What goes down must come up: symmetry in light-induced migration behaviour of Daphnia

During a short period of the year, Daphnia may perform a phenotypically induced diel vertical migration. For this to happen, light-induced swimming reactions must be enhanced both at dawn and at dusk. Enhanced swimming in response to light intensity increase can be elicited by fish-associated kairomone in the laboratory, if food is sufficiently available. However, during the light change at dusk the Daphnia are still in the hypolimnion, where no fish kairomone is present and both temperature and food availability is low. Still, what goes down must come up. This raises questions about how Daphnia tunes its light-induced swimming behaviour to prevailing conditions such that a normal diel vertical migration can be performed. We investigated the symmetry in behavioural mechanism underlying these diel vertical migrations in the hybrid Daphnia galeata × hyalina (Cladocera; Crustacea), with special interest for the environmental cues that are known to affect swimming in response to light increase. That is, we tested whether fish- associated kairomone, food availability, and temperature affected both swimming in response to light intensity increase and decrease similarly. We quantified swimming behaviour during a sequentially increased rate of light change. Vertical displacement velocity was measured and proved to be linearly related to the rate of the light change. The slope (PC) of the function depends on the value of the factors kairomone concentration, food availability, and temperature. The changes of the PC with kairomone concentration and with temperature were similar both at light intensity increases and decreases. The PC also increased with food concentration, although during light increases in a different way from during light intensity decreases. Low food availability inhibited swimming in response to light intensity increase, but enhanced swimming in response to light intensity decrease. Hence, ascent from the deep water layers with low food concentration at dusk is facilitated. These causal relations are part of a proximate decision-making mechanism which may help the individual Daphnia to tune migration to predation intensity and food availability.     

The effect of accelerations in light increase on the phototactic downward swimming of Daphnia and the relevance to diel vertical migration

The effect of relative increases in light intensity on photobehaviourwas studied in the hybrid Daphnia galeata x hyalina. We firstcarried out a series of experiments to study the influence offish kairomone on several response variables of light-inducedswimming. With fish kairomone present, an increase in the percentageof reacting daphnids to 100% was found at almost all ecologicallyoccurring relative light change rates that were above threshold.The relationship between the relative increase in light intensity(stimulus) and the time expiring between the onset of the stimulusand the start of the downward swimming response was not influencedby fish kairomone, nor did kairomone alter the functional relationshipbetween stimulus strength and downward displacement velocity,although velocity increased. During the previous experiments,various light change rates were applied, but per test run theserates were constant. The natural relative light increase inthe early morning consists of continuously increasing relativelight change rates, turning into decreasing rates after themaximum is reached     

Swimming of Daphnia galeata x hyalina in response to changing light intensities: Influence of food availability and predator kairomone

Experiments showed that phototactic downward swimming in Daphnia galeata x hyalina as caused by a relative increase in light intensity (stimulus) is influenced by predator kairomone and food availability. The swimming responses at four different combinations of food availability and fish kairomone were analysed. Addition of both food and kairomone led to a significant increase in percentage of animals that responded to the light stimulus, but there was no significant interaction effect.We also found that kairomone and food had significant impact on displacement velocity and on the time between start of the stimulus and onset of the response.     

Light-induced swimming of Daphnia: can laboratory experiments predict diel vertical migration?

Vertical displacement velocity of a Daphniagaleata × hyalina clone was quantified inrelation to changes in the relative rate of lightchange. An increase in the latter variable triggers anenhanced swimming response, and this response is againelicited when a second increase in the rate ofrelative light increase is applied. Decreases in therate of light increase affect phototactic swimming ina similar way. The acceleration/deceleration assistedstimulus-response system is an extension of the ideaof phototaxis as the underlying behavioural mechanismfor vertical migration, and suggests that continuousaccelerations in light change also affect verticaldisplacements observed in the field. A simple dielvertical migration simulation model was used tocalculate the vertical displacement of Daphniain relation to the natural light change at sunrise.The calculated vertical displacement fits nicely inthe temporal range of the observed averaged downwardmigration of adult Daphnia in Lake Maarsseveen.The calculated migration amplitude, however, islarger than the change in mean population depthobserved in nature.     

Two examples of the interplay between field observations and laboratory experiments from 35 years of research with planktonic organisms

Two studies of complicated ecological phenomena in Lake Maarsseveen (The Netherlands) are presented to illustrate that a combination of field and laboratory analysis might be a successful approach. In the first one, the yearly varying ratio of population abundance of two diatoms, Asterionella formosa and Fragilaria crotonensis during early spring is explained by a combination of abiotic and biotic factors. A fungal parasite blocks population development of the first species at temperatures higher than 3–5 °C . The second species then competes successfully for phosphate and develops a large population. The interaction between Asterionella and the fungus proved to be intricate. Several physiological mechanisms operate together and combined with variable climatic factors makes the prediction of what species will be dominant, impossible. In the second example diel vertical migration of the hybrid Daphnia galeata × hyalina is analysed in the field and the laboratory. Migration is confined to six to seven weeks in June–July when large shoals of juvenile perch are present in the pelagic zone of the lake. These fish exudate a kairomone that enhances the reaction of Daphnia to relative increases in light intensity of dawn. Especially accelerations in rate of relative increases enhance swimming velocity. The extent of enhancement depends, however, on the concentration of the kairomone and on food concentration. A Decision Making Mechanism is introduced and the relation between the response mechanism and the adaptive relevance of diel vertical migration is made.Finally considerations about research strategies and the importance of fundamental research are made. It is concluded that the development of limnology and aquatic ecology might be in danger when the present tendency to give prevalence to applied research continues.     

Ultrastructure of the epidermis of Meara stichopi (Platyhelminthes,Nemertodermatida) and associated extra-epidermal bacteria

This preliminary mechanistic model of normal swimming and phototactic behaviour in individual Daphnia was constructed using data and assumptions based on experiments and observations. Swimming under constant light intensity is characterized by short periods of upward movements alternating with equal periods of downward movements. Two oscillators are proposed that generate these phases in swimming. Unexpected shifts in depth, as observed in D. magna and D. longispina, are also present in the swimming of the computer daphnid and thus seem to be inherent to the underlying mechanism. As in real daphnids, during relative decreases in light intensity of low velocity, positive phototactic upward swimming is stepwise. With increasing velocity in the change in light, these steps disappear. When the model is triggered by a natural increase in light at dawn, a small downward movement results. Migration distance can be increased to commonly found depths of migrating Daphnia by the introduction of a fish exudate factor into the model, which enhances the phototactic response. Since attenuation of light in the water affects the phototactic swimming response, it also influences migration distance. The results of model calculations agree quite well with an empirical relationship between Secchi disc depth and amplitude of diel vertical migration in a number of lakes.     

Relationship between fish kairomone concentration in a lake and phototactic swimming by Daphnia

Light-induced swimming behaviour of Daphnia can lead to dielvertical migration. When this occurs, Daphnia may escape frompredation by juvenile 0+ fish. For this to happen, swimmingin response to the change in light intensity at dawn and duskmust be enhanced. This enhanced swimming reaction can be elicitedby fish-associated kairomone in the laboratory. We studied theeffect of these fish-associated kairomones in Lake Maarsseveen,The Netherlands. A bioassay was conducted in which light-inducedswimming reactions of Daphnia were used to quantify the kairomonesignal strength. In two successive years, 1998 and 1999, waterfrom Lake Maarsseveen was tested weekly in early summer. Sampleswere taken from the epilimnion at 3 m depth and from the hypolimnionat 15 m depth. Simultaneously, we caught 0+ perch (Perca fluviatilis)with bongo nets to determine its abundance. The length and weightof individuals caught in 1998 were determined to establish alength–weight relationship. Using this relationship andthe information on perch density from the trawls, the relativefish biomass was calculated for these two years. It was shownthat water from the epilimnion layer increasingly enhanced light-inducedswimming reactions until the second week of June, then the effectgradually disappeared. Water from the hypolimnion had no sucheffect. In 1998, these changes in signal strength correlatedwith the relative biomass of the 0+ perch, but in 1999, themaximum of the enhancement lagged 2 weeks behind the maximumof the biomass of the 0+ perch. This lag may be due to a differentdevelopment of the thermocline. We conclude that kairomone concentrationmay well correlate with 0+ perch biomass and thereby might informDaphnia not only about the presence, but also about the abundance,of juvenile perch.     

Proximate control of diel vertical migration in Phyllosoma larvae of the Caribbean spiny lobster Panulirus argus

Phyllosoma larvae of the spiny lobster Panulirus argus undergo diel vertical migration (DVM), in which they are at depth during the day and nearer the surface at night. This study determined the visual spectral sensitivity of Stage I larvae and investigated whether light plays a proximate role in DVM as an exogenous cue and as an entrainment cue for an endogenous rhythm in vertical migration. Under constant conditions, larvae have a circadian rhythm (24.5-h period) in vertical swimming that resulted in a twilight DVM pattern. The behavioral response spectrum and electroretinogram recording indicated two photoreceptor spectral classes with maxima at 360 and 486 nm. When stimulated in an apparatus that simulated the underwater angular light distribution, dark-adapted larvae showed only positive phototaxis, with a threshold intensity of 1.8 × 10(13) photons m(-2) s(-1) (3.0 × 10(-5) μmoles photons m(-2) s(-1)). They have an avoidance response to predator shadows in which they descend upon sudden decreases in light intensity of more than 69%. When stimulated with relative rates of decrease in light intensity as occur at sunset they ascended, whereas they descended upon relative rates of light intensity increase as occur at sunrise. Thus, the DVM pattern is controlled by both an endogenous circadian rhythm in swimming and behavioral responses to light at sunrise and sunset.     

Predator specificity of kairomones in diel vertical migration of Daphnia: a chemical approach

Daphnia responds to chemical cues released by fish with diel vertical migration (DVM) as a behavioural predator avoidance. We used a bioassay to characterize the chemical nature of the kairomone. Cues released from stickleback ( Gasterosteus aculeatus , Gasterosteidae) and a piscivorous pike ( Esox lucius , Esocidae) were enriched from incubation water by reversed-phase sorbent extraction and were reversibly inactivated by acetylation. HPLC yielded only one active fraction with identical retention times for the kairomones of both species. Chemical features did not differ from those previously reported for Cyprinidae, indicating that the active compounds are very similar if not identical. From further investigation of the kairomone released by crucian carp ( Carassius carassius , Cyprinidae), glucuronic acids and carboxy-, sulphate- and phosphate-groups can be excluded as essential for biological activity. The response of Daphnia increased with increasing concentrations of extracted kairomone. The kairomone was not released from mucus by digestion with hyaluronidase. Adsorption of the kairomone to food particles seems to be of minor importance suggesting that the cue is perceived as a freely dissolved molecule.     

Temperature selection in Brook Charr: lab experiments,field studies,and matching the Fry curve

Zooplankton diel vertical migration (DVM) is an ecologically important process, affecting nutrient transport and trophic interactions. Available measurements of zooplankton displacement velocity during the DVM in the field are rare; therefore, it is not known which factors are key in driving this velocity. We measured the velocity of the migrating layer at sunset (upward bulk velocity) and sunrise (downwards velocity) in summer 2015 and 2016 in a lake using the backscatter strength (VBS) from an acoustic Doppler current profiler. We collected time series of temperature, relative change in light intensity chlorophyll-a concentration and zooplankton concentration. Our data show that upward velocities increased during the summer and were not enhanced by food, light intensity or by VBS, which is a proxy for zooplankton concentration and size. Upward velocities were strongly correlated with the water temperature in the migrating layer, suggesting that temperature could be a key factor controlling swimming activity. Downward velocities were constant, likely because Daphnia passively sink at sunrise, as suggested by our model of Daphnia sinking rate. Zooplankton migrations mediate trophic interactions and web food structure in pelagic ecosystems. An understanding of the potential environmental determinants of this behaviour is therefore essential to our knowledge of ecosystem functioning.

     

Enhancement of the phototactic reaction in Daphnia hyalina by a chemical mediated by juvenile perch (Perca fluviatilis)

Experiments with Daphnia hyalina demonstrated that the phototacticreaction to changes in light intensity is strongly enhancedin the presence of a chemical substance mediated by juvenileperch. In constant light intensity, the daphnids were foundclose to the bottom of the experimental aquarium. The phenomenonmay result in a diel vertical migration. This would decreasepredation pressure near the surface.     

Control mechanisms of diel vertical migration: theoretical assumptions

We explore control mechanisms underlying the vertical migration of zooplankton in the water column under the predator-avoidance hypothesis. Two groups of assumptions in which the organisms are assumed to migrate vertically in order to minimize realized or effective predation pressure (type-I) and to minimize changes in realized or effective predation pressure (type-II), respectively, are investigated. Realized predation pressure is defined as the product of light intensity and relative predation abundance and the part of realized predation pressure that really affects organisms is termed as effective predation pressure. Although both types of assumptions can lead to the migration of zooplankton to avoid the mortality from predators, only the mechanisms based on type-II assumptions permit zooplankton to undergo a normal diel vertical migration (morning descent and evening ascent). The assumption of minimizing changes in realized predation pressure is based on consideration of DVM induction only by light intensity and predators. The assumption of minimizing changes in effective predation pressure takes into account, apart from light and predators also the effects of food and temperature. The latter assumption results in the same expression of migration velocity as the former one when both food and temperature are constant over water depth. A significant characteristic of the two type-II assumptions is that the relative change in light intensity plays a primary role in determining the migration velocity. The photoresponse is modified by other environmental variables: predation pressure, food and temperature. Both light and predation pressure are necessary for organisms to undertake DVM. We analyse the effect of each single variable. The modification of the phototaxis of migratory organisms depends on the vertical distribution of these variables.     

The photobehaviour of Daphnia spp. as a model to explain diel vertical migration in zooplankton

Many pelagic animal species in the marine environment and in lakes migrate to deeper water layers before sunrise and return around sunset. The amplitude of these diel vertical migrations (DVM) varies from several hundreds of metres in the oceans to approx. 5–20 m in lakes. DVM can be studied from a proximate and an ultimate point of view. A proximate analysis is intended to reveal the underlying behavioural mechanism and the factors that cause the daily displacements. The ultimate analysis deals with the adaptive significance of DVM and the driving forces that were responsible for the selection of the traits essential to the behavioural mechanism. The freshwater cladoceran Daphnia is the best studied species and results can be used to model migration behaviour in general. Phototaxis in Daphnia spp., which is defined as a light-oriented swimming towards (positive phototaxis) or away (negative phototaxis) from a light source, is considered the most important mechanism basic to DVM. A distinction has been made between primary phototaxis which occurs when light intensity is constant, and secondary phototaxis which is caused by changes in light intensity. Both types of reaction are superimposed on normal swimming. This swimming of Daphnia spp. consists of alternating upwards and downwards displacements over small distances. An internal oscillator seems to be at the base of these alternations. Primary phototaxis is the result of a dominance of either the upwards or the downwards oscillator phase, and the direction depends on internal and external factors: for example, fish-mediated chemicals or kairomones induce a downwards drift. Adverse environmental factors may produce a persistent primary phototaxis. Rare clones of D. magna have been found that show also persistent positive or negative primary phototaxis and interbreeding of the two types produces intermediate progeny: thus a genetic component seems to be involved. Also secondary phototaxis is superimposed on normal swimming: a continuous increase in light intensity amplifies the downwards oscillator phase and decreases the upwards phase. A threshold must be succeeded which depends on the rate and the duration of the relative change in light intensity. The relation between both is given by the stimulus strength versus stimulus duration curve. An absolute threshold or rheobase exists, defined as the minimum rate of change causing a response if continued for an infinitely long time. DVM in a lake takes place during a period of 1-5-2 h when light changes are higher than the rheobase threshold. Accelerations in the rate of relative increase in light intensity strongly enhance downwards swimming in Daphnia spp. and this enhancement increases with increasing fish kairomone and food concentration. This phenomenon may represent a ‘decision-making mechanism’ to realize the adaptive goal of DVM: at high fish predator densities, thus high kairomone concentrations, and sufficiently high food concentrations, DVM is profitable but not so at low concentrations. Body axis orientation in Daphnia spp. is controlled with regard to light-dark boundaries or contrasts. Under water, contrasts are present at the boundaries of the illuminated circular window which results from the maximum angle of refraction at 48–9° with the normal (Snell's window). Contrasts are fixed by the compound eye and appropriate turning of the body axis orients the daphnid in an upwards or an obliquely downwards direction. A predisposition for a positively or negatively phototactic orientation seems to be the result of a disturbed balance of the two oscillators governing normal swimming. Some investigators have tried to study DVM at a laboratory scale during a 24 h cycle. To imitate nature, properties of a natural water column, such as a large temperature gradient, were compressed into a few cm. With appropriate light intensity changes, vertical distributions looking like DVM were obtained. The results can be explained by phototactic reactions and the artificial nature of the compressed environmental factors but do not compare with DVM in the field. A mechanistic model of DVM based on phototaxis is presented. Both, primary and secondary phototaxis is considered an extension of normal swimming. Using the light intensity changes of dawn and the differential enhancement of kairomones and food concentrations, amplitudes of DVM could be simulated comparable to those in a lake. The most important adaptive significance of DVM is avoidance of visual predators such as juvenile fish. However, in the absence of fish kairomones, small-scale DVMs are often present, which were probably evolved for UV-protection, and are realized by not enhanced phototaxis. In addition, the ‘decision-making mechanism’ was probably evolved as based on the enhanced phototactic reaction to accelerations in the rate of relative changes in light intensity and the presence of fish kairomones.     

Diel vertical migration: modelling light-mediated mechanisms

Light is generally regarded as the most likely cue used by zooplanktonto regulate their vertical movements through the water column.However, the way in which light is used by zooplankton as acue is not well understood. In this paper we present a mathematicalmodel of diel vertical migration which produces vertical distributionsof zooplankton that vary in space and time. The model is usedto predict the patterns of vertical distribution which resultwhen animals are assumed to adopt one of three commonly proposedmechanisms for vertical swimming. First, we assume zooplanktontend to swim towards a preferred intensity of light. We thenassume zooplankton swim in response to either the rate of changein light intensity or the relative rate of change in light intensity.The model predicts that for all three mechanisms movement isfastest at sunset and sunrise and populations are primarilyinfluenced by eddy diffusion at night in the absence of a lightstimulus. Daytime patterns of vertical distribution differ betweenthe three mechanisms and the reasons for the predicted differencesare discussed. Swimming responses to properties of the lightfield are shown to be adequate for describing did vertical migrationwhere animals congregate in near surface waters during the eveningand reside at deeper depths during the day. However, the modelis unable to explain how some populations halt their ascentbefore reaching surface waters or how populations re-congregatein surface waters a few hours before sunrise, a phenomenon whichis sometimes observed in the field. The model results indicatethat other exogenous or endogenous factors besides light mayplay important roles in regulating vertical movement.     

A mechanism of predator-mediated induction of diel vertical migration in Daphnia hyalina

Predator avoidance is generally thought to be the most importantultimate reason for diel vertical migration in zooplankton.Combining field observations and experimental results, it isshown, first, that a phototactic reaction to relative changesin light intensity is at the base of a diel vertical migrationand, secondly, how this phototaxis is enhanced by a chemicalmediated by juvenile perch.     

Fish and mucus-dwelling bacteria interact to produce a kairomone that induces diel vertical migration in Daphnia

1. Bacterial populations associated with fish have previously been documented to be crucial for the production of chemical signals governing the interactions between predator fish and zooplankton prey. 2. In this study, we investigated the roles of fish and mucus‐dwelling bacteria in kairomone production by conducting two sets of experiments related to elimination of bacteria with antibiotics and using fish mucus in bioassays of Daphnia pulex’s diel vertical migration. 3. Daphnia’s migratory response to the antibiotic‐treated fish was about half the strength of the response to the fish cue treatment. Furthermore, when the same antibiotic‐treated fish were removed from the antibiotic‐containing water and transferred into control water for 24 and 48 h, the extent of D. pulex’s migration depended on the length of the incubation period, apparently corresponding to the regeneration of bacterial colonies associated with mucus. The migration pattern observed in the 24 h treatment was similar to that of antibiotic‐treated fish. On the other hand, a pronounced migration occurred in the 48 h following antibiotic treatment; here, we found a higher density of fish surface dwelling bacteria than at the start of the experiment. 4. In the experiment involving fish mucus, the mucus‐enriched control water induced a weak response similar to antibiotic‐treated fish. 5. On the basis of the results from the two experiments, we suggest that both fish and fish mucus‐dwelling bacteria interact in the release of kairomone in ecologically relevant quantities.     

Vertical distribution of Chlamydomonas changes in response to grazer and predator kairomones

Individuals in aquatic communities frequently assess their biotic environment through infochemicals. In particular, kairomones are commonly involved in interactions between predator and prey. However, the relationship between individuals and chemicals produced by other organisms that are not direct predators, but may indicate the presence of a predator, is not well characterized. We used experimental microcosms to test whether the unicellular green alga Chlamydomonas reinhardtii alters vertical migration patterns in response to kairomones produced by zooplankton ( Daphnia ) and planktivores (fish). Our results suggested that phototaxis in C. reinhardtii was strongly affected by the type of kairomone present, the concentration of the kairomone, and the duration of exposure to the kairomone. Kairomones generally increased phototaxis in C. reinhardtii . The adaptive significance of such behavioral changes in natural settings would depend largely on local community composition. The similarity in phototactic responses of C. reinhardtii to Daphnia and fish kairomone suggest that, in at least this species of phytoplankton, the underlying genetic elements responsible for kairomone detection may be responsive to a broad range of chemical stimuli, allowing this species to adjust its phototaxis in response to not only the presence of its grazers, but also to predators of its grazers.     

Role of melatonin in the control of depth distribution of <Emphasis Type="Italic">Daphnia magna</Emphasis>

Previous studies confirmed the presence of melatonin in Daphnia magna and demonstrated diurnal fluctuations in its concentration. It is also known that in several invertebrate species, melatonin affects locomotor activity. We tested the hypothesis that this hormone is involved in the regulation of Daphnia diel vertical migration (DVM) behaviour that is well recognized as the adaptive response to predation threat. Using ‘plankton organs’, we studied the effect of three concentrations of exogenous melatonin (10⁻⁵, 10⁻⁷, 10⁻⁹ M) on DVM of both female and male D. magna in the presence or absence of chemical cue (kairomone) of planktivorous fish. Depth distribution was measured six times a day, using infrared-sensitive closed circuit television cameras. Our results showed a significant effect of melatonin on the mean depth of experimental populations, both males and females, but only when melatonin was combined with fish kairomone. Females stayed, on average, closer to the surface than males, both responding to the presence of kairomone by descending to deeper strata. In the presence of exogenous melatonin and with the threat of predation, Daphnia stayed closer to the surface and their distribution was more variable than that of individuals, which were exposed to the kairomone alone. Approaching the surface in the presence of predation threat seems to be maladaptive. We postulate the role of melatonin as a stress signal inhibitor in molecular pathways of response to predation threat in Cladocera.     

Diel pattern of pelagic distribution and feeding in planktivorous fish

Summary The spatial distribution of juvenile roach (Rutilus rutilus), rudd (Scardinius erythrophtalamus), bream (Abramis brama) and bleak (Alburnus alburnus) was registered by echosounding during two years in small Bavarian lakes. The gut contents of the fish were analysed in order to reconstruct their rhythmicity of food intake. Fish were found in the pelagic zone only during the night, feeding before midnight with maximal rate. The vertical position of the fish was dependent on the water transparency following a light intensity in the order of 10^-3 Lux. The diel migration between littoral and pelagic zones was a mainly horizontal one with a vertical component. Both the speed of increase of fish density in the open water and of the vertical movement was correlated to the speed of change of light intensity. Shoals broke up into single fish during the offshore migration. The span of presence in the pelagic zone was identical with the duration of the dark period. During winter no diel migration nor periodical feeding took place.     

From inducible defences to population dynamics: modelling refuge use and life history changes in Daphnia

We investigated the relative importance of a behavioural defence (refuge use through diel vertical migration) and a life history change (a reduced size at first reproduction) that are used by daphnids to decrease the risk of predation by visually hunting fish. We used an individual based model of a Daphnia population in a stratified lake to quantify the effects of these inducible defences on Daphnia predation-mortality and the resulting Daphnia population dynamics. Our analysis shows that diel vertical migration (DVM) confers a much stronger protection against fish predation than a reduced size at first reproduction (SFR). DVM allows daphnids to withstand a higher predation pressure in the epilimnion and it decelerates a Daphnia population decline more strongly than a reduced SFR. DVM effectively reduces the (P/B) flow of carbon from daphnids to fish.
Many theoretical studies have only considered the fitness benefits of DVM above 'staying up' in the epilimnion of a lake. Our results suggest that 'staying down' in the hypolimnion would confer an even stronger fitness benefit to Daphnia than DVM at times of peak predation risk. Daphnids that remain in the hypolimnion avoid the predation suffered by migrating daphnids around dusk and dawn. Staying down could prevent a Daphnia population decline, while DVM and a reduced SFR can only decelerate the decrease of Daphnia population densities under heavy fish predation. Staying down at high concentrations of fish infochemicals has in fact been observed within a variety of Daphnia clones and species, both in the laboratory and in stratified lakes.