Constraints on the plasticity of Daphnia magna. influenced by fish-kairomones

Transcription

1 Functional Ecology 2000 Constraints on the plasticity of Daphnia magna Blackwell Science, Ltd influenced by fish-kairomones H. STIBOR* and D. MÜLLER NAVARRA Max-Planck Institut für Limnologie, Postfach 165, D Plön, Germany Summary 1. Daphnia magna reproduced at a smaller size (body mass) when exposed to fishkairomone than D. magna growing up without this cue. 2. The total egg mass for first reproduction was the same in both groups, but the proportion of clutch mass to total body mass was about 11% higher in daphniids exposed to fish-kairomone. This difference was about the same for low and high food concentrations (0 5 or 1 5 mgc l 1 ). 3. Although egg mass was the same, triglyceride (TAG) content of the egg material was significantly lower when daphniids were exposed to fish-kairomones for both food concentrations. 4. The TAG content of the clutch was lowered to the extent that the TAG proportion in the clutch in relation to the somatic mass of the animal was the same for animals treated with fish-kairomone and controls (5% of body mass at first reproduction). 5. The lower TAG content per clutch resulted in a lower quality of the individual egg in daphniids exposed to fish-kairomone. The resulting higher susceptibility to starvation under low food conditions is a substantial cost of the plastic response to fish-kairomones. 6. This cost may set a limit on the plastic response in resource allocation of Daphnia magna exposed to fish-kairomones. However, daphniids living in a system with fish predation are more likely to be eaten by fish than to starve to death. Key-words: Costs of defences, life history, phenotypic plasticity, reproductive allocation Functional Ecology (2000) Ecological Society Introduction The ability of an organism to modify its phenotype is an effective way to increase its fitness in a variable environment ( Dodson 1989; Stearns 1989). The ability of an organism to produce a range of different phenotypes in multiple environments has many obvious ecological benefits. However, factors that limit the evolution of this ability are more concealed. Lack of sufficient genetic variation (Via & Lande 1985) and other inherent costs of plasticity may be important in limiting the evolution of plasticity, a topic of recent interest in evolutionary ecology (DeWitt, Sih & Wilson 1998). In lakes, which are generally known to be variable environments for food availability and predation pressure, zooplankton of the genus Daphnia are paradigms of phenotypic plasticity ( Dodson 1989). Daphnia reproduces mainly parthenogenetically and has a generation time between one and several weeks, depending *Present address: Zoologisches Institut; LMU Muenchen, Karlstrasse 23 25, D München, Germany. Present address: Department of Environmental Science and Policy; University of California Davis, One Shields Avenue, Davis, CA 95616, USA. on food and temperature regime. Both invertebrate (e.g. Chaoborus larvae or copepods) and vertebrate (mainly fish) predators include daphniids in their diets. It has been shown that fish prefer to eat larger daphniids whereas invertebrate predators prefer mostly small ones (Zaret 1980; Townsend et al. 1986; Campbell 1991; but see Dodson & Havel 1988 and Stibor & Lüning 1994 for exceptions). Predator-induced changes in the phenotype serve as effective predator-defence mechanisms in environments with seasonal changes in predation pressure (see reviews by Larsson & Dodson 1993; Dodson et al. 1994; Tollrian & Harvell 1998). Daphnia can change their morphology, behaviour and life-history traits in response to chemical cues from predators so-called kairomones (see review in Larsson & Dodson 1993; Tollrian & Harvell 1998). Up-to-date information about basic physiological processes behind kairomone-induced phenomena is scarce. Hence, costs that are an important reason why such defences are not permanent will have their mechanistic base in the physiology of such processes. Costs of predator-induced changes in prey morphology and behaviour have been measured several times for aquatic organisms, whereas costs of life-history changes remain largely unknown (Harvell 1986; Havel 455

2 456 H. Stibor & D. Müller Navarra & Dodson 1987; Walls & Ketola 1989; Lüning 1992; Tollrian 1995). It is difficult to separate costs related to facultative life-history shifts from costs of indirect effects. Costs of indirect effects are related to predatorinduced changes in behaviour or morphology that affect the carbon budget, and thereby influence lifehistory parameters such as growth and reproduction (Ball & Baker 1996). Life-history changes in Daphnia influence the size at maturity depending on the size selectivity of the predator (Stibor & Lüning 1994). Daphniids exposed to chemical signals of predators that prefer small Daphnia (invertebrates), delay maturity until the animals are larger in size. Animals exposed to chemicals released by predators that prefer large daphniids (fish) do the opposite. Daphnia react to chemical cues from fish by maturing earlier, at a smaller size, and by using a higher proportion of net production for reproduction (Machacek 1991; Stibor 1992; Weider & Pijanowska 1993). Under laboratory conditions which excluded predation, neither life-history shifts induced by invertebrate nor vertebrate predator cues lowered Daphnia s fitness (measured as rate of population increase (r); Stibor 1995; Tollrian 1995). Under normal conditions the induced morphs do not suffer any disadvantages connected to their phenotypic changes. Therefore, it has been argued that a higher risk of mortality after a change in predation regime (with contrasting size selection by the new predators) is the main cost of induced life-history changes in Daphnia (Stibor & Lüning 1994; Tollrian 1995; Boersma, Spaak & DeMeester 1998). Smaller size at maturity lowers the risk of mortality when exposed to fish predation, especially since most Daphnia exposed to heavy fish predation have the chance to reproduce only once (Lampert 1993). Thus putting as much effort as possible into this reproductive event (which means higher resource allocation for reproduction) seems to be an effective strategy. Analyses of the carbon budget indicate that there are no costs of these shifts in terms of lower carbon assimilation or higher respiration rates (Stibor & Machacek 1998). Changes in the energy-allocation pattern for reproduction are a major result of changes induced by fishkairomone in Daphnia. We do not know if those changes in carbon allocation (quantitative changes) are also accompanied by changes in the biochemical composition (qualitative changes) of the reproductive material. Hence, this could be a very important source for potential costs as it is known that changes in egg composition can dramatically change the survival of Daphnia neonates under certain environmental conditions (Tessier et al. 1983; Gliwicz & Guisande 1992). Therefore, costs of fish-induced life-history changes could have a disadvantage for the next Daphnia generation, depending on egg quality. Here, a comparison of the quality of the reproductive material was conducted. As our measurement of quality, we used the fat content of the egg material. Fats are the major energy store and are known to be important in resisting starvation and therefore ensuring survival of young Daphnia (Tessier et al. 1983). The fat content was measured as triglycerides ( TAG) which are the main storage lipids found in Cladocera. Methods Some 160 neonate daphniids (Daphnia magna, Straus), not older than 4 h, were randomly split among four treatments, control and those exposed to fish-kairomone kept under two different food conditions, respectively. Animals were kept in membrane-filtered (0 45 µm) aged lake water from Lake Schöhsee. The fish-kairomone was enriched from water in which fish were previously kept for 24 h according to the method described in Loose, von Elert & Dawidowicz (1993). To exclude potential costs influenced by food availability, the experiments were conducted at high and low food conditions. The green alga Scenedesmus acutus ( Meyen) grown in a chemostat was used as the food source. The high food concentrations of 1 5 mg C l 1 represented non-limiting conditions and the low concentration of 0 5 mg C l 1 represented a near-limiting food level for this Daphnia species under present culture conditions (batch-culture). New media were prepared daily and the food levels were adjusted spectrophotometrically. Some 10 ml of the fish-kairomone extract was added to 1 l aged lake water to prepare the culture media for the animals exposed to fish-kairomone. The 40 daphniids per treatment came from the third clutch of mothers that had been kept under constant conditions at the Max-Planck-Institut in Plön. Experiments were performed under dim light at a controlled temperature of 20 C. During the experiment animals were raised individually in 200-ml glass-jars and transferred daily to new culture media until they produced their first clutch. Eggs were then carefully taken out of the brood chamber of the animals. The animals were dried for 24 h at 60 C and weighed individually to the closest µg. The eggs of a clutch were split into the following groups. One group was dried and weighed. The other group was analysed for TAG and homogenized (Ultrasonic) in 100 µl 0 1 M P-buffered NaCl solution (P-4417, Sigma, St Louis, MO, USA). The TAG measurements were done photometrically according to McGowan et al. (1983). The method is based on the digestion of TAG with a lipase (incubation at 20 C for 30 min). The resulting free glycerine is then phosphorylized and converted to quinominine and measured photometrically (at 540 nm). After correction for free glycerine of the sample prior to digestion and determination of the digestion efficiency (GPO Trinder reagent, Sigma), TAG contents can be quantified according to the absorption at 540 nm of the chemically treated sample, using glycine of known concentrations as standards. GPO Trinder reagent is normally used for the quantitative enzymatic determination of TAG in serum or plasma.

3 457 Plasticity of Daphnia magna Fig. 1. Total body mass (left bars) and clutch mass (right bars) at first reproduction of Daphnia magna. CH = control Daphnia at high food (1 5 mg C l 1 ), FH = Daphnia treated with fishkairomone at high food; CL = control Daphnia at low food (0 5 mg C l 1 ), FL = Daphnia treated with fish kairomone at low food. Error bars represent the 95% confidence limits. Fig. 2. Triglyceride content (%) of the eggs of Daphnia magna. CH = control Daphnia at high food (1 5 mg C l 1 ), FH = Daphnia treated with fish-kairomone at high food; CL = control Daphnia at low food (0 5 mg C l 1 ), FL = Daphnia treated with fish kairomone at low food. Error bars represent the 95% confidence limits. Results Daphniids grown in media containing fish-kairomone had less total dry body mass than the control animals (Fig. 1). In the high food treatments (1 5 mg C l 1 ), mean masses (± SE) were 204 µg (6 22) (controls) and 154 µg (13 39) (fish-kairomone treatment). In low food treatments (0 5 mg C l 1 ), they averaged 120 µg (3 96) (controls) and 86 µg (3 61) (fish-kairomone treatment). The clutch masses for these two groups were similar regardless of the presence of fish-kairomone (Fig. 1). At high food levels, clutch masses were 57 µg (2 92) for controls and 59 µg (8 05) for daphniids in the fish treatments. Under low food concentration, clutch masses were 39 µg (2 08) in the controls and 38 µg (1 78) for daphniids exposed to fish-kairomone. In relative terms, control daphniids invested 28% (high food) and 33% (low food) of their total mass in the first clutch. Daphniids treated with fish-kairomones invested 38% (high food) and 44% (low food) of their body mass to their first clutch. The difference in the investment between controls and fish treatments was about 11%, which agrees well with results from Machacek (1991) and Stibor (1992). Controls and fish-cue treatments differed in the TAG content of their clutch. Animals treated with fish-kairomone had lower relative amounts of TAG in their egg mass than did control animals, independent of food conditions. At the high food concentration, the mean concentration of TAG in the total egg mass was 6 8 µg for the control animals and 4 4 µg in the animals treated with fish-kairomone. These amounts correspond to a proportion of TAG in the eggs that was about 12% in the controls and only 7 5% in the eggs of daphniids treated with fish-kairomone (Fig. 2). The difference was significant (t-test: t(78) = 5 47, P < 0 01). At the low food concentration, controls Fig. 3. Triglyceride investment for first reproduction as proportion of maternal somatic body mass. CH = control Daphnia at high food (1 5 mg C l 1 ), FH = Daphnia treated with fish-kairomone at high food; CL = control Daphnia at low food (0 5 mg C l 1 ), FL = Daphnia treated with fishkairomone at low food. Error bars represent the 95% confidence limits. had about 2 8 µg (7% of total egg mass) as TAG in their eggs, whereas fish-kairomone exposed animals had only about 1 8 µg (4 7%) (Fig. 2). The difference was significant (t-test: t(78) = 3 73, P < 0 01). The investment of TAG into eggs in relation to the somatic mass of the mother at first reproduction did not differ significantly between control and fish-kairomone treated daphniids. At high ration both the control and animals treated with fish-kairomone invested about 4 5% of their somatic body mass as egg triglycerides (Fig. 3; t-test: t(78) = 0 28, P = NS). At low ration it

4 458 H. Stibor & D. Müller Navarra was about 3 5% (Fig. 3; t-test: t(78) = 0 66, P = NS). Minimum detectable differences (Cohen 1988) at a power of 0 8 were 2 2% in the experiments with 1 5 mg C l 1 food conditions and 1 7% in the experiments with a food condition of 0 5 mg C l 1. Discussion Daphnia exposed to fish-kairomones generally increase their reproductive output by reducing the individual size of their progeny and increasing the allocation of net production to reproduction (Machacek 1991; Stibor 1992). Although clutch mass was unchanged by the presence of fish-kairomone, TAG content per clutch was reduced in the fish-kairomone treatments at both food concentrations. This held for both the absolute amount per clutch as well as the relative amount per clutch mass. Fish-kairomones result in smaller neonates (smaller eggs) of daphniids (Machacek 1991; Stibor 1992) and a lower amount of fat in the reproductive material. This results in neonates with lower individual fat amounts than neonates from daphniids that are not influenced by fish-kairomones. Field evidence of effects of fish predation on egg lipid allocation also exists. Two populations of the cladoceran Holopedium gibberum which faced different predation regimes in their natural environments were investigated for differences in reproductive parameters (Arts & Sprules 1988). In the population where the animals were exposed to fish predation, the eggs of mothers were smaller and had fewer fat droplets in the yolk. In relation to their body mass, animals in both populations invested the same amount of fat droplets into their eggs. Our results show that this reproductive allocation pattern can be an effect of phenotypic plastic responses induced by fish-kairomones and need not necessarily be a result of clonal succession driven by selective mortality. The drop of the fat content in the eggs of Daphnia induced by the fish-cue is similar to the change of daphniid s eggs when food conditions become poor. The observed range of the decrease in the TAG content of the egg material can lead to reduced survival of young Daphnia as described by Tessier et al. (1983), Cleuvers, Goser & Ratte (1997) and others. Animals treated with fish-kairomone have less body mass and use more material for reproduction in relation to their body mass. This leads to a lower TAG amount in the reproductive material for daphniids treated with fish-kairomone in comparison with untreated animals, independent of the size and number of eggs produced by the mothers. But why do Daphnia produce eggs with lower fat content and thus lower quality in response to the fish cue even if food conditions seem to allow them to produce higher-quality eggs? One answer is that even though the TAG content per clutch is lower in the animals treated with fish-kairomone, the proportion of TAG in the reproductive material was, independent of the presence of fish-cue, always around 5% of the maternal body mass at first reproduction. There might be a constraint for a lower limit of fat that has to be retained in the mother s body for survival or further reproductive events. If so, this would be a real inherent cost and would set the limits for the degree of possible phenotypic plasticity. In our experiments the amount of TAG which was allocated to egg material was a conservative portion of Daphnia magna body mass. We investigated only one clone of D. magna in favour of a high number of replicates per treatment. Clonal differences in this parameter are to be expected as in other traits influenced by fish-kairomones (Boersma et al. 1998). In a study by Boersma, De Meester & Spaak (1999) D. magna clones differed in the strength of the responses to fish-kairomones. However, the general direction of the life-history responses to fishkairomones was the same in all investigated clones. This may be a strong hint that life-history shifts of D. magna in response to fish-kairomones follow a general pattern. Additionally, our findings seem to hold true for other Daphnia species (D. Müller Navarra, unpublished data). TAG investment in eggs is dependent on somatic mass. Thus, to grow larger and to reach a higher somatic mass also means to be able to invest more TAG into eggs and to channel a higher proportion of net production into reproduction without having to lower egg quality. This constraint of body size on egg quality seems to be very important and should therefore be taken into account when searching for a trade-off between growth and reproduction in Daphnia (Lynch 1989, 1992; Spitze, Burnson & Lynch 1991; Yampolsky & Ebert 1994). It also might explain the fact that under normal culture conditions Daphnia are lazy (S. I. Dodson, personal communication). Daphniids probably have a higher resource allocation than they show under normal culture conditions, e.g. when exposed to fish-kairomones. The gain from having more offspring when only one reproductive event is probable (as under high fish predation, Lampert 1993) must match the costs of lower resistance to starvation of the offspring. In this case, costs of defence mechanisms are affecting the offspring and do not necessarily affect the treated individual or even appear in its life span. Such maternal effects have also been considered when searching for costs in life-history traits, contrary to costs for morphological changes induced by predators, which always directly affect the phenotypically altered form (Wiackowski & Szkarlat 1996). Recent research also produced evidence that morphological defence mechanisms of Daphnia cucullata in response to predator kairomones have strong maternal components (Agrawal, Laforsch & Tollrian 1999). The gain from having more offspring with low investments of energy reserves (TAG) might be advantageous when the probability of mortality due to starvation is low but the risk of mortality through predation is high. Interestingly, high predation pressure often interacts with better food conditions for daphniids

5 459 Plasticity of Daphnia magna because daphniid abundance is lowered by predation and thus competition for common resources (food) is not so predominant. In contrast, under low predation pressure one can assume that prey (Daphnia) concentrations are higher and that therefore food resources are more exploited and thus lower. Therefore, the trade-off in Daphnia reproduction is between maximization of the reproductive output on the one hand and their resistance to starvation on the other hand. The allocation pattern can be adjusted to the predation level (mortality due to predation), which is coupled to the food level (mortality due to starvation) in a way that optimizes the solution for the above-mentioned trade-off. References Agrawal, A., Laforsch, C. & Tollrian, R. (1999) Transgenerational induction of defences in animals and plants. Nature 401, Arts, M.T. & Sprules, W.G. (1988) Evidence for indirect effects of fish predation on maternal lipid investment in Holopedium gibberum. 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