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Keywords:

  • functional morphology;
  • phenotypic plasticity;
  • predation;
  • predator–prey interactions

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

1. Phenotypic plasticity in defensive traits has been proven to be effective in ecosystems with frequently changing predator regimes. However, if a single dominant predator exerts predation pressure on each ontogenetic stage, prey should adapt by developing defensive traits for each life stage within a cost-benefit framework. This may require a change of defensive mechanisms between juvenile and adult life stages.

2. In this study, we examined the morphological defences of the cladoceran Daphnia magna Strauss induced by the tadpole shrimp Triops cancriformis Bosc. We tested for the induction of morphological defences by directly exposing daphnids to the predator and conducted a life span experiment to determine if the expression of the induced morphological defences varies throughout the life span of D. magna. In addition, we studied the adaptive value, i.e. the effectiveness, of the Triops-induced morphological defences in D. magna by conducting predation trials.

3. We found that, in D. magna, the expression of an array of inducible morphological defences, which act synergistically to provide effective protection, changes during the daphnids lifetime in response to the tadpole shrimp T. cancriformis. This gradual switch in the protective function of single traits between juvenile and adult stages represents a novel functionality and complexity of inducible defences. Both direct contact with the predator and chemical cues (kairomones) released by T. cancriformis induce an increased body length, body width and an elongation of the tail spine in D. magna. This study is the first to show that kairomones released by a predator can induce ‘bulkiness’ as a defensive mechanism in Daphnia. Finally, we demonstrate the effectiveness of the Triops-induced morphological defences (i.e. an elongated tail spine and increased bulkiness) by conducting predation trials.

4. Our study provides rare evidence for morphological defences in D. magna, and in addition shows that prey species gradually switch between plastic traits to maintain effective defences throughout their entire lifetime. Hence, our results help to shed light on the mechanisms governing phenotypic plasticity within natural populations.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Predation is one of the major factors driving natural selection. As a result, prey species have evolved a variety of defensive mechanisms to cope with the selection pressure exerted by predators. Phenotypic plasticity in defensive traits has evolved in response to spatial and temporal heterogeneity in predator regime and predation risk within natural habitats. This phenomenon has attracted scientists from an ecological and evolutionary point of view because inducible defences have impacts on a variety of important processes from community ecology to ecosystem function (Tollrian & Harvell 1999; Kishida et al. 2010).

In freshwater ecosystems the induction of defences is mediated primarily by info-chemicals, known as kairomones, released by predators. Textbook examples for the study of inducible defences are water flea of the genus Daphnia (Crustacea: Cladocera). In Daphnia, alterations in life history, e.g. an altered size or age at maturity (Weider & Pijanowska 1993; Riessen 1999), behaviour, e.g. diel vertical migration (Dodson 1988; Lampert 1989) and morphology, such as helmets or spine like structures (for review see Laforsch & Tollrian 2009), have been reported as defences.

Most studies on inducible morphological defences in Daphnia were conducted using fish (e.g. Tollrian 1994), Chaoborus larvae (e.g. Krueger & Dodson 1981; Spitze 1992) or, more rarely, Notonecta (e.g. Dodson & Havel 1988; Barry 2000) as predators. However, Petrusek et al. (2009) discovered that the tadpole shrimp Triops cancriformis (Crustacea: Notostraca) induces the formation of morphological defences, the so called ‘crown of thorns’, in the Daphnia atkinsoni species complex. For the study of inducible defences in pond dwelling Daphnia species, Notostracans are particularly interesting as they are top predators in ephemeral waters and can play an important role in structuring macroinvertebrate communities (Yee, Willig & Moorhead 2005). Given their predatory nature and their ability to reach high densities (Boix, Sala & Moreno-Amich 2002) they are a potential threat to pond dwelling Daphnia. Since T. cancriformis is a species that has not changed its morphology for about 220 Myr (Kelber 1998), it is likely that several Daphnia species have adapted to this predator by forming distinct defensive strategies.

We used the pond dwelling species Daphnia magna, which serves as an important model organism for several research areas including ecotoxicology as well as environmental and evolutionary functional genomics. In this species, studies on inducible defences have focused primarily on the effects of fish. The outcome of these studies has shown that the presence of fish can induce a variety of life-history shifts, alterations of behaviour and rarely morphological defences (e.g. Weider & Pijanowska 1993; Pijanowska & Kowalczewski 1997a; Boersma, Spaak & De Meester 1998). However, there is sparse information concerning the reaction to invertebrate predators. Pijanowska & Kowalczewski (1997b) and Coors & De Meester (2008) found that D. magna responds to kairomones exuded by predatory cyclopoids and Chaoborus larvae with an increase in body size at first reproduction. Here, we present a study on Triops-induced morphological defences in D. magna, a species which was not known to respond to a single predator with an array of inducible morphological defences that act synergistically to form an effective protection.

In particular, we addressed three questions: (i) Does D. magna respond to the presence of the tadpole shrimp T. cancriformis by the formation of inducible morphological defences? (ii) Does the formation of the morphological defences vary over the whole lifetime of the daphnids? Since T. cancriformis threatens all size classes of D. magna the daphnids may switch from an easy-to-build start-up defence in juveniles to a more comprehensive defence in adult life stages. (iii) Do the Triops-induced traits offer an effective protection against the predator?

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Induction experiment

We used a laboratory cultured clone (K34J) of D. magna for our experiment, isolated from a former fishpond near Munich, Germany. A laboratory cultured clonal line of T. cancriformis from the University of Vienna served as the predator. The induction experiment was carried out in glass aquaria (30 × 20 × 20 cm) filled with 10 L of artificial medium based on ultrapure water, phosphate buffer and trace elements. The bottom of each aquarium was covered with sterilized sand. This experiment was conducted in a climate chamber at 20 ± 0·5 °C under fluorescent light at a constant photoperiod (15 h day : 9 h night). The experiment was started by randomly placing 10 primiparous, age-synchronized daphnids (F0 generation) into each aquarium. Age-synchronized cohorts of Daphnia were grown prior to the experiments by collecting primiparous mothers with freshly deposited eggs. The first brood of these mothers was used for the experiments. A single T. cancriformis with a body length of 25–30 mm was placed into each aquarium of the induction treatment. Each treatment was replicated three times. The daphnids were fed daily with 1·0 mg C L−1 green algae (Scenedesmus obliquus). Moreover, we added five pellets of fish food per day (JBL Grana Discus; JBL GmbH & Co. KG, Neuhofen, Germany) to each of the aquaria as a food source for T. cancriformis. The same amount of fish food was added to the control treatment, although preliminary experiments have shown that fish food does not induce defences in Daphnia. Faeces, exuviae and fish food remnants were removed every day. As we used third generation individuals for the analysis, the daphnids of the F0, F1 and the F2 generation were removed from the experiment after releasing their first clutch. After each removal, the Daphnia density was kept at a maximum of 50 individuals by randomly removing supernumerary neonates from the aquaria. For the analysis, primiparous individuals of the F3 generation were preserved in 70% ethanol (Black & Dodson 2003). The following morphological parameters were recorded, using a digital image-analysis system (cell^P; Olympus, Hamburg, Germany); body length, defined as the distance between the upper edge of the compound eye and the base of the tail spine; body width, defined as the maximum distance between the dorsal and the ventral edge of the carapace; and the length of the tail spine, defined as the distance between the base and the tip of the tail spine.

Life span experiment

The life span experiment was conducted under the same conditions as the induction experiment, using again the D. magna clone K34J. Prior to the experiment, an age-synchronized cohort of daphnids was raised as described above. Based on the results of the induction experiment, the individuals destined for the induction treatment were pre-induced with T. cancriformis in order to take transgenerational effects into account (Agrawal, Laforsch & Tollrian 1999). We kept the daphnids in direct contact with one T. cancriformis of 20 mm body length in two glass aquaria containing 10 L of artificial medium until the F3-generation was born. The daphnids of the F0 and F1 generation were removed after releasing their first clutch. After each removal the Daphnia density in the aquaria was kept at a maximum level of 50 individuals by randomly removing supernumerary individuals. The neonates released by the F2 generation were then used in the experiment.

In order to distinguish single individuals, each Daphnia was placed in a labelled 50 mL welted glass, which was closed with a gauze cap (mesh width: 400 μm). Four randomly chosen glasses, each containing a single neonate daphnid, were placed into each aquaria filled with 10 L of artificial medium. The glasses rested with their gauze cap towards an air stone (Aeras Micro; JBL GmbH & Co.) connected to an air-pump. The current produced by the air stone guaranteed a constant exchange of medium, predator cues and algae inside the glasses. The bottom of each aquarium was covered with sterilized sand. Three T. cancriformis, with a body length over 20 mm, were placed into each aquarium of the induction treatment. The experiment was replicated five times with the aquaria serving as replicates.

The daphnids were fed daily by adding S. obliquus at a concentration of 0·7 mg C L−1. In addition to fish food we used freshly killed D. magna (clone K34J) as a food source for T. cancriformis to maximize the expression of morphological defences because predators consuming conspecific prey are known to increase the formation of inducible defences (Stabell, Ogbebo & Primicerio 2003; Laforsch, Beccara & Tollrian 2006). The daphnids were killed prior to feeding using carbonated water. Daily, 10 pellets of fish food and 10 freshly killed D. magna were added to both the induction and the control treatment. The aquaria were cleaned daily of exuviae, faeces, fish food remnants and dead Daphnia. Medium and aquaria were exchanged every 4 days. During that procedure the glasses and the air stones were cleaned. To maintain a constant level of kairomones during the experiment, we used medium that was conditioned with T. cancriformis. For that purpose, we kept three T. cancriformis for 1 day in 30 L of medium. The daphnids were measured alive after each moult, for which we checked twice per day. Thereby, the same morphological parameters were recorded as described for the induction experiment.

Predation trials

The predation trials were carried out in a temperature controlled room at 20 ± 0·5 °C under fluorescent light (15 h day : 9 h night). In order to generate a sufficient number of induced and non-induced D. magna (clone K34J), 10 adult females were placed in a 12 L glass aquarium with 10 L of artificial medium. Each of the aquaria contained a net cage, including one T. cancriformis (> 20 mm) in the induction treatment. Each treatment was replicated 10 times. The daphnids were fed daily by adding 0·7 mg C L−1. Daily, five pellets of fish food and five freshly killed D. magna were placed in each of the net cages to feed the T. cancriformis and the same amount was also added to the net cages in the control treatment. The net cages were cleaned daily from exuviae, faeces, fish food remnants and dead daphnids. Every 4 days half of the medium was exchanged. After releasing their first clutch out of the brood pouch, the adult daphnids were removed from the aquaria. In the following 21 days, the number of daphnids in each aquarium was kept at a constant level of 100 individuals by randomly removing supernumerary daphnids. After this period, randomly picked daphnids with a body length of 1·5 ± 0·2 (juvenile) and 3·3 ± 0·2 mm (adult) were exposed to T. cancriformis with a body length of 20 ± 2 mm in the predation trials. The body length of T. cancriformis was defined as the distance between the cranial edge of the carapace and the caudal end of the body, excluding the furca. Prior to the predation trials, the daphnids were labelled with blue and red food colouring (Schwartauer Werke GmbH & Co. KGaA, Bad Schwartau, Germany) to ease the process of distinguishing between induced and non-induced daphnids. To assure that the colour has no influence on the predator’s preference, the colours for induction and control were interchanged after the first half of the predation trails. For each predation trial, 10 individuals of the induced and 10 individuals of the non-induced daphnids were placed in a glass bowl containing 100 mL of artificial medium. The trial started by placing one T. cancriformis into the bowl and was terminated after 30 min or, alternatively, when half of the daphnids were captured by the predator. Afterwards the number of surviving induced and non-induced daphnids was determined using a stereomicroscope. The predation trials were replicated 10 times for each Daphnia size class.

Statistical analysis

The data were analysed using the software package spss v15·0 (SPSS Inc., Chicago, USA). To compensate for size-dependent differences in the recorded parameters, relative values were calculated for body width and tail spine length by dividing the respective trait length by body length. Prior to analysis, relative values were arcsine-square-root-transformed (Sokal & Rohlf 1995) and all residuals were tested for normality. For analysis of the induction experiment a nested anova with treatment as a fixed factor and replicates as random factor, was performed to test for treatment effects between induced and non-induced daphnids. The data collected in the life span experiment were analysed using a repeated measures anova to test for time–treatment interactions, indicating divergent growth and treatment effects. We only analysed the data from moult 0 (i.e. data recorded from the neonates) to moult 12 because sample size strongly decreased after the 12th moult due to mortality. The data recorded from juvenile (moult 0–4) and adult daphnids (moult 5–12) were analysed separately to contrast the expression of inducible defences across life stages. For body width and tail spine length the analysis was performed using both absolute and relative values. The endpoint data collected in the predation trials were analysed using a two-factorial glm with the number of surviving individuals as dependent variable and the treatment and size class as fixed factors.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

In the induction experiment, we tested the expression of morphological defences in D. magna by exposing them to the invertebrate predator T. cancriformis. In comparison to their non-induced conspecifics, primiparous induced individuals had an increased body length (nested anova; F1,4 = 31·989, = 0·005; Table 1). Furthermore, the relative body width (nested anova; F1,4 = 340·897, < 0·001; Table 1) and the relative length of the tail spine (nested anova; F1,4 = 32·963, = 0·005; Table 1) were significantly increased in daphnids exposed to the predator.

Table 1.   Body length, body width and length of the tail spine of induced and non-induced primiparous D. magna from the induction experiment and corresponding data from the life span experiment. Data show means ± 1 SE
 Induction experimentLife span experiment
ControlInducedControlInduced
Body length (μm)2739·84 ± 18·273216·26 ± 22·352911·18 ± 31·543181·04 ± 18·92
Body width (μm)1896·00 ± 13·102283·74 ± 16·452029·32 ± 24·312271·04 ± 14·61
Relative body width (%)69·20 ± 0·1371·00 ± 0·1269·71 ± 0·3071·40 ± 0·28
Tail spine length (μm)961·37 ± 8·251305·13 ± 9·56854·24 ± 17·891261·46 ± 14·10
Relative tail spine length (%)35·20 ± 0·3240·75 ± 0·2929·32 ± 0·8039·66 ± 0·43

During the life span experiment, the expression of the defensive traits was studied throughout the entire life span of D. magna. Concerning body length in juvenile daphnids we found a significant time–treatment interaction (repeated measures anova; F2·318 = 3·253, = 0·037, corrected using Greenhouse–Geisser estimates of sphericity), indicating divergent growth (Fig. 1a). Furthermore, induced juveniles had a significantly increased body length compared to control individuals (repeated measures anova, test for between subject effects; F1,34 = 13·445, = 0·001). In adult daphnids no significant time–treatment interaction could be found (repeated measures anova, F2·359 = 0·864; = 0·442, corrected using Greenhouse–Geisser estimates of sphericity), as the induced daphnids had a significantly greater body length than their undefended conspecifics throughout (repeated measures anova, test for between subject effects; F1, 34 = 101·025, < 0·001).

image

Figure 1.  Comparison of the body length (a), the body width (b) and length of the tail spine (c) between induced and non-induced D. magna in the life span experiment. The dashed lines indicate the transition from juvenile to adult life stages, the error bars indicate the standard error of mean (SE).

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In juvenile daphnids a significant time–treatment interaction concerning both absolute (repeated measures anova; F1·869 = 7·243, = 0·002, corrected using Greenhouse–Geisser estimates of sphericity) and relative values of body width (repeated measures anova; F3·345 = 4·229, = 0·005, corrected using Huynh–Feldt estimates of sphericity) could be found, indicating divergent growth of body width (Fig. 1b). Thus, induced daphnids had a significantly greater body width in terms of absolute (repeated measures anova, test for between subject effects; F1,34 = 53·941, < 0·001) and relative values (repeated measures anova, test for between subject effects; F1,34 = 75·596, < 0·001). Concerning body width in adult daphnids, no significant time–treatment interaction could be observed for absolute and relative values. However, the treatments differed significantly for both values (repeated measures anova, test for between subject effects; absolute body width: F1,34 = 138·396, < 0·001; relative body width: F1,34 = 75·596, < 0·001).

Concerning tail spine length in juvenile daphnids, we could observe a significant time–treatment interaction for both absolute (repeated measures anova; F1·628 = 47·437, < 0·001, corrected using Greenhouse–Geisser estimates of sphericity) and relative values (repeated measures anova; F= 10·484, < 0·001), indicating divergent growth of the tail spine (Fig. 1c). Juvenile, induced daphnids had a significantly longer tail spine compared to their undefended conspecifics (repeated measures anova, test for between subject effects; absolute tail spine length: F1,24 = 283·493, < 0·001; relative tail spine length: F1,22 = 343·438, < 0·001). In adult daphnids a significant time–treatment interaction could be observed for absolute (repeated measures anova; F3·213 = 4·170, = 0·009, corrected using Greenhouse–Geisser estimates for sphericity) and relative tail spine length (repeated measures anova; F2·688 = 4·567, = 0·009, corrected using Greenhouse–Geisser estimates for sphericity), indicating a convergent decline of the tail spine length (Fig. 1c). Thus, adult, induced daphnids had a significantly greater tail spine length in terms of absolute and relative values (repeated measures anova, test for between subject effects; absolute tail spine length: F1,17 = 169·358, < 0·001; relative tail spine length: F1,17 = 101·458, < 0·001).

The effectiveness of the induced morphological traits of D. magna was studied by conducting predation trials. A significantly higher number of induced daphnids survived compared to their undefended conspecifics (two-factorial glm; F= 21·810, < 0·001) (Fig. 2). Moreover, compared to juvenile daphnids, a significantly higher number of adult daphnids, induced and non-induced, survived (two-factorial glm; F= 23·886, < 0·001). The size class-treatment interaction effect was not significant (two-factorial glm; F= 0·106, = 0·746), indicating that the inducible morphological defences in juvenile and adult D. magna were equally effective against T. cancriformis.

image

Figure 2.  Comparison of the number of surviving individuals between induced and non-induced juvenile (1·5 ± 0·2 mm body length) and adult (3·3 ± 0·2 mm body length) D. magna, tested against T. cancriformis with a 2 cm body length. A significantly higher number of induced daphnids (< 0·001) survived compared to their undefended conspecifics and a significantly higher number of adult individuals, induced and non-induced, survived compared to juvenile daphnids (< 0·001). The error bars indicate the standard error of mean (SE).

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Temporary ponds, the habitat D. magna and T. cancriformis share, were considered as an environment where predation pressure is low because key predator species (e.g. fish and dragonfly larvae) are prevented from colonizing those ponds due to physical and environmental constraints (Wellborn, Skelly & Werner 1996). Nevertheless, predation can play an important role in temporary ponds (Brendonck et al. 2002). Recent studies indicate that tadpole shrimps can exert a high predation pressure in ephemeral ponds, especially on cladocerans and copepods (Boix et al. 2006). The fact that T. cancriformis feeds on cladocerans and reaches high densities in temporary ponds (Boix, Sala & Moreno-Amich 2002), indicates that they should at least temporarily impose strong predation pressure on D. magna. As predation is a major driver of natural selection, the development of defensive mechanisms should be favoured. Accordingly, D. magna should alter their diapausing behaviour, their morphology or both to cope with high predation pressure. Our results show that D. magna responds to the presence of this particular predator by the formation of morphological defences while gradually switching between defensive traits from juvenile to the adult life stages to maintain effectiveness. However, we could not observe the production of resting eggs during the time span of our experiment, indicating the adaptive value of the induced morphological defences.

The induction experiment revealed that the invertebrate predator T. cancriformis induces increases in body length and width as well as an elongation of the tail spine in primiparous D. magna (Table 1) resulting in an increased bulkiness of defended individuals. In this study, we compared daphnids of the same developmental stage rather than of the same age and although the focus of this study was not on life-history traits, we observed that predator-exposed daphnids matured earlier compared to control individuals. Hence, a comparison of individuals of the same age should result in even more pronounced differences between treatments (which also applies for the life span experiment). The observed expression of Triops-induced morphological defences in D. magna is in accordance with Petrusek et al. (2009) who recently found that D. atkinsoni is capable of building morphological defences in response to chemical cues released by T. cancriformis.

Since T. cancriformis is an omnivorous predator with a relatively broad range of prey sizes (Boix et al. 2006; Golzari, Khodabandeh & Seyfabadi 2009), life-history shifts in prey, such as an altered size at first reproduction should be less effective. Furthermore, this predator searches for food in all micro-habitats leaving no refuges where D. magna may hide, e.g. via diel vertical migration. Thus, the expression of inducible morphological defences is the most efficient way to cope with this predator. The effectiveness of the Triops-induced bulkiness is presumably based on the predator’s mode of feeding. It catches its prey with its nine anterior leg pairs and takes it to the midventral food groove built by the basal endites of the legs (Gruner 1993). Through this food groove the prey is transported to the mouth where it is crushed by sharp mandibles and ingested. Studies dealing with prey choice in related Notostraca species have found this group to be gape limited predators; limited specifically by the size of the food groove and the opening width of the mandibles, For instance, it could be demonstrated that T. longicaudatus, a close relative of T. cancriformis with a very similar morphology, selectively preyed upon smaller mayfly larvae (Walton, Tietze & Mulla 1991). Furthermore, predation trails in the closely related species Lepidurus arcticus revealed that smaller prey (D. pulex) were preferred over larger prey due to easier handling (Christoffersen 2001). T. cancriformis should show an analogous preference when preying upon D. magna due to their similar morphology and feeding mode. Thus, increased bulkiness in D. magna induced by T. cancriformis likely interferes with the feeding apparatus of this predator.

The results of our life span experiment uncovered that the expression of the protective traits varied over time and life stages, resulting in a successive change of different defensive traits. In juvenile D. magna the elongated tail spine is the most prominent defensive structure. We could observe that the tail spine is already enlarged in the neonate offspring of Triops-induced D. magna. In situations in which the neonates are already threatened by the predator, protective morphological traits should be developed before the offspring are released from the brood pouch of the mother. Due to limited space in the brood pouch, large-sized morphological defences cannot be formed during embryonic development. However, during development the tail spine is folded caudally to the ventral edge of the carapace and then experiences a rapid post-hatching expansion followed by a hardening of the cuticle rendering it a perfect start-up defence (Laforsch & Tollrian 2004). In addition, our results are in concordance with the findings of Caramujo & Boavida (2000) who found that an elongated tail spine in juvenile daphnids of the D. hyalina × galeata species complex offers effective protection against predatory copepods. This protective effect can be explained by the fact that a longer tail spine contributes to an increased overall length of the daphnids which makes them less susceptible to gape limited predators.

In our experiment the tail spine was longer in induced daphnids, as reflected in significantly increased absolute and relative values in both juvenile and adult individuals. Furthermore, we observed a divergent growth and decline, respectively, of the tail spine throughout the experiment. In juvenile induced daphnids the tail spine grew faster than in the non-induced individuals. However, in both treatments the absolute length of the tail spine was continuously reduced after the sixth moult, soon after reaching maturity (Fig. 1c). The reduction was stronger in the induced than in the non-induced daphnids. This reduction of the tail spine in adult D. magna, together with the fact that the relative length of the tail spine is continuously reduced from the neonate stage on, suggests that the protective function of this trait is especially important in juvenile daphnids. Our results indicate that the protective function of the tail spine is gradually taken over by both body length and width, resulting in an increased bulkiness of adult D. magna. A significant divergence in body length and width between predator exposed and non-predator exposed animals emerged early during the juvenile stage. However, the difference in body length and width is most pronounced in adult individuals although no significant divergent growth concerning these traits could be observed in adults (Figs. 1a,b). Here, the protective effect of increased body length is intensified by the increased dorsoventral width of the carapace, which is in accordance with the Chaoborus induced increase of body width seen in D. pulex (Tollrian 1995).

We could observe that inducible morphological defences in D. magna are present throughout the entire life span of the daphnids. This can be explained by the fact that different size classes of T. cancriformis are usually present in the same habitat (Boix, Sala & Moreno-Amich 2002), thus exerting predation pressure on all size classes of D. magna and favouring the expression of defensive traits across all life stages. A similar pattern can be observed in D. cucullata, which is threatened by several size-selective invertebrate predators and accordingly develops elongated helmets and tail spines at each life stage (Laforsch & Tollrian 2004). In contrast, the Chaoborus-induced expression of neckteeth in D. pulex is restricted primarily to the juvenile instars (Krueger & Dodson 1981; Riessen & Trevett-Smith 2009) which are the only instars susceptible to Chaoborus predation. Hence, evolution favours the expression of inducible morphological defences in Daphnia only in those life stages threatened by the predator.

An essential feature of inducible defences is that they provide an efficient protection against the predator (Harvell & Tollrian 1999). Our predation trials revealed that induced juvenile and adult D. magna are better protected than control individuals, i.e. we could observe a significantly higher number of surviving daphnids of the induced morph (Fig. 2). In our experiment the body length of the induced and non-induced D. magna were the same as we used individuals of approximately the same size for each stage (i.e. juvenile and adult). However, the results of the life span experiment showed that induced animals differed for body width (adults) and length of the tail spine (especially in juveniles but also in adults). In juveniles the protective effect is presumably based on the elongated tail spine but also partly on the already emerging bulkiness of the induced individuals. In adult daphnids, the protection is likely based on the increased bulkiness of the induced individuals. Since the switch between defensive traits shown in the life span experiment is a gradual and dynamic process, it is likely that the elongated tail spine still plays a role for adult individuals. The synergistic effect of both traits should decrease the predator’s ability to handle the induced D. magna, i.e. to transport individuals through the food groove and to crush them with the mandibles. Moreover, we speculate that the kairomones released by T. cancriformis induce increased stability of the carapace in D. magna. Increased stability and thickness of the carapace has been observed in D. pulex and D. cucullata, exposed to Chaoborus-kairomones (Laforsch et al. 2004). The expression of such a hidden defence would render the induced daphnids less susceptible to being crushed by the mandibles of T. cancriformis. Together with the increased bulkiness this provides effective morphological defence for the prey.

Our results demonstrate that D. magna has not only adapted to the predator T. cancriformis by forming of an array of inducible morphological defences, it also gradually changes the expression of the single defensive traits. We conclude that this successive switch has evolved because D. magna is threatened by this predator throughout its entire life span and thus a transition from less costly start-up defences in juvenile daphnids to other defences in adults may be required to maintain effective protection against T. cancriformis. Given that the studied predator–prey system is not the only one where a predator is threatening its prey across all life stages, our results suggest that a comparable shift in defensive traits could also be present in other species.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

We thank M. Kredler, E. Ossipova, J. Fischer, Q. Herzog and J. Wickjürgen for help during the experiments, R. Boucher and J. Lohr for linguistic improvements as well as the editor and three anonymous reviewers for valuable comments on the manuscript. M. R. was funded by a research scholarship provided by the Universität Bayern e.V.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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