Coexisting Daphnia Species Research Articles

Formation of morphological defences in response to YOY perch and invertebrate predation in two Daphnia species coexisting in a mesotrophic lake

This study examined the formation of morphological defences by two coexisting Daphnia species, the large-sized D. pulicaria (2 mm) and the small-sized D. mendotae (1.4 mm), in response to the presence of young-of-the-year (YOY) yellow perch (Perca flavescens) and invertebrate predators (Chaoborus, Leptodora) during summer in a mesotrophic lake. We hypothesized that due to differential size-selective predation risk by YOY fish and invertebrates, the large-sized and the small-sized Daphnia species would show different morphological responses to predation threats. We followed changes in two morphological traits (relative length of the tail spine in D. pulicaria and of the helmet in D. mendotae) among different periods during summer according to YOY fish and invertebrate predation. We defined four YOY fish predation periods based on the presence of YOY perch in the pelagic zone of the lake and the relative abundance of Daphnia preys in their gut contents, and two invertebrate predation periods based on exclusive or mutual occurrence of the invertebrate predators. The large-sized (D. pulicaria) and the small-sized (D. mendotae) species showed different morphological responses to YOY fish and invertebrate predators, respectively. The tail spine ratio of the juveniles and adults of D. pulicaria did not change in response to YOY fish predation or to invertebrate predation. A gradual increase in the helmet ratio was observed in the small-sized D. mendotae over the summer period. This change was related to the co-occurrence of the invertebrate predators (Chaoborus and Leptodora) and to YOY fish predation. The warmer temperature cannot be accounted for helmet elongation since it was constant across depths, and not related with the co-occurrence of D. mendotae and YOY perch.

Alternative antipredator responses of two coexisting Daphnia species to negative size selection by YOY perch

Our study showed for the first time in nature that two coexisting Daphnia adopted alternative life history and behavioural strategies to cope with negative size-selection predation by gape-limited young-of-the-year (YOY) perch. We evaluated the phenotypic plasticity in life history and behavioural traits of two coexisting Daphnia species, D. pulicaria (2 mm) and D. galeata mendotae (1.4 mm), in response to seasonal changes in predation by YOY yellow perch (Perca flavescens) in a mesotrophic lake. We expected that the large-sized D. pulicaria, the most likely subjected to sizeselective predation by YOY perch, will show stronger antipredator responses than the small-sized D. galeata mendotae. To test this hypothesis, we examined changes in life history and behavioural traits in juveniles and adults of both species during four YOY fish predation periods that were selected based on the presence of YOY perch in the pelagic zone and the relative abundance of Daphnia prey in their gut contents. Our study supports the scenario of negative size-selective predation by gape-limited YOY perch on both Daphnia species. The electivity index indicated that no daphnids with a body length .1.75 mm were predated by YOY yellow perch. Coexisting Daphnia exhibited phenotypic plasticity in their antipredator defenses based on their vulnerability to seasonal changes in size-selective predation of YOY perch. Juvenile Daphnia were the targeted prey and they responded by a decreased body length. Behavioural defenses were the dominant strategy used by both adult Daphnia populations to withstand high predation. A decreased size at maturity was not employed by Daphnia, except at the very end of the predation period. Behavioural defenses are short-term strategy adopted to avoid predation. Both antipredator defenses became unnecessary expenses and were no longer sustained after the predation period.

A trophic cascade with Chaoborus: population dynamics of ex-ephippial generations of Daphnia

Changes in population dynamics and competitive outcome between Daphnia pulex and D. rosea in response to severe predation by Chaoborus larvae were investigated in a field experiment. In a nearly fishless lake with unusually high densities of Chaoborus flavicans and C. obscuripes, the two coexisting Daphnia species developed morphological defense structures to a different degree. An enclosure-exclosure setup was applied to investigate the population dynamics of both species in presence and absence of the predators. The field study followed the population growth of freshly hatched juveniles of D. pulex and D. rosea as they naturally emerged from ephippia in the lake's sediment during spring. Both Daphnia species have a large body size and, in enclosures with Chaoborus only, developed neckteeth and elongated tail spines as inducible defenses. Nevertheless, Chaoborus accounted for high mortality rates in ephippial neonates and strongly reduced the chance of individuals to reach maturity, especially in D. rosea, the dominant species in enclosures without Chaoborus. It is concluded that, in the first generation, when otherwise large and morphologically defended Daphnia are synchronized by ephippial origin to a population of only juveniles, a trophic cascade with Chaoborus may become highly effective.

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history altera- tions of coexisting Daphnia species responding to environmental temperature and predator cues. In a laboratory experiment, we measured Daphnia life history plasticity under different predation risk and temperature treatments that simulate changing envi- ronmental conditions. Daphnia pulicaria abundance and size at first reproduction (SFR) declined, while ephippia (resting egg) formation increased at high temperatures. Daphnia mendotae abundance and clutch size increased with predation risk at high temperatures, but produced few ephippia. Thus, each species exhibited phenotypic plasticity, but responded in sharply different ways to the same environmental cues. In Glen Elder reservoir, Kansas USA, D. pulicaria dominance shifted to D. mendotae dominance as temperature and predation risk in- creased from March to June in both 1999 and 2000. Field estimates of life history shifts mirrored the laboratory experiment results, suggesting that similar phenotypic responses to seasonal cues contribute to seasonal Daphnia population trends. These results illustrate species-specific differences in life history plasticity among coexisting zooplankton taxa.

On the different nature of top‐down and bottom‐up effects in pelagic food webs

SUMMARY 1. Each individual planktonic plant or animal is exposed to the hazards of starvation and risk of predation, and each planktonic population is under the control of resource limitation from the bottom up (growth and reproduction) and by predation from the top down (mortality). While the bottom‐up and top‐down impacts are traditionally conceived as compatible with each other, field population‐density data on two coexisting Daphnia species suggest that the nature of the two impacts is different. Rates of change, such as the rate of individual body growth, rate of reproduction, and each species' population growth rate, are controlled from the bottom up. State variables, such as biomass, individual body size and population density, are controlled from the top down and are fixed at a specific level regardless of the rate at which they are produced.2. According to the theory of functional responses, carnivorous and herbivorous predators react to prey density rather than to the rate at which prey are produced or reproduced. The predator's feeding rate (and thus the magnitude of its effect on prey density) should hence be regarded as a functional response to increasing resource concentration.3. The disparity between the bottom‐up and top‐down effects is also apparent in individual decision making, where a choice must be made between accepting the hazards of hunger and the risks of predation (lost calories versus loss of life).4. As long as top‐down forces are effective, the disparity with bottom‐up effects seems evident. In the absence of predation, however, all efforts of an individual become subordinate to the competition for resources. Biomass becomes limited from the bottom up as soon as the density of a superior competitor has increased to the carrying capacity of a given habitat. Such a shift in the importance of bottom‐up control can be seen in zooplankton in habitats from which fish have been excluded.

Morphological defenses induced in situ by the invertebrate predator Chaoborus: comparison of responses between Daphnia pulex and D. rosea.

The presence of plankton predators may induce altered morphology in their potential prey. To date, the mechanism of induction and adaptive value of such defensive responses have been examined in the laboratory. This study investigated the morphological defense structures induced by the invertebrate predator Chaoborus in two coexisting Daphnia species, D. pulex and D. rosea, in the field. In Piscivore Lake (Gräfenhain, Germany), continuous and intense biomanipulation had led to near elimination of planktivorous fish and greatly increased abundances of Chaoborus (up to >10 larvae l-1). Here, the density of Chaoborus was manipulated within the lake by an enclosure/exclosure setup and resulting morphological responses of Daphnia spp. were investigated in situ. Three replicate enclosures (4.6 m3) contained no Chaoborus (predator exclusion bags), whereas Chaoborus entered three others at ambient densities (predator enclosures). In both species of Daphnia, formation of neckteeth and elongation of the tail spine were recorded in the predator enclosures, but not in the predator exclusion treatments. Additionally, D. rosea responded to predator inclusion with an increase of the size at first reproduction. Despite the induced defense structures, the presence of Chaoborus caused increased mortality of both Daphnia species. In addition, Chaoborus affected the coexistence of the two populations of Daphnia by causing higher relative mortality in D. rosea. Neckteeth formation was always more pronounced in D. pulex than in D. rosea of the same size. Neckteeth were induced specifically in vulnerably sized juvenile instars of D. pulex, but were not found in all vulnerable instars of D. rosea. In D. rosea, neckteeth were few or absent in the ephippial hatchlings, and neckteeth formation ceased before juveniles reached a body size outside the range that larger larval stages of Chaoborus could ingest. This study provides the first experimental demonstration in the field of the inducibility of morphological defense structures in Daphnia at ambient densities of Chaoborus larvae, and quantifies these in situ responses. This expands on earlier observations of a correlation between predator density in the field and the expression of neckteeth in Daphnia. The term "maximum size for neckteeth formation" (MSNF) is defined as the limit in body size above which no production of neckteeth was evident. This limit was used to distinguish the size classes of Daphnia that show a sensitive response to Chaoborus kairomone. This new term may be used for further comparisons among species and among different types of predator-induced responses as well as for the evaluation of the adaptive value of defense structures.

Daphnia populations in three interconnected lakes with roach as the principal planktivore

Data on two co-existing Daphnia species, D.cucullata (G.O.Sars) and D.hyalina (Leydig), from three neighboring and interconnected lakes in northern Poland, revealed an extremely confined range of population density fluctuations throughout summer in each of the two species in each of the three lakes, with D.cucullata being an order of magnitude more abundant than D.hyalina (mean of 11.5 ind. l -1 for D.cucullata and 1.1 ind. l -1 for D.hyalina with 95% confidence limits of ±2.5 and ±0.5 ind. l -1 , respectively), in spite of markedly changing fecundity. There was no apparent phase of decline or increase that could be related to the distinct phases of low or high fecundity resulting from different food levels. Analysis of the gut contents of roach (Rutilus rutilus), the dominant planktivore in the three lakes, revealed identical selectivity for each of the two Daphnia species, suggesting that the lower density of the D.hyalina population was compensated for by the greater conspicuousness of individuals of this species. It is concluded that the population density of these Daphnia species remains far below the carrying capacity of the habitat and does not depend on food levels. Food avail- ability merely sets the rate of population increase, while the actual population density in the lakes studied reflects the species' vulnerability to predation by planktivorous fish.

Adaptation to oxygen deficiency: Contrasting patterns of haemoglobin synthesis in two coexisting Daphnia species

Low oxygen concentrations (<3 mg O 2 l −1) in the surrounding water body usually cause a marked decline of respiration and filtering rates in Daphnia (Crustacea, Cladocera). Typically during summer, when low-oxygen conditions develop in the metalimnion, the abundance of Daphnia in thermally stratified eutrophic lakes is restricted to the high-oxygen environment of the epilimnion. At the same time, food resources are depleted in the epilimnion and accumulate in the metalimnion due to sedimentation of phytoplankton and detritus. This study investigated the spatial distribution and possible physiological adaptation of Daphnia spp. in a eutrophic lake at Gräfenhain (Germany). The described conditions resulted in an almost complete vertical separation of the two coexisting Daphnia species ( D. pulex and D. rosea). D. pulex amounted to <10% of all daphnids in the epilimnion and to >96% in the low-oxygen layer around the metalimnion. This pattern was consistent, independent of time of day. Both Daphnia spp. were able to produce haemoglobin (Hb). D. pulex intensely enhanced the Hb concentration during individual growth up to 125 mg Hb g −1 dry weight, which is 5-fold the concentration found in the neonates. In contrast, D. rosea did not enhance Hb concentration during ontogenesis. Birth rates in the metalimnetic D. pulex were high in comparison to those of the epilimnetic D. rosea. The ability to produce Hb appears to allow D. pulex to colonize low-oxygen layers and to exploit food resources therein.

Contrasting patterns of body size for Daphnia species that segregate by habitat.

We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.

Elemental composition of two coexisting Daphnia species during the seasonal course of population development in Lake Constance.

In 1986 and 1987 the seasonal course of population density, life history traits and biomass composition (CHN-elemental and ash) of two coexisting Daphnia species from L. Constance (Überlinger See) were studied. The two daphnids are morphologically similar, but demonstrate different seasonal variation of behaviour (i.e. diurnal vertical migration). With the exception of nitrogen content, the mean biomass composition of adults and that of eggs were significantly different: the dry biomass of eggs (adults) contained on average 55.6 (46.1) % C, 7.9 (6.5) % H, 9.7 (9.7) % N, and 10.0 (23.8) % ash. These values were similar to those of other daphnids; it may be concluded that the average daphnid composition is not species specific, but is determined mainly by the position in the life cycle and by the nutritional state of the animal. A distinct seasonal variation in the elemental composition of the biomass of adult daphnids was observed. It related significantly to the concentration of food, as indicated by the epilimnetic chlorophyll-a concentrations and by Secchi depth. From the amplitude of seasonal variation of elemental composition and from the shape of relationships between the elemental composition of daphnids and their food, it can be concluded that in summer the non-migrating D. galeata lives under stronger food limitation than the migrating H. hyalina. In spring, there were no differences in the biomass composition of either species. In summer, the adult fcmales of D. galeata contained significantly less carbon, and hydrogen than those of D. hyalina. In most parameters of biomass (i.e. biomass composition of adults) and of life history (i.e. clutch size, mean adult weight), D. galeata displayed higher coefficients of variation. A tendency in D. galeata to have peaks of population development carlier in the season and in D. hyalina to have them towards autumn was observed. These contrasting responses of the populations development, the seasonal variability of the biomass composition seem to support the view of Geller (1986) who called D. galeata and D. hyalina "exploitative" and "conservative" strategists respectively.

Instar-specific mortalities of coexisting Daphnia species in relation to food and invertebrate predation

Instar-specific mortalities of coexisting Daphnia species in relation to food and invertebrate predation

Instar-dependent mortality rates of coexisting Daphnia species in Lake Vechten, The Netherlands

Instar-dependent mortality rates of coexisting <i>Daphnia</i> species in Lake Vechten, The Netherlands

Periodicity of Egg Laying and Egg Age Distributions in Planktonic Cladocera

In natural populations of planktonic Cladocera, I observed significant diel, molt cycle, and cohort periodicity of egg laying. Diel cycles of egg laying were pronounced in one population of Daphnia galeata mendotae; most females laid eggs only at dawn. There was a strongly negative correlation between diel egg laying cycles of coexisting Daphnia species. Egg laying in two Daphnia populations cycled with a period approximating the egg development times, suggesting a synchronization of individual molt cycles. Noncontiguous reproduction was characteristic of a population of Holopedium gibberum. Adults reproduced synchronously and then died; there was little overlap of generations.