource interactions are ofcourse crucial for processes at the population and com-munity level and, thus, if chemical communication af-fects such interactions we must learn more about it tofully understand the dynamics of natural systems. Be-low, we shortly review this, as well as other areas wherechemical communication has shown to be important inaquatic systems, and introduce the contributed papersto this Special section of Oikos.