Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of food and predation

Journal of Plankton Research Vol.18 no.6 pp.863-880.19% Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of f...
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Journal of Plankton Research Vol.18 no.6 pp.863-880.19%

Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of food and predation Z.Maciej Gliwicz and Maria Jos6 Boavida1 Department ofHydrobiology, University of Warsaw, Nowy Swiat 67, Warsaw, Poland and'Departamento de Zoologia e Antropologia, Centro de Biologia Ambiental, Universidade da Lisboa, Campo Grande 1700, Lisboa, Portugal Abstract. Field data from seven alpine lakes in Serra da Estrela. Portugal, show that reproduction in Daphnia may be as efficiently controlled by fish predation and copepod predation on eggs in brood cavities as it is by food limitation. Body length and clutch size estimates in Daphnia pulicaria revealed high inter- and intra-population variability in maturation size (body size at first reproduction), and in number of eggs per clutch. Daphnia at first maturation in lakes stocked with rainbow trout were half the size of those found in fishless lakes (body length of 0.86-0.95 and 1.55-1.81 mm. respectively). The mean number of eggs per clutch was reduced to a similar degree by food limitation, predation by fish and copepod predation on eggs in brood cavities, but the underlying mechanisms of this reduction were different. Food limitation caused smaller clutch sizes in all individuals, so variation remained the same. Fish predation caused the selective removal of individuals with maximum clutches, so variation decreased. Copepod predation caused removal of eggs from brood cavities of randomly infested females, so that variation increased, particularly at a high food level when non-infested females carried large clutches of eggs.

Introduction

The intra-specific and intra-clonal variability in clutch size in Daphnia species is tremendous. The number of eggs per clutch increases linearly with body size, beginning with the first clutch, which usually consists of one or two eggs (Lynch, 1980a,b). There is a maximum egg number per clutch at each adult body size, since clutch size responds to an increase in food level in a saturation-type fashion (Lampert, 1978). Therefore, it is generally assumed that clutch size variability within field Daphnia populations should be explained by body size and food levels (Taylor, 1985; DeMott, 1989). The variability in clutch size in Daphnia of a given body size has also been explained by various levels of vertebrate and invertebrate predation. Planktivorous fish selectively remove more conspicuous females that carry larger clutches, so that mean clutch size becomes reduced in a Daphnia population that is exposed to fish predation; this has only been suggested for Daphnia (Gliwicz, 1981), but confirmed for planktonic copepods (Dawidowicz and Gliwicz, 1983; Vuorinnen et al., 1983). Juvenile copepods enter Daphnia brood cavities to feed on eggs and embryos, so that the number of eggs per clutch becomes reduced in infested females (Gliwicz and Stibor, 1993). Another source of variability in Daphnia clutch size may be associated with the phenotypic plasticity in size at maturation, which is also related to the intensity of vertebrate and invertebrate predation. In the presence offish that selectively feed on large prey, early reproduction in small females should be favored, while invertebrate predators preferring small prey should favor large body size at first repro© Oxford University Press

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Z.M.GIiwicz and M J.BoavkJa

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Fig. 1. Effects of food limitation and predation on size at first reproduction and number of eggs per clutch in Daphnia (top), and hypothetical differences in the mechanisms of clutch size reduction due to food limitation, fish predation and invertebrate predation on eggs in brood cavities (bottom). Solid lines: mean clutch size on body length; dotted lines: upper and lower limits of clutch size reflecting equal coefficient of variation for different body lengths. Although each of the three factors leads to a decrease in the mean number of eggs per clutch in every class of body size, the nature of the mean's decrease seems to be different: food limitation should result in a symmetrical shift of all clutches in the population (left); fish predation should result in a decrease of the clutch's upper limit due to selective removal of females with a greater number of eggs (middle); invertebrate predation should result in a decrease of the clutch lower limit due to clutch size reduction in a fraction of randomly infested females (right). Fish predation would also result in a shortening of the regression line as an effect of a decrease in the maximum body length due to selective removal of the largest individuals from the population.

duction (Lynch, 1980a,b). The change in size at first maturation stems from the plasticity of life histories in each individual in the population. The change in clutch size within a given body size stems from a shift induced in the frequency distribution of females with different numbers of eggs per clutch. Hypothetically, each of the three factors—food limitation, fish predation and invertebrate predation on eggs in brood cavities—should produce a similar decrease in the mean clutch size of Daphnia of a given body size within a Daphnia population, although the mechanisms differ (Figure 1). At high, unlimiting food levels and in the absence of predation, clutch size increases to its maximum values for each consecutive body size, its variability being relatively low within the entire range of body sizes (Gliwicz and Lampert, 1994). It can be assumed (Figure 1, bottom) that a decrease in the mean clutch size for a given body size should result from a decrease in the upper and lower limits of the number of eggs per clutch when food level is reduced, therefore the variability should remain the same (left). Variability should be affected, however, when the mean clutch size decrease stems from fish predation (variation decreased due to selective removal of females with a greater number of eggs; middle) or invertebrate predation on eggs in brood cavities (variation increased due to clutch size reduction only in a fraction of randomly infested females; right). 864

Food and predation in Daphnla reproduction

Experimental studies have shown that shifts in clutch sizes and shifts in the size atfirstreproduction can be induced either as a change in the clonal composition of the population by changing levels of predation (Arts and Sprules, 1988; Ebert, 1991; Tessier and Consolatti, 1991; Glazier, 1992; Tessier et al, 1992) or as phenotypic change by chemical cues on the intensity of predation (Machdcek, 1991; Stibor, 1992; Weider and Pijanowska, 1993). An attempt to prove that they occur in field populations has been less successful because the levels of predation in natural habitats are not easy to evaluate and usually their lake-to-lake as well as their seasonal changes have to be subjectively assumed (e.g. Lampert, 1993). Moreover, in locations with high predation intensity by planktivorous fish, large Daphnia species are absent and replaced by small-bodied cladocerans, thus making comparisons between different habitats impossible. A unique situation is that in Grosser Binnensee, a coastal lake in northern Germany. Large Daphnia magna have been found to survive in this habitat under variable fish predation due to a rapid phenotypic shift in size atfirstreproduction, which decreases at the time of an assumed increase in fish predation (Lampert, 1993). Such a unique situation has also been found in alpine lakes of Serra da Estrela, Portugal, where large Daphnia pulicaria survivefishstocking and can be found in lakes with all four combinations of predation regime and food limitation: high or low food at high or low fish predation. We examined D.pulicaria from seven Estrela lakes to see whether the effects of fish predation and invertebrate predation on clutch size and the size at first reproduction were consistent with the predictions from Figure 1. Method

The Estrela lakes vary in their trophic status and predation level, but their zooplankton communities are similar to each other (Table I) as well as to zooplankton in other alpine lakes of Europe and North America (Gliwicz and Rowan, 1984; Gliwicz, 1985). As in mostfishlesslakes of the Tatra, the Alps and Colorado Rockies, the diversity of zooplankton communities in the Estrela fishless lakes is extremely low. Each community is composed of a single large cladoceran, D.pulicaria, one overwhelmingly dominant cyclopoid copepod (Tropocyclopsprasinus in Estrela), and less abundant rotifers of two genera, Keratella and Polyarthra. As in other alpine lakes, in most Estrela lakes that are stocked by rainbow trout (Salmo gairdneri), D.pulicaria becomes exterminated and is replaced by more evasive or smaller cladocerans (Diaphanosoma brachyurum and Chydorus sphaericus in Estrela, Bosmina elsewhere) and rotifers (Gliwicz, 1985). Unlike in other alpine lakes stocked with fish, however, in three stocked lakes of Estrela D.pulicaria persists in abundant populations despite high densities of rainbow trout into whose diet it is included as a marginal food (we found only 15 Daphnia specimens in the gut of an adult, 22 cm total length, accidentally angled from Escura, that was filled mostly with airborne insects; we assume, however, that younger trout rely more on zooplankton). Daphnia pulicaria was found in only seven of the 14 Estrela lakes that we sampled on four occasions: 10,16 and 26 September, and 4 October, 1993. Zooplankton samples were taken as vertical hauls (three replicate samples that, 865

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Chlorophyll a (jig 1 ') Algal densities (101 cells m|-')J Predation level Fish density as total number of Salmo gairdntri ha ' Stocked in 1992 Stocked in 1993 Copepod density (number of T.prasimis copepodites per one Daphnia)' Densities of dominant zooplankters (ind. 1 ')• Daphnia pulicaria Diaphanosoma brachyurum Chydorus sphaeriais Tropocyclops prasinus* Keratella cochlearis Polyarthra vulgaris

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