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

Summary 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.Authorship order alphabetical     

Seasonal dynamics and interspecific competition in Oneida Lake Daphnia

I investigated the population dynamics and competitive interactions of two species of the suspension-feeding crustacean Daphnia in Oneida Lake, N.Y. Both species have persisted in the lake for decades, but their water-column densities are negatively correlated. The larger Daphnia pulicaria dominates in some years, the smaller D. galeata mendotae in others, and in some years one species replaces the other seasonally. Although this pattern results in part from annual variation in vertebrate predation pressure, predation alone cannot explain the irregular daphnid dynamics. In 1992–1995, I examined the water-column abundances, birth and death rates of both species. D. pulicaria dominated in two years, D. galeata mendotae was replaced by D. pulicaria in one year and in 1994, both species persisted in low numbers. To test the effect of temporal changes on the strength of intra- and interspecific competition on both juvenile and adult daphnids, I manipulated a series of field enclosures in 1994 and 1995. The outcome of competition varied within and between years, and its effects were most evident at the highest densities and lowest resource levels. For adults of both species, the effects of interspecific competition were detected more often than those of intraspecific competition. Lipid reserves (a metric of fitness) among juveniles were generally low, with those of D. galeata mendotae often being less than those of D. pulicaria. Contrary to the results of other studies examining competition in daphnids, spatial segregation and predictable within-year reversals in competitive dominance most likely do not play a large role in fostering coexistence of the Oneida Lake daphnids. Instead, coexistence of these competitors is promoted by interannual variation and long-lived diapausing eggs. Received: 20 July 1997 / Accepted: 21 November 1997     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Larval fish-induced phenotypic plasticity of coexisting Daphnia: an enclosure experiment

1. The indirect effects of predators on lower trophic levels have been studied without much attention to phenotypically plastic traits of key food web components. Phenotypic plasticity among species creates phenotypic diversity over a changing environmental landscape. 2. We measured the indirect effects of planktivorous larval walleye (Stizostedion vitreum) on phytoplankton biomass through their effects on the dominant herbivore species, Daphnia pulicaria and D. mendotae. 3. Fish had no effect on phytoplankton biomass or overall Daphnia density. We observed a compensatory response to predation by functionally comparable species within a trophic level in the form of shifting dominance and coexistence of Daphnia species. We hypothesized that this phenotypically plastic response to predation decoupled a potential trophic cascade in this freshwater pelagic system. Daphnia pulicaria density decreased over time with fish predation, but D. mendotae density increased over time with fish predation. 4. Phenotypically plastic life history trait shifts and reproductive rates differed between species in fishless and fish enclosures, accounting for population trends. Daphnia pulicaria were also proportionally higher in walleye larvae stomachs than in the enclosures, indicating that walleye preferred to feed on D. pulcaria over D. mendotae. The resultant shift in dominance may partially explain the overall benign effect of fish on grazers and supports the hypothesis that trophic level diversity can decouple a trophic cascade.     

Temperature and kairomone induced life history plasticity in coexisting Daphnia

We investigated the life history alterations 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 environmental 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 increased 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.     

Influence of two Daphnia species on summer phytoplankton assemblages from eutrophic lakes

We conducted grazing experiments to test whether larger-bodiedDaphnia pulicaria have a different effect from smaller-bodiedDaphnia galeata mendotae on the composition of summer algalassemblages in eutrophic lakes. Three separate cubitainer experimentswere run for 5 days in a replicated factorial design utilizingtwo algal community types and the two Daphnia species. Inorganicphosphorus and nitrogen were added to prevent nutrient limitationof the algae. Both edible and inedible size fractions of chlorophylla increased in cubitainers without Daphnia spp. Grazer additionusually resulted in a reduction in edible chlorophyll; reductionswere greater in D.pulicaria cubitainers. Grazing by Daphniaspp. on presumed inedible chlorophyll was variable. Algal sizewas not always a good predictor of grazeability. The resultsof this study indicate that D.pulicaria, because of its greaterfiltration potential and ability to ingest larger particles,provides a stronger control on inedible-sized algae when comparedto equal numerical densities of D.g.mendotae. However, Aphanizomenonincreased as a response to heavy grazing pressure by D.pulicariaon other algal species. This suggests that biomanipulation effortsthat promote large-bodied Daphnia may not produce desirableresults if nutrient inputs remain high.     

Formation of morphological defences in response to YOY perch and invertebrate predation in two <Emphasis Type="Italic">Daphnia</Emphasis> 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. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Characterizing a zooplankton neighbourhood: small-scale patterns of association and abundance

• 1 This study compares small-scale (i.e. neighbourhood) associations among distributions of phytoplankton, zooplankton, and zooplankton grazing rates using a combination of correlations and analyses of spatial patchiness.
• 2 All zooplankton and phytoplankton populations had patchy distributions. On average, individuals experienced members of their same taxon at a density that was 42% greater than the mean sample density.
• 3 The various zooplankton taxa experienced different average neighbourhoods, even within the same stratum of the lake. For example, Daphnia galeata mendotae (Birge) experienced phytoplankton concentrations that were nearly 50% greater than those experienced by Daphnia pulicaria (Forbes) at the same depth.
• 4 The distributions of the various phytoplankton taxa were positively correlated with each other, but the distributions of zooplankton were negatively correlated, or more often, unrelated to each other.
• 5 Phytoplankton abundance was negatively correlated with zooplankton grazing rates.
• 6 We hypothesize that while phytoplankton distributions are driven by common external factors, the factors that determine horizontal distributions of zooplankton are species specific. In addition, zooplankton grazing appears at least partially responsible for the generation of patchiness in the distribution of phytoplankton.

     

The effect of suspended sediments on Lake Texoma Daphnia: field distributions and in situ incubations

1. We measured the abundance and eggs per female of four Daphnia species in turbid and relatively clear regions of Lake Texoma (Oklahoma‐Texas, U.S.A.) on 12 dates over the course of 5 years. 2. Two species, Daphnia lumholtzi and Daphnia parvula, occurred and reproduced in turbid locations, but two other species, Daphnia mendotae and Daphnia pulicaria, occurred almost exclusively in relatively clear conditions. 3. To test the hypothesis that interference with foraging excludes clear‐water Daphnia species from turbid locations, we incubated adult D. mendotae at both a clear and a turbid site. In three successive experiments D. mendotae individuals incubated at the turbid site carried as many or more eggs than individuals incubated at the clear site.     

Habitat segregation and interactive effects of multiple predators on a prey assemblage

1. In a field experiment we examined the interactive effects of two common predators of zooplankton, bluegill sunfish (Lepomis macrochirus) and Chaoborus spp. on the growth rate and habitat use of three congeneric prey species (Daphnia). Bluegill and Chaoborus both consume Daphnia, but bluegill also prey on Chaoborus. The prey species, Daphnia pulicaria, D. rosea and D. retrocurva, differed in body size and vertical distribution. We expected the largest species, D. pulicaria, to be most vulnerable to fish predation and the smallest species, D. retrocurva, to be most vulnerable to Chaoborus predation. 2. As we expected, the population growth rate of D. pulicaria was significantly reduced by fish. However, Chaoborus also significantly reduced the growth rate of this species. No significant interaction effect was detected, indicating that the effect of these predators was additive. The growth rates of D. rosea and D. retrocurva were significantly reduced by Chaoborus, but a significant interaction effect indicated that the effect of Chaoborus was stronger in the absence of fish than when fish were present. Therefore the impact of Chaoborus and fish on D. rosea and D. retrocurva was non-additive. The interactive effect of the two predators on D. retrocurva was greater in magnitude than on D. rosea. 3. In the absence of predators, the three Daphnia species showed no differences in mean habitat depth between day and night. Both predators significantly affected diel habitat use of D. pulicaria and D. rosea. Fish caused both of these Daphnia species to move deeper during the day, whereas Chaoborus caused Daphnia to move into shallower water at night. Daphnia retrocurva tended to migrate upwards at night in all predator treatments, but no significant differences in migration were observed among the predator treatments. The effects of predators on habitat use were not interactive for any prey species. 4. Our results suggest that body size, habitat use and the diel migratory response to predators are important factors mediating the interactive effects of multiple predator types on zooplankton.     

Influence of different sized Daphnia species on chlorophyll concentration and summer phytoplankton community structure in eutrophic Wisconsin lakes

We conducted field observations on 13 eutrophic Wisconsin lakesdominated by either the larger bodied Daphnia pulicaria or thesmaller bodied Daphnia galeata mendotae. While Daphnia numericaldensities were not significantly different between groups oflakes, pulicaria lakes had much higher Daphnia biomasses andfiltration potentials than galeata lakes. Although we foundsignificant differences in chlorophyll (Chl) a between bothgroups of lakes during June, on a seasonal basis populationsof different sized Daphnia were not associated with significantdifferences in Chl a. Filtration potential per se was the majordeterminant of Chl a, regardless of which Daphnia species dominated.However, in pulicaria lakes, the clear-water phase started earlier,lasted longer, and was usually characterized by greater Secchidisc readings than in galeata lakes. For large blue-green algaesuch as Aphanizomenon, D.pulicaria appeared to delay bloom conditions,but ultimately did not prevent the alga from growing. Our resultssuggest that high densities of large-sized Daphnia are a desirablegoal of biomanipulation because they can attain filtration potentialshigh enough to increase summer water clarity in eutrophic lakes.     

Testing the ecological relevance of Daphnia species designations

1. Molecular approaches have increasingly revealed hidden genetic structure within ecologically important species, leading to the creation of sibling species whose ecological relevance is often unclear. A prime example is Daphnia galeata mendotae, which was split into D. dentifera and D. mendotae based on differences at two allozyme loci. 2. In a set of lake populations in Michigan USA, we test the geographical and temporal consistency of the genetic structure underlying this species split. We also test the morphological relevance of this molecular variation and its ecological significance in lakes. In essence, we ask: does recognition of these new species provide valuable information for plankton ecologists? 3. We found that D. dentifera and D. mendotae represent morphologically and ecologically distinct forms that are distributed among lakes in non‐random fashion, which were remarkably stable over 6 years. Key differences between the species concern their body and head shape, vertical habitat use within lakes and distribution among lakes of different size. We hypothesise that these differences represent specialisation to habitats that differ in risk of invertebrate predation. 4. Reproductive barriers alone are insufficient to explain the pattern of genetic structure; in some lakes complete introgression is apparent. However, parent species and hybrids exhibit a stable co‐existence in many lakes, which suggests that ecological specialisation reinforces divergence within this taxon.     

Phylogenetics and Evolution of theDaphnia longispinaGroup (Crustacea) Based on 12S rDNA Sequence and Allozyme Variation

Although members of the crustacean genusDaphniahave been the target of much research, there is little understanding of the group's evolutionary history. We addressed this gap by inferring a phylogeny for one of the major species groups (longispina) using nucleotide sequence variation of a 525-bp segment of the mitochondrial 12S rDNA and allozyme variation at 21 loci. We identified the major lineages and their relationships, assessed the phylogenetic utility of the few morphological characters in the group, and examinedDaphniaphylogeography. Nuclear and mtDNA phylogenies were generally concordant in recognizing the same four species complexes. An exception was the position ofDaphnia galeata mendotae.The allozyme tree paired this species with theDaphnia rosealineage, whereas the mtDNA trees groupedD. g. mendotaewithDaphnia galeata galeata.This discordance was consistent with the reticulate evolution of nuclear genes supporting the hypothesis thatD. g. mendotaerepresents a case of homoploid hybrid speciation. Striking morphological stasis in thelongispinagroup was evidenced by its very limited morphological divergence over an estimated 100 MY, and by the unusual transitional saturation of the conservative 12S rRNA gene within a species group. Phylogenetic inference also provided evidence that similarities in cephalic crest shape likely resulted from convergent or parallel evolution among species. Endemism at the continental level was indicated for previously cosmopolitan species, but the estimated times of these divisions were inconsistent with vicariance events suggesting recent dispersal among continents. A significant role for divergent selection in new habitats during speciation was suggested by the neighboringly sympatric distributions of four sister species pairs over broad geographic areas.     

New microsatellite primers for Daphnia galeata mendotae

In order to study genetic changes in populations of Daphnia galeata mendotae, I characterized seven polymorphic microsatellite loci. Primers to amplify these loci were tested on individual eggs from the resting egg bank of Onondaga Lake, NY. Levels of polymorphism and cross‐amplification in D. g. galeata indicate that they will be useful markers for ecological genetic studies on both adults and diapausing eggs of these species.     

Predator-induced life-history changes and the coexistence of five taxa in a Daphnia species complex

Interspecific hybridization is common in water fleas of the Daphnia galeata species complex (e.g. D. galeata , D. cucullata , D. hyalina and their interspecific hybrids). We studied the effect of fish on the life histories of five taxa of this species complex originating from the Plußsee, northern Germany. Using four clones per taxon, we found that fish kairomones reduce size at birth and size at maturity significantly. For size at maturity larger taxa showed a significantly stronger reaction to fish kairomones than the smaller taxa. With respect to the intrinsic rate of increase, r , we compared two predation regimes (positive size selective and not selective). We found that under the fish predation regime most clones had a higher r when cultured with fish kairomones, leading to a higher r , and a stronger reaction for the smaller taxa. We conclude that fish predation might be an important factor influencing the co-occurrence of Daphnia parental taxa with their hybrids.     

Contrasting migration behaviour of Daphnia pulicaria and D. galeata x hyalina, in avoidance of predation by 0+perch (Perca fluviatilis)

In spring and early summer, a small population of the large-bodied Daphnia pulicaria coexists with a much larger population of the medium-sized hybrid Daphnia galeata × hyalina in the epilimnion of Lake Maarsseveen (The Netherlands). When large shoals of juvenile perch (Perca fluviatilis) appear in the open water, both species start to migrate vertically. Since D. pulicaria has a larger body-size than D. galeata × hyalina, and is therefore competitive dominant over the hybrid, it is unlikely that both species interact via their common food resource, but they react both to fish predation. However, since they differ in size, and therefore in vulnerability for fish predation, both species adopt different strategies. The smaller bodied, and less vulnerable D. galeata × hyalina exhibits diel vertical migration ascending to the surface at dusk, and staying there during the night. In this way, benefiting from the higher temperatures of the surface layers. In contrast, the large-bodied, and more vulnerable D. pulicaria selects the deep cold hypolimnion water layers as refuge against fish predation. In this way it benefits from a safe habitat, free from fish predators, but on the other hand suffers from low water temperatures, which decrease its fitness. It is likely that the relatively higher temperature in the upper water layers serves as a proximate factor for the downward migration of D. pulicaria.     

Impact of fish predation on coexisting Daphnia taxa: a partial test of the temporal hybrid superiority hypothesis

Fish predation was tested as a factor mediating the coexistence of Daphnia taxa in the shallow, hypertrophic Lake Blankaart. Naturally co-occurring populations of D. galeata and the hybrid D. galeata x cucullata were subjected to different levels of fish predation in in situ enclosures. In control enclosures without fish, the largest taxon D. galeata rapidly became dominant over the intermediate sized D. galeata x cucullata, mainly as a result of higher birth rates. In enclosures with fish, population densities of D. galeata dropped relative to D. galeata x cucullata, due to higher mortality rates. These results are in concordance with the `temporal hybrid superiority hypothesis', and can be explained by a higher vulnerability of the large and more conspicuous D. galeata to the size selective predation exerted by visually hunting planktivorous fishes. After approximately one month, however, population growth rates of D. galeata and D. galeata x cucullata in the enclosures with fish converged, due to a relative reduction in the mortality rate of D. galeata. This suggests that, in the presence of fish, D. galeata may co-exist with hybrids due to a decrease in its relative vulnerability to visual predation with time. Indeed, both D. galeata and the hybrid showed strong reductions in adult body size in the enclosures with fish, but this size reduction tended to be stronger in D. galeata than in D. galeata x cucullata. In addition, turbidity increased in the enclosures with fish and may additionally have reduced the relative advantage of D. galeata x cucullata with regard to mortality caused by visual predation.     

Density and size distribution of Daphnia populations at different fish predation levels

Density and size structure changes of natural daphniids populations were studied in enclosures with a different level of fish predation. Daphnia pulicaria was totaly removed in high predation variants, and the differences of the mean body length both in adults and juveniles are apparent between low predation enclosure and enclosure without fish. Daphnia galeata was replaced by D. magna and D. pulicaria in the enclosure without fish. The decline of densities and the substantial (30–50%) and fast (during 12 days) shift of mean body length both in adults and juveniles of D. galeata was induced by the fish (carp fry) introduction to the high predation enclosures.     

Phenotypic Plasticity in Life-history Traits and Feeding Appendages in Two Species of Daphnia Fed a Natural Phytoplankton Assemblage

The ability of two cladoceran species – Daphnia pulicaria and D. mendotae – to utilize a natural phytoplankton assemblage dominated by cyanobacteria was investigated experimentally. Reference animals were fed high quality green algae. The natural phytoplankton assemblage originated from Oneida Lake (NY, USA). Both cladoceran species occur in the lake, but their densities vary temporally. As expected, lake phytoplankton were a poor food source. Fecundity decreased, and age at first reproduction and offspring size increased. The magnitude of these effects varied between and within species. The increase in offspring size differs from studies using toxic cyanobacterial strains, but agrees with expectations on adaptive responses to low food availability. Both species performed equally on the lake phytoplankton, but D. pulicaria appeared to be the superior species in the high quality food treatment.     

Selective predation on an introduced zooplankter ( Bythotrephes cederstroemi ) by lake herring ( Coregonus artedii ) in Harp Lake,Ontario

1. Bythotrephes cederstroemi (Crustacea: Onychopoda: Cercopagidae) invaded Harp Lake, Ontario in 1993, since when the zooplankton community has shifted from dominance of small-bodied to large-bodied species. During 1995 diets of adult lake herring (Coregonus artedii), Harp Lake’s primary planktivorous fish, were examined to determine the extent to which this conspicuous zooplankter has become integrated into the lake’s foodweb and whether fish condition has been affected in consequence. 2. Bythotrephes and Daphnia galeata mendotae were strongly preferred prey, whilst Holopedium gibberum and calanoid and cyclopoid copepods were negatively selected by lake herring. Predation on Bythotrephes and Holopedium was not size-selective, although D. galeata mendotae and calanoid copepods (Leptodiaptomus minutus and L. sicilis) consumed by herring were significantly larger than co-occurring conspecifics in the lake. 3. Caudal spines of Bythotrephes may form boluses in lake herring stomachs. However, the number of caudal spines in fish digestive tracts did not differ significantly from the number of Bythotrephes’ mandible pairs, indicating that the former were not differentially retained. 4. Lake herring weight-at-length relationships in lakes in Muskoka, Ontario, invaded by Bythotrephes did not differ from those in adjacent non-invaded lakes, indicating that Bythotrephes invasion of lakes apparently has not affected condition of lake herring.