The combined effects of hypoxia and fish kairomones on several physiological and life history traits of Daphnia

Abstract

1. Numerous studies have tested the combined effect of the threat of predation by fish and low oxygen concentrations on the phenotypic plasticity of Daphnia.These studies assessed the trade-off between minimising predation risk and the negative effects of oxygen deficiencies in the context of depth selection behaviour. We tested whether this trade-off also affects physiological and life history traits. We expected an interactive effect between the threat of fish predation and low oxygen concentrations, such, that the net effect of both stressors would be antagonistic (lower than the sum of each of the stressors acting separately), rather than additive (or synergistic) on the majority of traits investigated, but we predicted synergistic effects on heat shock proteins (HSPs). 2. To test this, we performed life table experiments in different oxygen concentrations (normoxia and hypoxia) and levels of predation threat (the presence and absence of fish kairomones) on HSP70 and putative HSP110, haemoglobin concentration and life history traits with small-bodied Daphnia galeata and large-bodied Daphnia pulex originating from waterbodies where there were different risks of fish predation. 3. As predicted, the net effect of both stressors was antagonistic for most of the physiological and ecological variables studied. The presence of kairomones resulted in decreased body size of adults, egg size, egg size in relation to brood chamber volume, and in increased clutch size in relation to body size. These effects were weaker in hypoxia than in normoxia, which may suggest an existence of adaptive responses caused by a lower perceived risk in hypoxia than in normoxia, as the foraging abilities of fish are limited by oxygen deficiencies. 4. The presence of kairomones hampered the production of haemoglobin in hypoxia for the clones of larger-bodied species, which suggests the existence of a trade-off between reduced visibility under positive-size selective predation risk and increased efficiency of oxygen transport to body tissues. The presence of kairomones and hypoxia resulted in an increased level of putative HSP110, and the effect of kairomones was stronger in hypoxia than in normoxia. More complex results were obtained for the effect of both stressors on the level of HSP70. | 2205 WILCZYNSKI et al.

Knowing the Enemy: Inducible Defences in Freshwater Zooplankton

“…Even though there is evidence that prey organisms are able to react differently to some fish species [222], there are contradictory studies that still find no difference in defence induction when Daphnia were exposed to different fish species (e.g., [220,231]). As pointed out in the former sections, the response to fish predators is usually expressed in accordance with a general pattern, i.e., reduced size and increased reproduction (e.g., [203,210,232]). Thus, the reaction norm may depend on kairomone concentration [220,221], which would not only explain the differences in the reaction norm between fish species (bigger fish might produce more kairomone) but also the response differences between small juvenile and fully-grown adult fish [199].…”

“…Food availability plays a role in the expression of induced defences in freshwater zooplankton, indicating the existence of allocation costs. They are represented by direct metabolic demands, e.g., due to increased respiration rate [23,203], or direct energy and material costs for forming and maintaining defences, such as morphological structures or increased reproduction (e.g., [99,[194][195][196]). Several studies already reported increasing demands of nutrients with increasing predation risk, or reduced defence expression under nutritional constraints [99,[204][205][206][207][208].…”

Combined effects of elevated epilimnetic temperature and metalimnetic hypoxia on the predation rate of planktivorous fish

“…The second explanation concerns the differences in the Daphnia and fish species used in the studies, with medium-bodied D. longispina and rudd in our study, and the larger-bodied Daphnia pulicaria and ide ( Leuciscus idus) in the study of Larsson and Lampert (2011). The latter explanation seems likely in the light of observations from numerous studies indicating a great variability in the vulnerability to hypoxia of different Daphnia (Heisey and Porter, 1977; Sell, 1998; Wilczyński et al ., 2019) and fish (Ekau et al ., 2010) species, with D. pulicaria as particularly resistant (Larsson and Lampert, 2012) and ide as a particularly vulnerable fish species to oxygen deficiencies (Mann, 1996).…”

“…Vanderploeg et al ., 2009a), two arguments may suggest that the mortality risk from fish would be greater at a decreased oxygen concentration in the metalimnion. The first argument is that persisting in an environment with a low oxygen concentration has high physiological and in turn demographic costs for zooplankton (Kobayashi and Hoshi, 1984; Hanazato and Dodson, 1995; Larsson and Lampert, 2011; Wilczyński et al ., 2019). This is consistent with the field observations that maximum zooplankton density during the day is usually in the top part of the metalimnion with relatively high oxygen concentration, even when the light intensity allows fish to forage in this layer (e.g.…”

“…Nikinmaa and Rees, 2005), which would outweigh the adaptations of Daphnia to oxygen deficiencies. For instance, contrary to fish, planktonic animals such as Daphnia would not be able to produce sufficient amounts of hemoglobin (the only protein responsible for the transport of oxygen in Daphnia ; Hebert et al ., 1999), because an increase of its content in the hemolymph results in a reddish body coloration, making Daphnia more conspicuous to fish (Confer et al ., 1978; Wilczyński et al., 2019). Moreover, long-term (duration of few days) acclimation of fish to hypoxia would increase their hunting abilities in hypoxic conditions and thus increase their pressure on zooplankton.…”