were treated with the antibiotic ampicillin, this enhancement was significantly decreased. Therefore, not fish, but bacteria associated with fish, seem to produce ...
Journal of Plankton Research Vol.20 no.9 pp.1847-1852, 1998
SHORT COMMUNICATION Do bacteria, notfish,produce 'fish kairomone'? Joop Ringelberg and Erik Van Gool1 N 10O-Centre for Limnology, Rijksstraatweg 6,3631 AC Nieuwersluis and Department of Aquatic Ecology, University of Amsterdam, Amsterdam, The Netherlands 1
To whom correspondence should be addressed
Abstract. Fish-associated chemicals enhance phototactic downward swimming in Daphnia. If perch were treated with the antibiotic ampicillin, this enhancement was significantly decreased. Therefore, not fish, but bacteria associated withfish,seem to produce this kairomone.
Over the past decade, chemical communication in predator-prey relationships has received much attention in terrestrial ecology and aquatic ecology (Larsson and Dodson, 1993; Harvell and Tollrian, 1998). The effect of fish kairomones (for terminology, see Dicke and Sabelis, 1988) on Daphnia has been described in a large number of papers. These chemical substances induce changes in morphology (Tollrian, 1990; Machacek, 1993), life history traits (Weider and Pijanowska, 1993; Stibor and Liming, 1994; Reede and Ringelberg, 1995; Reede, 1997) and behaviour (Dodson, 1988; Ringelberg, 1991a; De Meester, 1993; Forward and Rittschof, 1993; Loose, 1993; Van Gool and Ringelberg, 1995). In the pelagic food web of lakes, Daphnia are staple food for fish and the induced changes in vulnerability are certainly important in the dynamics and composition of the community. The Daphnia-hsh relationship has become a model for evolutionary studies (De Meester, 1994; De Meester et al, 1998). Nevertheless, the precise origin of the kairomone and its chemical identity are still unknown, although several characteristics have been revealed (Von Elert and Loose, 1996). The phenotypically induced, predation-diminishing strategies are disadvantageous to the predator and selection against production of the infochemical might be expected. This selection does not seem to be very strong since kairomone production is widespread in freshwater fish and not related to Daphnia predation. Two alternative explanations may hold for the existence of fish kairomones: (i) these kairomones are essential to the functioning of the predator and the negative effects are counterbalanced; (ii) the kairomones are not produced by the predator at all. In the latter case, we must assume that some other organism, closely associated with the predator, is responsible for their production. Indeed, using an antibiotic, we demonstrated that, not fish, but bacteria associated with fish, are involved. A bioassay was used to test for the presence of kairomones, with the phototactic downward swimming response, caused by increases in light intensity, as a behavioural test criterion. Fish kairomones enhance phototactic downward swimming (Ringelberg, 1991b). The apparatus included a series of six cylinders, 7 cm © Oxford University Press
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in diameter and 100 cm in length, placed in a water bath (20°C) and illuminated from above by a row of incandescent lamps (adaptation light intensity 1 umol nr 2 s"1). The stimulus consisted of an increase in light intensity lasting for 13 min, with a final intensity of 16 umol nr 2 s"1. One day before the observations were started, 30 mature females of the hybrid D.galeata X hyalina were transferred to each cylinder with water from Lake Maarsseveen and Scenedesmus acutus (0.3 mg C I"1) as food source. To stimulate breakdown of possible traces of kairomone, the water from Lake Maarsseveen had been circulating over a sand filter for at least 3 days. Prior to use, the water from the sand filter was filtered over a 0.45 um acetate filter to remove debris. Water with kairomones was taken from a small aquarium (81) with one juvenile perch (Perca fluviatilis, -4 cm length). A juvenile perch was placed in Lake Maarsseveen water either without or with the antibiotic ampicillin (20 mg I"1) 2 days before the experiments. Water from these aquaria was diluted eight times to test for effects on photobehaviour. Photobehaviour was measured for different treatments: filtered lake water (Co), lake water with ampicillin added (CoBa), fish water with (FiBa) and without ampicillin (Fi), and fish water with ampicillin added after it was taken from the aquarium (pseudoFiBa). Observations were started around noon on the day after the introduction of daphnids into the cylinders. First, the vertical distribution of the daphnids, before the start of the light increase, was established by three successive counts of the numbers present in 10, equally sized, compartments of each cylinder. Then, the light intensity increase was started. After 11 min, when the reactive downward swimming had almost stopped, vertical distributions were determined again. The original light intensity was then restored and, after 1.5 h of adaptation, the procedure was repeated. In this way, observations on the same animals were made three times (repeated measures, a = 3; however, a = 4 for treatment F; see Table I). For each cylinder and count, the initial mean population depth was calculated. The test criterion, the reactive vertical displacement, was determined as the difference between the average initial mean population depth (n = 3) and the final mean population depth (n = 1). Per experiment, two treatments were present in two cylinders each, randomly distributed over the six cylinders, and two controls (Co). A complete experiment was performed twice (treatment A, B, C; Table I), with different perch and different Daphnia and several weeks apart, or once (treatment D, E, F; Table I). We used repeated measures ANOVA to test for significant treatment effects (SYSTAT, Version 5.2). The sequence in counting cylinders was introduced as a cofactor in the analysis. The results are presented in Figure 1 and Table I. The large increase in reactive vertical displacement in the presence of fish water (Fi), as compared with the control (Co), is evident (Ho: Fi = Co, P < 0.001; Table IA). Obviously, a chemical substance, excreted into the aquarium by the perch, enhances strongly downward swimming upon the light intensity increase. This effect is reduced markedly if ampicillin is added to such an aquarium for 2 days (Figure 1; H o : FiBa = Fi, P < 0.001; Table IB), although the enhancing effect had not disappeared completely (Figure 1; H
