to maternal size and clutch number, and analysed effects of treatments on residuals from the models. We also measured variation in per offspring investment.
Journal of Plankton Research Vol.21 no.8 pp.1553–1564, 1999
Reproductive allocation in Daphnia exposed to toxic cyanobacteria Marko Reinikainen1,2,3, Jaana Hietala2 and Mari Walls2 1University of Helsinki, Tvärminne Zoological Station, FIN-10900 Hanko and 2University of Turku, Department of Biology, Section of Ecology, FIN-20014 Turku, Finland 3To
whom correspondence should be addressed at: Umeå University, Department of Ecology and Environmental Science & Umeå Marine Sciences Centre, S-901 87 Umeå, Sweden
Abstract. We investigated experimentally how resources were allocated to reproduction in Daphnia pulex and Daphnia longispina when varying levels of toxic Microcystis were added to higher quality food. We used multiple regression models to estimate mean offspring size and clutch size in relation to maternal size and clutch number, and analysed effects of treatments on residuals from the models. We also measured variation in per offspring investment. At a high cyanobacterial level, D.pulex was virtually unable to reproduce. At a lower level, D.pulex produced small clutches. However, the regression model residuals indicated that the presence of cyanobacteria increased the portion of available resources allocated to reproduction. The observed allocation may be a means to maximize reproduction under diminished longevity. Effects on mean offspring size were marginal in D.pulex, but variation in per offspring investment sometimes decreased in cyanobacterial exposures. Daphnia longispina was affected by a higher cyanobacterial level only, where offspring size was reduced. Deviations from the regression model indicated that effects on maternal size alone do not explain this effect. Clutch size residuals and per offspring investment were unaffected by treatments in D.longispina. The observed responses differ from theoretical models on reproductive allocation under food limitation.
Introduction The performance and competitive ability of an individual are affected by the way it allocates resources to reproduction in different environments. It is often assumed that there is a fixed amount of resources available for reproduction at a given time of an individual’s life history (Smith and Fretwell, 1974; Ebert, 1994) and that increased reproductive investment takes place at the cost of somatic investment [Williams, 1966; see Tuomi et al. (1983) for alternative allocation systems]. When these assumptions are true, offspring size, offspring number and maternal size are interrelated parameters that vary within limits set by morphological and other constraints (e.g. Glazier, 1992). The relative importance of these life history parameters depends on the given environmental conditions. Variation in food quantity is the most studied environmental parameter in papers concerning reproductive allocation in Daphnia. Small offspring size has, for instance, been shown to be associated with low starvation resistance (e.g. Threlkeld, 1976; Tessier et al., 1983; Goulden et al., 1987). Producing large clutches and consequently smaller offspring should, therefore, be advantageous at high food levels only. According to Smith and Fretwell (1974), it should furthermore be important to invest resources evenly, so that all offspring are close to the size that is optimal for a given environment. © Oxford University Press
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It is important to note, however, that reproductive allocation is not affected by food level only; effects of food quality are equally important (Brett, 1993). Cyanobacteria are generally regarded as a poor-quality food source, and several studies have shown that exposure to toxic strains results in markedly reduced clutch sizes in Daphnia and other zooplankton (e.g. Porter and Orcutt, 1980; Fulton and Paerl, 1987; Hanazato and Yasuno, 1987; Gilbert, 1990). Effects of toxic cyanobacteria on offspring quality, however, have not been studied previously. In natural waters, food quality fluctuates as markedly as does food quantity (Sommer et al., 1986). In particular, the often rapid occurrence of cyanobacteria as a part of the phytoplankton community is believed to have a negative effect on large cladocerans (e.g. Edmondson and Litt, 1982; Infante and Abella, 1985; Jarvis et al., 1987). According to Sivonen et al. (1990), ~50% of dense populations of cyanobacteria are toxic, containing neurotoxic alkaloids and/or hepatotoxic peptides. Both of these groups of toxins have been shown to affect Daphnia in laboratory studies (DeMott et al., 1991; Haney et al., 1995). In this study, we report how a peptide toxin-producing strain of the cyanobacterium Microcystis aeruginosa affects reproductive allocation in Daphnia. Our study includes several improvements over earlier papers concerning life history effects of toxic cyanobacteria on Daphnia: (i) we studied both offspring size and offspring number; (ii) we considered the possibility that maternal size and reproductive instar affect reproductive allocation (see Glazier, 1992); (iii) we used five genotypes (clones) of Daphnia, originating from two different species (Daphnia pulex and Daphnia longispina). The study of different genotypes is important because interclonal variation in responses is a requisite for selection (Glazier, 1992; Boersma, 1995). Furthermore, there is large variation in the susceptibility of different Daphnia clones to toxic substances (Baird et al., 1990). Therefore, conclusions derived from studies of a single clone may be of limited value. Method Individuals from three clones of D.pulex (P1, P2 and P3) and two clones of D.longispina (L1 and L2), kept as stock cultures in our laboratory, were used in the experiment. These clones have been isolated from small, fishless ponds near Turku in SW Finland. All clones derive from separate ponds. The phytoplankton composition and other environmental characteristics of the pond habitats are often unstable (C.Laurén-Määttä, unpublished data), and we will not attempt to link our results to the history of the Daphnia clones. Experimental animals were obtained from the second or third clutch of mothers that had been born into the experimental conditions [see Hietala et al. (1995) for details]. Filtered (Whatman GF/C) lake water from Lake Littoistenjärvi (SW Finland) was used as Daphnia medium in all cultures. The green alga Scenedesmus obtusiusculus was used as food in the Daphnia cultures, as well as during the experiment. The cyanobacterium used was M.aeruginosa (strain PCC7820). This cyanobacterial strain produces at least one variant of cyclic peptide toxins called microcystins. One or more unknown toxins, 1554
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that are probably more toxic to Daphnia than microcystins, may also be present (Reinikainen et al., 1994). The cyanobacterial treatments consisted of a mixture of 20 000 cells ml–1 of S.obtusiusculus (0.260 mg C l–1) and 10 000 or 40 000 cells ml–1 of cyanobacteria (0.076 and 0.304 mg C l–1, respectively). Unexposed control animals were fed Scenedesmus only. All animals were reared individually in 20 ml vessels, at 20°C, and on a 16 h:8 h light:dark cycle. They were transferred daily into fresh media. Ten replicate animals of each clone were used per cyanobacterial treatment and in the controls. At the beginning of the experiment, the animals were
