Complex effects of a kairomone of Chaoborus and an insecticide on Daphnia pulex

Daphnia pulex were reared in Chaoborus-conditioned water containingthe insecticide carbaryl, and their life history parametersand morphologies were investigated. The insecticide inhibitedthe animals' growth and reproduction and delayed their maturationtime more intensely in the chaoborus-conditioned water thanin the control Chaoborus-free water, indicating that a kairomoneof Chaoborus made the Daphnia more sensitive to the insecticide.The Chaoborus conditioned water induced neckteeth formationof D.pulex in instars 1–2 and elongated the intermoultingperiod of juveniles. The moulting to the spined morphs and elongationin duration of juvenile stages seemed to increase the risk ofdamage from the insecticide. The potential population growthrate of D.pulex in treatments was estimated as a possible fitnessindicator of the animals. It was reduced synergistically bythe kairomone of Chaoborus and the insecticide. Some individualskept neckteeth until the third or fourth instar stage when theywere exposed to sublethal concentrations of the insecticidein the Chaoborus-conditioned water. This was considered as aresult of synergistic effects of both the kairomone and theinsecticide. Insecticides may be a factor inducing further developmentof protuberant structures in cyclomorphic Daphnia in naturalwater bodies.     

Multiple predator defence strategies in Daphnia pulex and their relation to native habitat

Daphnia may respond with an array of anti-predator defences(behavioural, morphological and life history) to a chemicalcue (kairomone) exuded by its predators: fish and Chaoborus.Given the wide array of potential responses, it is an interestingquestion whether anti-predator defences are coupled or independentof each other. Since anti-predator responses are costly andeven possessing the genetic information to respond to a certainpredator might involve a cost, clones may only react to predatorsthey co-occur with in nature. In this study, we provide evidencefor an uncoupling of responses by Daphnia pulex in several anti-predatordefences against Chaoborus. We were unable to detect a correlationbetween behavioural (migration), morphological (neck-spine induction)and life history [growth rate, neonate size and size at firstreproduction (SFR)] responses. Furthermore, anti-predator responsesdid not always comply with what is commonly believed. We foundthat Daphnia clones can migrate up or down when exposed to fishor Chaoborus kairomone and that population growth rate, neonatesize and SFR can increase or decrease in response to Chaoboruskairomone. We also show patterns in anti-predator defences thatseem to relate to the habitat from which clones were derived.Daphnia clones that were collected in habitats with Chaoborusas the dominant predator tended to react strongly to Chaoboruskairomone by migrating upward and producing neck-spines. Themigration behaviour against fish kairomone in these clones wasoften an unexpected upward migration. The Daphnia clone thatco-existed with fish predators showed a downward migration inthe presence of fish as well as Chaoborus kairomone. Clonesthat had occurred with either both or no predators had mixedresponses. We sometimes found an upward migration in combinationwith smaller body size as a response to Chaoborus kairomone.This may be interpreted as a behavioural defence against Chaoborusand a life-history defence against fish. Daphnia seem not toexhibit defence behaviour against predators they do not co-occurwith. It might be costly for Daphnia to maintain genetic informationto respond to these predators and protect that information fromgenetic drift.     

When and how can <Emphasis Type="Italic">Daphnia</Emphasis> prepare their offspring for the threat of predation?

The life history of Daphnia exposed to fish kairomone at different developmental stages was examined in a laboratory experiment. The strongest life history response to the applied predation threat was observed in females exposed during the 4th instar. Compared to Daphnia experiencing the presence of fish at earlier or later instars, these individuals reached maturity at a smaller size and released fewer neonates. Moreover, their offspring also demonstrated the strongest reaction to predation threat, exhibiting the broadest phenotypic plasticity in the life history response to predation. The breadth of their reaction norm was, on average, two times larger comparing with individuals from other treatments. Broader phenotypic plasticity may offer clear selective advantages under the unpredictable predation regime. This finding highlights the adaptive role of maternal effect in shaping life history of cladocerans.     

Instar-dependent mortality rates of coexisting Daphnia species in Lake Vechten, The Netherlands

In Lake Vechten, population parameters were determined for twocoexisting Daphnia species, D.hyalina and D.cucullata, fromApril 23 to December 10, 1986. Daphnia hyalina is dominant inspring and D.cucullata in summer and autumn. Size frequencydistributions were converted into instar frequency distributionsby using experimental growth curves and field measurements onnewborn size and the size at first reproduction. A discreteevent computer model was used to calculate the instar mortalityrates. Relative importances of instar mortality rates for thepopulation mortality rate were calculated and discussed. Themortality of the juvenile stages was found to be more importantthan adult mortality during most of the sampling season, despiteegg mortality when egg-bearing females are killed. Only in Septemberand October was the mortality of adult instars more important.Correlation coefficients between differences in the rates ofincrease and differences in mean brood size and instar mortalitiesshow that the former are caused mainly by differences in juvenilemortality, the main factor determining the replacement of D.hyalinaby D.cucullata during early summer.     

Life history shifts in response to different levels of fish kairomones in Daphnia

Life history parameters, such as age and size at maturity, neonatesize and number of eggs in the first clutch, of a clone of thehybrid Daphnia galeata x hyalina were determined at six levelsof fish kairomones. It was found that the age and size at maturitydecreased gradually with increasing kairomone level, while thenumber of eggs in the first clutch increased linearly. At thesame time, neonate size decreased linearly with increasing kairomonelevel. In a starvation experiment, newly born neonates of mothersnot cultivated in the presence of kairomones survived the longest.These results show that Daphnia is able to adjust its life historytraits to different levels of kairomones, and thus to differentfish densities.     

Endosulfan-enhanced crest induction in Daphnia longicephala: evidence for cholinergic innervation of kairomone receptors

Daphnia iongicephala were reared from early embryogenesis inthe presence or absence of crest-inducing kairomones releasedby Anisops gratus (Notonectidae) and from birth in one of fiveconcentrations of the organochiorine pesticide, endosulfan (0,0.1, 1.0, 10 or 100 g 1^–1 The morphologyof the daphnidswas measured 3 days after birth and on production of the firstbrood of eggs. The reproductive parameters, first brood size,age at first reproduction, mean egg volume and total egg volume,were also measured. Endosulfan significantly induced crest developmentin 3-day-old daphnids at 0.1, 1.0 and 10 g ^1– in thepresence and absence of Anisops kairomone, although crest sizein the absence of kairomone was only marginally greater thancontrols. A concentration of 10 pg 1^–1 endosulfan significantlyenhanced crest growth of kairomone-exposed daphnids at maturity.The Anisops kairomone alone induced a large crest in D.iongicephola,reduced first brood size, mean egg volume and total egg volume,and increased age at maturity. The coefficient of variationof mean egg volume was significantly reduced by Anisops kairomone.It is hypothesized that endosulfan enhances crest developmentbyinhibition of .     

Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of food and predation

Field data from seven alpine lakes in Serra da Estrela. Portugal.show that reproduction in Daphnia may be as efficiently controlledby fish predation and copepod predation on eggs in brood cavitiesas it is by food limitation. Body length and clutch size estimatesin Daphnia pulicaria revealed high inter- and intra-populationvariability in maturation size (body size at first reproduction).and in number of eggs per clutch. Daphnia at first maturationin lakes stocked with rainbow trout were half the size of thosefound in fishless lakes (body length of 0.86–0.95 and1.55–1.81 mm. respectively). The mean number of eggs perclutch was reduced to a similar degree by food limitation, predationby fish and copepod predation on eggs in brood cavities, butthe underlying mechanisms of this reduction were different.Food limitation caused smaller clutch sizes in all individuals,so variation remained the same. Fish predation caused the selectiveremoval of individuals with maximum clutches, so variation decreased.Copepod predation caused removal of eggs from brood cavitiesof randomly infested females, so that variation increased, particularlyat a high food level when non-infested females carried largeclutches of eggs.     

Does trimethylamine induce life-history reactions in Daphnia?

Laboratory experiments were carried out to investigate life-history reactions of Daphnia to trimethylamine (TMA), a substance, which has recently been found to induce a similar phototactic reaction in Daphnia as fish kairomones. The effects of different treatments (control, fish kairomone and TMA) on the life-history traits of five clones of D. magna and one clone of D. galeata and D. hyalina, respectively, were studied. Only D. magna exhibited significant reactions to TMA and to fish kairomone. D. galeata showed no significant responses to either TMA or to fish kairomone. D. hyalina reacted to the fish treatment with two traits (size and number of eggs at maturity). The comparison of the reaction norms to TMA and fish kairomone demonstrated that the directions of some responses to both factors were similar but were different for others such as for size at maturity. TMA resulted in a larger size at maturity in D. magna, whereas fish kairomone had no significant influence on this trait. We, therefore, conclude that trimethylamine is not the primary causative chemical agent in fish induced life-history adaptations in Daphnia.     

<Emphasis Type="Italic">Daphnia</Emphasis> growth is hindered by chemical information on predation risk at high but not at low food levels

Daphnia hyalina × galeata (Dhg) and D. pulicaria (Dp) are ready to pay greater costs in terms of predation risk avoidance at high rather than at low food levels. Such costs are easier to assess in Daphnia than in large long-lived and difficult-to-handle herbivores, since they can be precisely determined in a few-day experiment as the reduced growth (P=A−R) resulting from diminished assimilation (A) and/or increased respiration (R). In experiments with Daphnia grown for six days from the neonate to the first clutch of eggs, which were given different levels of algal food (Scenedesmus at concentrations from 0.05 to 1.60 mg C l⁻¹), individual growth was lower in the presence of fish kairomone (chemical information on fish predation; present at a concentration that induces antipredator defensive behavior and life histories) than in the absence of kairomone (control). The difference from the control was negligible at the lowest food levels, and gradually increased with increasing food concentration. At a food concentration of 1.6 mg C l⁻¹, growth was reduced by 9–32 and 0–8% in Dhg and Dp, respectively, compared to the controls. A similar reduction was observed in the body length of six-day-old animals (Dhg 6–19%, Dp 0–14%), but not in the first clutch reproductive effort (clutch volume). Daphnia had a greater number of eggs per clutch in the presence of the kairomone, but smaller eggs, so that the total volume of eggs in a clutch was the same with and without kairomone. The amplification of the effect of the kairomone due to high food levels was weaker in Dp, a species that rarely coexists with planktivorous fish in natural habitats.     

Life history responses of <Emphasis Type="Italic">Daphnia longispina</Emphasis> to mosquitofish (<Emphasis Type="Italic">Gambusia holbrooki</Emphasis>) and pumpkinseed (<Emphasis Type="Italic">Lepomis gibbosus</Emphasis>) kairomones

In the present study, the effect of chemical cues from two fish species (mosquitofish and pumpkinseed), at different concentrations, was tested in life history experiments with Daphnia longispina. The two fish species used represent the most abundant planktivores of many Mediterranean shallow lakes (SW Europe), where the indigenous fish communities have been replaced by such exotic assemblages. Results have shown a similar response of D. longispina to both fish species: kairomones stimulated daphnids to produce more offspring, which resulted in higher fitness (r), relatively to a fishless control. Fish presence also induced an earlier first reproduction, a smaller size at maturity of daphnids, and the production of smaller-sized neonates. Significant correlations with fish concentration (indirect measure of fish kairomone concentration) were found for size at maturity and neonate size, for both fish species. These results are in accordance to the “positive response” observed by other authors, which represents a defence mechanism to face losses caused by fish predators. The chemically mediated size reduction of mature females and neonates is an adaptive response to the size-selective predation exerted by fish. Pumpkinseed introduction is very recent in the lake of origin of the daphnids used in the experiments and its kairomone produced similar effects to mosquitofish in the life history of D. longispina. These results are contrary to the existence of a species-specific kairomone and support the hypothesis of a general fish kairomone. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

How do migrating daphnids cope with fish predation risk in the epilimnion under anoxic conditions in the hypolimnion?

The vertical distribution of Daphnia in experimental tubes isinfluenced by fish kairomone concentration in the ‘epilimnion’and oxygen conditions in the ‘hypolimnion’. Daphnidstrade off reduced predation mortality in a refuge against disadvantagesdue to unfavourable oxygen conditions.     

Is the midsummer decline of Daphnia really induced by age-0 fish predation? Comparison of fish consumption and MDaphnia mortality and life history parameters in a biomanipulated reservoir

We analysed the temporal pattern of a Daphnia galeata populationand the development of the age-0 Ash community in a long-termbiomanipulated lake (Bautzen reservoir, Germany) during springand early summer of two successive years. In Bautzen reservoir,the age-0 Rsh-Daphnia interaction is a key process within thefood web due to the low abundance of adult zooplanktivorousfish. Daily consumption of daphnids by age-0 fish, as estimatedby a bioenergetics model, was compared to daily mortality ratesof daphnids. In addition, we estimated life history parametersof Daphnia that may indicate predation impact by fish. A midsummerdecline of daphnids occurred only in July of 1996, whereas in1995 the Daphnia biomass remained >4 mg wet weight 1^–1for the entire summer. The percentage of total Daphnia mortalitydue to fish predation before the onset of the midsummer declinewas –2% day^–1. Temporal patterns of individual size,clutch size and size at maturity also indicated that the 1996midsummer decline of daphnids was not the exclusive consequenceof age-0 fish predation. Instead, low reproductive capacityof daphnids also contributed significantly to the decline. Consequently,year-to-year variation of the Daphnia dynamics may be determinedby a fine-tuned ‘timing’ between the period of reducedfecundity and the time of the strongest predation impact byage-0 fish.     

Morphological responses of four species of cyclomorphic Daphnia to a short-term exposure to the insecticide carbaryl

Four species of cyclomorphic Daphnia (D.pulex, D.galeata mendotae,D.retrocurva, D.lumholtzi) were exposed to the insecticide carbarylfor a short term (8–4 h) from the final embryonic stageto the first instar. Daphnia pulex formed neckteeth, and theremaining three Daphnia species developed high helmets and longtailspines. The results suggest that the development of suchprotuberant structures (anti-predator morphologies) in responseto the insecticide exposure is a general phenomenon in Daphnia,and that stimuli on the nervous system of Daphnia may inducethe morphological changes, which originally evolved as a responseto predator kairomone. Two clones of D.pulcx were examined anda clone which was more sensitive to the predator Chaobonts kairomonethan another developed more marked neckteeth in response tocarbaryl, suggesting that the sensitivity in morphological responseto the insecticide may be related to the sensitivity to thekairomone. ¹Present and permanent address: Regional Environment Division,National Institute for Environmental Studies Onogawa, Tsukuba,Ibaraki 305, Japan     

Morphological defenses induced in situ by the invertebrate predator Chaoborus : comparison of responses between Daphnia pulex and D. rosea

The presence of plankton predators may induce altered morphology in their potential prey. To date, the mechanism of induction and adaptive value of such defensive responses have been examined in the laboratory. This study investigated the morphological defense structures induced by the invertebrate predator Chaoborus in two coexisting Daphnia species, D. pulex and D. rosea, in the field. In Piscivore Lake (Gr?fenhain, Germany), continuous and intense biomanipulation had led to near elimination of planktivorous fish and greatly increased abundances of Chaoborus (up to >10 larvae l^–1). Here, the density of Chaoborus was manipulated within the lake by an enclosure/exclosure setup and resulting morphological responses of Daphnia spp. were investigated in situ. Three replicate enclosures (4.6 m³) contained no Chaoborus (predator exclusion bags), whereas Chaoborus entered three others at ambient densities (predator enclosures). In both species of Daphnia, formation of neckteeth and elongation of the tail spine were recorded in the predator enclosures, but not in the predator exclusion treatments. Additionally, D. rosea responded to predator inclusion with an increase of the size at first reproduction. Despite the induced defense structures, the presence of Chaoborus caused increased mortality of both Daphnia species. In addition, Chaoborus affected the coexistence of the two populations of Daphnia by causing higher relative mortality in D. rosea. Neckteeth formation was always more pronounced in D. pulex than in D. rosea of the same size. Neckteeth were induced specifically in vulnerably sized juvenile instars of D. pulex, but were not found in all vulnerable instars of D. rosea. In D. rosea, neckteeth were few or absent in the ephippial hatchlings, and neckteeth formation ceased before juveniles reached a body size outside the range that larger larval stages of Chaoborus could ingest. This study provides the first experimental demonstration in the field of the inducibility of morphological defense structures in Daphnia at ambient densities of Chaoborus larvae, and quantifies these in situ responses. This expands on earlier observations of a correlation between predator density in the field and the expression of neckteeth in Daphnia. The term ”maximum size for neckteeth formation” (MSNF) is defined as the limit in body size above which no production of neckteeth was evident. This limit was used to distinguish the size classes of Daphnia that show a sensitive response to Chaoborus kairomone. This new term may be used for further comparisons among species and among different types of predator-induced responses as well as for the evaluation of the adaptive value of defense structures. Received: 10 April 1999 / Accepted: 6 April 2000     

Enhanced growth at low population density in Daphnia: the absence of crowding effects or relief from visual predation?

1. It has been suggested that chemical information from crowded populations of an animal such as Daphnia carries a cue indicating imminent food limitation, and we suggest that in the presence of fish kairomones, it may also convey a hint of the need to enhance antipredation defences. 2. We performed two‐factorial experiments with Daphnia grown in flow‐through plankton chambers in medium containing high levels of Scenedesmus food plus chemical information on either low or high population density levels and in the presence or absence of fish chemical cues (kairomones) and recorded (i) the effects on Daphnia growth rate and reproduction, and (ii) the effects on Daphnia depth selection. Further depth‐selection experiments were performed to test the reaction of Daphnia to crowding information at different Daphnia concentrations and to test its effect on daytime and night‐time depth selection by different Daphnia instars in the presence of kairomones. 3. The effects of crowding information alone (in the absence of kairomones) were weak and were not significantly strengthened by the addition of kairomones. The effects of kairomones alone (in the absence of crowding information) were much stronger and were increased by the presence of crowding chemicals: Daphnia selected greater depths in daylight (the later the instar and the larger its body size, the greater the depth), their body growth was slower and daily reproductive investment reduced, compared with Daphnia grown in the absence of crowding information. This suggested that crowding chemicals carry a cue indicating the need to invest more into antipredation defences. 4. The adaptive significance of these effects was confirmed by the differential vulnerability to predation of the Daphnia when offered as prey to live roach after being grown for 6 days either in the presence (higher vulnerability) or in the absence (lower vulnerability) of information on high density. 5. The strong interaction between crowding information and fish kairomones may be explained either as the reaction to a cue indicating impending food stress or as the reaction to a signal of increased predation risk. While the former scenario is already known from crowding studies, the latter is a novel idea that stems from the old concept of ‘low‐density anti‐predation refuge’. The two scenarios are not mutually exclusive: each stems from the need to invest in survival rather than in growth and reproduction [Corrections were made to this paragraph after first online publication on 4 April 2012].     

Genetic differentiation in life history between Daphnia galeata populations: an adaptation to local predation regimes?

Species of the water flea Daphnia exhibit constitutive as wellas phenotypically inducible anti-predator defence strategies,involving life history, morphological and behavioural traits.We explored the hypothesis of genetic differentiation in anti-predatordefence strategies using Daphnia clones originating from twodifferent water bodies: Tjeukemeer (the Netherlands) and FishPond (Belgium). Both water bodies are inhabited by zooplanktivorousfish. In contrast to Tjeukemeer, Fish Pond is also inhabitedby larvae of the phantom midge Chaoborus. The life history responsesof the two sets of clones to kairomones from fish (Perca), tokairomones from Chaoborus, and to a mixture of both were compared.Clones from Tjeukemeer and Fish Pond showed strong responsesto the presence of fish kairomone, with reductions in adultand neonate body length, in age at first reproduction and inthe total number of neonates produced during the first threeadult instars. Responses to Chaoborus kairomone were much lesspronounced, although there was a tendency towards an increasein the number of neonates in the first brood. Significant inter-populationgenetic differences were found for all the investigated traits.However, there was no indication for genetic adaptation of theFish Pond clones to negative size-selective predation by Chaoborus.Compared to Tjeukemeer clones, Fish Pond clones had a lowersize at first reproduction, produced smaller neonates and produceda higher number of neonates in their first brood. This suggestsadaptation to positive rather than to negative size selectivepredation. Genetic differences between populations were observedmainly for constitutive life history traits, rather than forphenotypic shifts in response to the presence of predator kairomones.     

Combined effects of food shortage and oxygen deficiency on life history characteristics and filter screens of Daphnia

Daphnia magna were reared in environments with different concentrationsof food and dissolved oxygen, and their life history characteristicsand filter-screen areas were analyzed. Both low food concentrationand low oxygen concentration degraded Daphnia fitness (as measuredby individual growth and reproduction), and synergism was detectedstatistically for the effects of the two factors together. Lowfood concentration induced the development of large filter screensin Daphnia, but this development was suppressed by low oxygenconcentration, suggesting that low oxygen concentration reducesthe filtering rate of Daphnia in a food-limited environment.Therefore, energy uptake by Daphnia seems to be reduced by foodshortages and oxygen deficiencies jointly, and this reductionin energy uptake may be a possible mechanism causing the synergisticeffects of the two environmental stresses on the life historyof Daphnia.     

Body size distribution in <Emphasis Type="Italic">Daphnia</Emphasis> populations as an effect of prey selectivity by planktivorous fish

We test the hypothesis that size distribution of a Daphnia population reflects the vulnerability of each size category (instar) to predation by planktivorous fish. We hypothesize that due to the different reaction distances from which separate prey categories can be seen by a foraging fish, each category is preyed upon until its density is reduced and its size-specific apparent density level (number of prey within a hemisphere of radius equal to the reaction distance) or encounter rate (number of prey encountered per time within a tube with a cross section of radius equal to the reaction distance) become equal to those of other size categories. An experiment was performed with populations of Daphnia hyalina and D. pulicaria grown at two Scenedesmus + Chlamydomonas food levels (0.2 and 0.05 mg C per liter) in outdoor mesocosms (1000 l tanks) with predation by invertebrates (phantom midge) prevented by mosquito netting. Once the populations had become established, roach were added to the tanks at dusk each day and allowed to feed for 3 h, while control tanks were kept fish-free. After 20–60 days, while D. pulicaria was at low density level, the densities of D. hyalina in fish tanks were high enough to see that the age structure and size distribution matched those from simulations with the age-structured population model based on size-specific encounter rate. This match, however, remained only up to the point of first reproduction when—in contrast to the size/age distribution predicted by the model—the percentage share of adult instars in the total population decreased rapidly with age. This deviation from the predicted densities of adult instars suggests that neither encounter rates nor apparent densities derived from instar-specific reaction distances are sufficient to explain the instar-specific impact of a visual predator on planktonic prey. This implies that a foraging fish may temporarily change its feeding mode from the typical low-speed harvesting of small but abundant prey from within its visual field volume, to high-speed hunting for more scarce but larger ovigerous females when their abundance allows higher net energy gain. Shifting from one feeding mode to the other may be responsible for damping population density oscillations in Daphnia.     

Combined effect of the insecticide carbaryl and the Chaoborus kairomone on helmet development in Daphnia ambigua

The cladoceran Daphnia ambigua was exposed to both the insecticide carbaryl and the kairomone released from the predator Chaoborus simultaneously, and its morphological changes were analyzed. Daphnia developed helmets in response to the kairomone, but not in response to carbaryl at low (sublethal) concentrations (1–3 µg 1^–1). However, the carbaryl enhanced the development of high helmets and prolonged the maintenance period of the helmets over instars in the presence of the kairomone. These results suggest that sublethal concentrations of the insecticide alter predator-prey interactions by inducing helmet formation in Daphnia, which may reduce vulnerability of the Daphnia to predation.     

Depth selection and life history strategies as mutually exclusive responses to risk of fish predation in <Emphasis Type="Italic">Daphnia</Emphasis>

We tested if pelagic crustaceans of the genus Daphnia use different anti-predator defences in environmental conditions that do or do not offer deep refuge from planktivorous fish. We kept Daphnia catawba in two series of 9-m deep enclosures with and without caged cyprinid fish Phoxinus eos. In one series of enclosures, Daphnia could select its depth of residence and hide in deep dark water layers to avoid anticipated fish predation, while in another series of enclosures, a plankton net barrier fixed at 2-m depth forced them to stay in subsurface zone exposed to fish kairomones. We compared depth residence and migratory behaviour strategies with life history strategies (body size and size at first reproduction, diapause induction) in Daphnia exposed or not to fish kairomones with or without deep refuge. In deep enclosures with fish, Daphnia spent daytime hours in deep dark layers while at night, they resided closer to the water surface. Yet, no change in life history parameters of migrating individuals was observed compared to the fish-free conditions. In enclosures with fish, where the net barrier forced Daphnia to reside in subsurface zone, they produced smaller offspring, matured at smaller size and achieved lower maximum body length compared to the fish-free conditions. However, they did not produce diapausing eggs. Our experimental study supports the hypothesis that diel vertical migration behaviour with daytime residence in deep, dark water are the preferred antipredator strategy chosen by Daphnia facing anticipated fish predation over life history changes such as reduced size and low growth rate which are used when dark deep refuge is not present or accessible.