ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY
40, 77—80 (1998)
ENVIRONMENTAL RESEARCH, SECTION B ARTICLE NO.
Adaptations and Resistance of Zooplankton to Stress: Effects of Genetic, Environmental, and Physiological Factors Marko Reinikainen,*,-,1 Jaana Hietala,- and Mari Walls-,‡ *Department of Biology, As bo Akademi University, BioCity, FIN-20520 Turku, Finland; -Laboratory of Ecology and Animal Systematics, Department of Biology, University of Turku, FIN-20500 Turku, Finland; and ‡Maj and Tor Nessling Foundation, Pohjoinen Hesperiankatu 3 A, FIN-00260 Helsinki, Finland Received July 25, 1996
conditions (Ebert, 1994). Similarly, some suspension feeders have evolved characteristics that allow coexistence with toxic prey. These characteristics include selective feeding by copepods: nutritious algae are preferred over toxic ones. Other zooplankton species, like many cladocerans, do not have this ability (DeMott and Moxter, 1991). As a consequence, toxic cyanobacteria seriously inhibit many cladocerans (Lampert, 1981). In the present study, how Daphnia clones differ in their sensitivity to toxic cyanobacteria is reported. How poor food conditions interact with cyanobacteria and affect the resistance of Daphnia also are described. Differences in reactions to a stress factor from a predator are exemplified.
The ability of a species to adapt to stress factors such as exposure to toxicants depends to a large extent on the presence of individuals that are able to respond to the exposure in a successful way. Several strategies can be employed to cope with different stress factors. Investments on growth and reproduction, for instance, can be varied to meet the requirements of the environment. Large individuals generally have a high resistance against stress, but a large body size is often achieved at the cost of other characteristics. In the present study, the resistance of several clones of Daphnia to different stress factors, such as toxic cyanobacteria, a predator released chemical, and starvation, was investigated. The focus was on interactions among different factors and whether observed responses can be regarded as evolved adaptations for the different conditions. ( 1998 Academic Press
MATERIALS AND METHODS
Clonal Differences in Responses of Daphnia to Stress
An experiment was conducted in which clones of Daphnia longispina, Daphnia magna, and Daphnia pulex from the habitats described in Table 1 were used. Individuals from each clone were exposed to toxic cyanobacteria (Microcystis aeruginosa strain PCC7820) for 4 days, using a method similar to that described by Reinikainen et al. (1994). The cyanobacterial strain used contains the peptide toxin microcystin-LR (about 0.2—0.8% dry wt). Seven concentrations, ranging from 0 (control) to 230,000 cells ml~1 (about 1.8 mg C liter~1), were used. Survivorship was recorded daily, and LC values were calculated by using the Probit analysis 50 (National Swedish EPA, 1989). Examples from the literature are furthermore used to illustrate clonal differences in resource allocation to growth in Daphnia exposed to cyanobacteria and to a predator released chemical.
Ricklefs (1973) wrote that adaptations in an organism reflect the past, whereas natural selection acts in the present. Consequently, evolved adaptations against stress factors with a short evolutionary history, such as most xenobiotica, are unlikely. Resistance to xenobiotica is, however, possible as a result of random genetic variation: some clones of the cladoceran Daphnia are resistant to some xenobiotica, whereas other clones are extremely sensitive (Baird et al., 1990; Barber et al., 1990). When stress factors with a long evolutionary history are studied, adaptations can, on the other hand, be expected. Starvation, predation, and naturally occurring toxins are examples on such stress factors. The adaptations include reasonable solutions to allocation problems. Many zooplankton species are, for instance, known to produce few and small clutches under starvation. The offspring produced are, however, relatively large and resistant to poor food
The Interaction of Food Level and Cyanobacterial Exposure 1 To whom correspondence should be addressed at present address: Sydvast Polytechnic, Forstinstitutv., FIN-10600 Tammisaari, Finland. Fax: #358 19 2227 300. E-mail: [email protected]
To demonstrate the importance of environmental factors to stress resistance, an experiment was conducted in which 77 0147-6513/98 $25.00 Copyright ( 1998 by Academic Press All rights of reproduction in any form reserved.
REINIKAINEN, HIETALA, AND WALLS
TABLE 1 List of Daphnia Clones Used in Toxicity Tests with Cyanobacteria Clone Linnunpa¨a¨ Eestiluoto Tuorla Seili Hormisto Lepikko
Daphnia longispina Daphnia longispina Daphnia longispina Daphnia magna Daphnia pulex Daphnia pulex
Farm pond Rock pool Woodland pond Rock pool Woodland pond Woodland pond
Note. The name of the pond from which each clone was isolated is used as clone name.
a clone of D. pulex was exposed to M. aeruginosa (PCC7820) for 7 days, as described by Reinikainen et al. (1995). Each exposure level, ranging from 0 (control) to 50,000 cells ml~1 (0.4 mg C liter~1), was combined with two levels of the green alga Scenedesmus obtusiusculus (10,000 and 100,000 cells ml~1, corresponding to about 0.1 and 1 mg C liter~1, respectively). There were seven replicates of each treatment combination. Mean survival times and their 95% confidence intervals (C.I.) were calculated for each treatment combination. Furthermore, how starvation prior to cyanobacterial exposure affects egg viability in Daphnia is discussed. RESULTS AND DISCUSSION
FIG. 1. LC values (cells ml~1) for Daphnia clones exposed to toxic 50 cyanobacteria for 96 h. The upper 95% confidence limits also are provided.
by the predator Chaoborus sp. Figure 2 illustrates how clone ‘A’ grew well in the cyanobacterial exposure (about 100% growth compared with controls) but poorly in the Chaoborus exposure (85% growth). For clone ‘B,’ the opposite was true: there was no effect on growth in the Chaoborus
Clonal Differences in Responses of Daphnia to Stress The cyanobacteria were lethally toxic to all Daphnia clones in a 96 h exposure (Fig. 1). The sensitivity was, however, strongly dependent on clonal origin. The most sensitive clone was D. pulex ‘Lepikko,’ whereas the D. longispina clone ‘Linnunpa¨a¨’ was the most tolerant one. There was a fivefold difference in the LC values among these 50 clones (about 30,000 cells ml~1 and 160,000 cells ml~1, respectively; Fig. 1). If clones within a species are compared, there are overlapping C.I. values in all cases, whereas comparisons among species suggest that D. pulex is the most sensitive species, D. longispina the most tolerant one, and D. magna intermediate in sensitivity (Fig. 1). However, the D. magna clone ‘Seili’ was different from the D. longispina clones ‘Linnunpa¨a¨’ and ‘Eestiluoto’ but not from clone ‘Tuorla.’ Thus, it is important to consider within-species variability when comparisons among species are made. [See Baird et al. (1990) and Barber et al. (1990) for a thorough discussion on problems associated with clonal variation in toxicity tests.] In addition to differences in mortality, clones differ in how they allocate resources under stress. In a study by Ohra-Aho (1992), clones of D. pulex were exposed to toxic M. aeruginosa and to water containing a chemical released
FIG. 2. The growth of two Daphnia clones exposed to cyanobacteria (Cya) and to a chemical from Chaoborus (Ch). Growth was measured as the difference between the first and the ninth instar sizes, and it is illustrated here in relation to growth in the controls [modified from Ohra-Aho (1992)].
RESISTANCE OF ZOOPLANKTON TO STRESS
tems, it cannot be verified that a response pattern should be seen as an adaptation.
exposure, whereas growth was reduced to 85% in the cyanobacterial exposure. In the case of Chaoborus exposure, energy that is not used for growth can, for instance, be used for the production of antipredatory neck spines (Walls and Ketola, 1989). In cyanobacterial exposures, energy may be needed for detoxification. However, there are no data available to verify this possibility, and it is also possible that reduced growth in cyanobacterial exposures should be seen as toxic inhibition. Whereas it can be demonstrated that clones respond differently to stress factors, it is still difficult to determine whether these responses are indicative of different adaptations or are caused by random genetic variation. It is generally believed that life-history effects of Chaoborus on Daphnia are caused by a specific chemical called kairomone (Parejko and Dodson, 1990). Thus, responses to Chaoborus exposure can be discussed in relation to predator populations in the field, and the neck-spine area provides an idea of whether the responses are ecologically meaningful (OhraAho, 1992). Responses to cyanobacteria are, on the other hand, more difficult to interpret. Sivonen et al. (1990) estimated that a significant portion of cyanobacterial blooms in Finnish waters contain peptide toxins, such as microcystinLR. However, other toxins may be more important to zooplankton [e.g., Jungmann (1992)]. Because nothing is known about the occurrence of these toxins in the ecosys-
Cyanobacterial exposure clearly reduced the survival times of D. pulex (Fig. 3). However, the food level during the experiment affected the results: at a high concentration of alternative food, low to intermediate levels of cyanobacteria had virtually no effect on survival times. The effect of alternative food can probably be explained by differences in the relative concentrations of toxic food. This phenomenon, which has previously been demonstrated in short-term exposure by Reinikainen et al. (1994), has consequences for the planning of experiments that include toxic food particles. Furthermore, varying phytoplankton communities in natural waters probably alter the effect of cyanobacteria. The concentration of alternative food affects not only the relative concentration of toxic food, but also the physiological state of the suspension feeder. Reinikainen et al. (1995) exposed both well-fed and nearly starved animals to cyanobacteria. Figure 4 reveals that egg mortality was higher in starved animals. Consequently, the resistance of zooplankton to stress is affected by starvation: mortality of adults and their eggs increases at low concentrations of high-quality food.
FIG. 3. Mean survival times of Daphnia exposed to cyanobacteria at two levels of an alternative food. The vertical bars indicate 95% confidence intervals.
FIG. 4. Egg mortality in Daphnia exposed to cyanobacteria after pretreatments at a high and a low food level, respectively. The vertical bars indicate SE [modified from Reinikainen et al. (1995)].
The Interaction of Food Level and Cyanobacterial Exposure
REINIKAINEN, HIETALA, AND WALLS
Genetic, environmental, and physiological factors affect the stress resistance of zooplankton. Differences in stress resistance under varying conditions can be seen as the material upon which natural selection acts. Evolved adaptive strategies can be demonstrated for well-known stress factors, such as Chaoborus predation, whereas less is known about adaptations against other stress factors, such as cyanobacteria. Ecotoxicological studies should more often consider the possibilities that long-term stress acts selectively on individuals and that future populations may be adapted to exposure by certain toxicants or to other adverse conditions. ACKNOWLEDGMENTS This study was financially supported by the Academy of Finland (M.R., M.W.) and by the Maj and Tor Nessling Foundation.
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