Environmental plasticity of fish avoidance diapause response in Daphnia magna

Organisms cope with harsh environmental conditions in various ways: either by tolerating environmental stress (through physiological adaptations), or by avoiding it in space (through migration) or time (diapause). Some species rely on a single strategy while others may choose from an array of options when facing different environmental stressors. Planktonic crustaceans may utilise different active (morphological, behavioural, life-history) or passive (diapause) defences to survive periods of high risk of fish predation. Recent evidence has indicated that resting egg production could be induced in Daphnia magna by chemical cues associated with fish predation. This suggests that contrary to most known cases of diapause, which are triggered well in advance of catastrophic events (here termed "predictive diapause"), fish avoidance diapause in D. magna may exhibit a "responsive nature" and be initiated only after intensive predation appears. Experimental evidence discussed here indicates that the reaction of D. magna to chemical signals of fish predation could be conditional and determined by key environmental conditions, which in nature affect relative gains of activity vs dormancy. At high risk of fish predation, the decision of Daphnia to produce resting eggs was disfavoured by high food concentration. This reaction was claimed adaptive since high food allows for higher reproductive rates and better survival of offspring. All this may assure higher benefits due to activity despite some risk of predation (once predation pressure is not fatal to all active descendants) and disfavour resting eggs production. Moreover, at moderate food conditions the decision of Daphnia to produce resting eggs was disfavoured by the availability of a dark refuge from fish visual predators and thus likely lowering the risk of being preyed upon. Furthermore, when food was at a moderate level and a dark refuge was not present the decision of Daphnia to produce resting eggs was favoured by low water temperature. This could be explained as an adaptive reaction again, once low water temperature (due to its effect on a rate of metabolism) should have affected the gains derived from active life and reproduction more seriously than ones of inactive stages. The evidence presented here indicates that a responsive diapause allows D. magna to maximise reproductive output by taking advantage of opportunities presented by an unpredictable environment.