Vulnerability of Daphnia middendorffiana to Parabroteas sarsi predation: the role of the tail spine

The predaceous calanoid copepod Parabroteas sarsi and Daphniamiddendorffiana co-exist in South Andes ponds. Daphnia middendorffianajuveniles have a tail spine with negative allometric development.A series ofexperiments was carried out with tailed and tail-removedjuveniles of three different instars. In all cases tested, feedingrates were significantly higher on tail-removed prey. Directobservations showed a higher frequency of unsuccessful attackson spined juveniles when compared with tail-removed juveniles.The proportion of dorsal attacks also increased in spined juveniles.Prey total length was a much better predictor of feeding ratethan prey body size. Three groups of juveniles with equal totallength, but differentage, body size and biomass, showed no significantdifferences in their vulnerability to predation.     

Assessment of the importance of fish predation versus copepod predation on life history traits of Daphnia hyalina

Daphnia hyalina is a cladoceran present throughthe whole year except for late summer in Maranhão,a meso-eutrophic reservoir in central Portugal. Apartfrom the influence of food, both vertebrate andinvertebrate predation pressures seem to have aneffect on D. hyalina population dynamics.Enclosure experiments were designed to assess therelative importance of both types of predation. Afterthe summer crash, D. hyalina reached highernumbers in the fishless enclosures than in the lakedespite of high predation pressure upon juveniles byAcanthocyclops robustus. Fish predation upon thelargest individuals, especially large egg bearingfemales, was responsible for the lower fertility ofthe open water population when compared with theenclosure population. In the enclosures an increase intail spine length was observed. The longer tail spineprobably offered protection from copepod predation,allowing at least some of the juveniles to coexistwith their potential predator and reach the adultstage, less susceptible to copepod predation.     

The costs of moulting in Daphnia; mineral regulation of carbon budgets

1. Zooplankton differ in their elemental contents. For the essential elements calcium (Ca) and phosphorus (P), Daphnia spp. have particularly high contents, and thus high demands, compared with other cladocera and copepods.
2. There are recent indications that the growth of zooplankton, notably Daphnia , may be directly constrained by the availability of P and Ca.
3. Major pools of P and Ca are associated with the carapace, and the moulting process represents a constant drain of these elements, as well as carbon (C), from the animals.
4. The demands for P and Ca for moulting may pose constraints on the carbon accumulation in the animals, and thus the extensive needs for these elements may translate into a total loss of C transported in the pelagic food web.
5. While the costs of moulting are substantial for the zooplankton, the export of minerals bound in the moult will play only a minor role for the epilimnetic element budgets in most lakes.
     

Response of a zooplankton community to the addition of unsaturated fatty acids: an enclosure study

1. The effect of the addition of emulsions with different fatty acid composition to a semi-natural zooplankton community was studied in enclosures.
2. The reactions of different taxa in the zooplankton community to the addition of the emulsions were different. The copepods showed almost no reaction, nor did the selective cladocerans ( Bosmina ) or rotifers ( Synchaeta or Polyarthra ). The non-selective filterfeeding cladocerans Daphnia and Ceriodaphnia , and the rotifer Keratella , showed responses to the addition of the emulsions.
3.  Keratella showed the highest density in the enclosures with high amounts of highly unsaturated fatty acids added, whereas both Daphnia and Ceriodaphnia reached the highest numbers in the enclosures where we added emulsions of saturated fatty acids only.
4. Our results suggest that different taxa may be limited by different factors, even though they use similar food sources. Hence, we conclude that it is very difficult to generalize on the limiting factors in aquatic systems.     

Effects of predation by the copepod Mesocyclops ogunnus on the sex ratios of mosquito Aedes albopictus

Cyclopoid copepods are important predators in many aquatic ecosystems and have been used as biological agents in successful programs to control mosquito larvae. However, the impacts of this predation on adult mosquito populations are still poorly understood. The present study compared the sex ratios and body sizes (measured as wing length) of Aedes albopictus mosquitoes emerging from recipients containing the copepod predator Mesocyclops ogunnus with control situations without this predator. We found that copepod predation significantly biased mosquito sex ratios toward females, and that both the males and females emerging from copepod-containing recipients were significantly larger than control insects. The ecological and epidemiological consequences of the changes induced by copepod predation on mosquito populations are discussed.     

Perch or plankton: top‐down control of Daphnia by yellow perch (Perca flavescens) or Bythotrephes cederstroemi in an inland lake?

1. Seasonal termination of the vernal clear-water phase in Long Lake, Grand Traverse Co., Michigan coincided with severe size-selective predation on juvenile Daphnia pulicaria from 0.8 to 1.8 mm in length. This could be caused by predation by age-0 yellow perch ( Perca flavescens ) or by the exotic predatory zooplankter Bythotrephes cederstroemi .
2. During the initial decline of Daphnia , Ivlev's electivity coefficient for yellow perch from 15.0 to 20.0 mm in length was 0.50 for copepods and −0.75 for D. pulicaria .
3. Bioenergetics modelling of both yellow perch and Bythotrephes demonstrates that, during the initial Daphnia decline, Bythotrephes consumed 1.5–5 times greater total mass than yellow perch. Furthermore, models in which Bythotrephes consumed juvenile Daphnia were more consistent with the timing of the Daphnia decline than those in which yellow perch consumed juvenile Daphnia .
4. The invasion of Bythotrephes into Long Lake seems to be a significant perturbation, introducing effects that propagate throughout the food chain. Bythotrephes created a possible bottleneck for age-0 yellow perch in late June by suppressing Daphnia .     

Availability,uptake and regeneration of phosphate in mesocosms with varied levels of P deficiency

A population of cyclomorphic Bosmina coregoni was studied in Lake Östersjön, southwestern Sweden and results from field samples collected in 1988, 1989, 1990 and 1991 are presented. Animals collected in summer have remarkably higher carapace and prolonged antennule compared to what we call the normal morph. In 1991 the extreme morph reach its maximum body length, body height and antennule length in July to September. The occurrence of the extreme morph coincide with the hatching of the predaceous cladoceran Leptodora kindtii.The two morphs fluctuate in abundance and in relation to each other. In early spring only the normal morph occurred in the samples followed by a period of about two months when the two morphs were found together, in July only the extreme morph was found. In September the two morphs were again present in the lake. As has been shown for other cladoceran, the conspicuous carapace and antennule could be an adaptive response that decreases mortality due to invertebrate predation. Spectacular features like these are likely also accompanied by some sort of costs.     

The effect of diapause emergence on the seasonal dynamics of a zooplankton assemblage

1. The role of seasonal phenology in the emergence of zooplankton from diapause in patterns of seasonal abundance in the water column was investigated in Oneida Lake, New York. Replicate emergence traps, placed in contact with the lake sediments at two locations (one at a shallow site and one at a deep site), were monitored between May and August.
2. Although six rotifer taxa showed a clear seasonal succession in the water column throughout the study period, all but one taxon emerged exclusively in spring. Three cladoceran and three calanoid copepod species, also present in the water column throughout the study period, again showed predominantly spring emergence. In contrast, three cyclopoid copepod species had distinct seasonal periods of emergence that corresponded, at least in part, to the timing of abundance peaks in the plankton.
3. These results for a single lake are largely consistent with patterns observed or inferred by other investigators for other lakes: variable dependence of abundance in the plankton on diapause emergence for species with long-lived diapausing eggs (i.e. rotifers, cladocerans and calanoid copepods), and much closer dependence for species with short-lived diapausing immature stages (i.e. cyclopoid copepods).     

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.     

Dynamics of the exotic Daphnia lumholtzii and native macro-zooplankton in a subtropical chain-of-lakes in Florida, U.S.A.

1. Seasonal dynamics of the exotic Daphnia lumholtzii and native macro-zooplankton species were studied for 2 years in six inter-connected lakes in Florida, U.S.A. The lakes ranged widely in pH, colour and trophic status, and were dominated by copepods.
2. All six lakes contained both D. lumholtzii and the native D. ambigua , but the two species did not overlap in time. D. ambigua was dominant in autumn–spring, coinciding with lower water temperature, higher transparency and lower nutrient and chlorophyll a (Chl a ) concentrations than in summer, when D. lumholtzii was dominant.
3. Based on the field observations, temperature optima were 24 °C for D. ambigua and 29 °C for D. lumholtzii , suggesting that temperature plays a role in determining dominance among the daphnids of subtropical Florida lakes.
4.  D. lumholtzii has not displaced native cladocerans but occupies a 'vacant' seasonal niche, unexploited due to the inability of native taxa to tolerate high temperature. Furthermore, D. lumholtzii did not significantly alter algal–zooplankton interactions. There was evidence of top–down control by grazing, but it was primarily attributable to the native taxa.     

A comparison of the effects of prey and non-prey neighbours on foraging rates of Epischura nevadensis (Copepoda: Calanoida)

SUMMARY. 1. Adults of the calanoid copepod, Epischura nevadensis , aggregate in situ near the thermocline in Lake Tahoe, California-Nevada. together with adults of another species of calanoid copepod, Diaptomus tyrelli and juveniles of both species. With a series of laboratory predation and algal clearance trials, we show that foraging rates of adult E. nevadensis are determined not only by the density of co-occurring potential prey (small copepods), but also by the presence of co-occurring non-prey neighbours (large, adult copepods). These effects occur at densities and in zooplankton assemblages found naturally, emphasizing the ecological importance of neighbours other than prey on zooplankton feeding.
2. Neighbours are distinguished primarily by size. Although predation rates increase linearly with the densities of small copepods. both algal clearance and predation rates decrease in the presence of large copepods. We also show, with a field predation experiment using small enclosures, that adults are size selective within species and that Diaptomus are selected over conspecifics of the same size.
3. We hypothesize that by reducing foraging rates in the presence of large zooplankton. E. nevadensis avoids predators and reduces predation risk at the cost of reduced energy consumption.     

Calanoid copepods from South Georgia,with special reference to size dimorphism within the genus Pseudoboeckella

Summary Three species of calanoid copepods, Boeckella michaelseni, Parabroteas sarsii and Pseudoboeckella poppei were recorded from 6 freshwater localities in the Husvik area, South Georgia. Of the latter species two distinct size morphs occurred, with no overlap in size even in closely situated populations. The large morph was recorded in lakes, the small was found in ponds. The small morph did not coexist with the large predatory P. sarsii, and we suggest predation pressure from this species as the major cause for the observed distribution of these morphs. The pronounced size segregation as well as small morphological dissimilarities suggest that these morphs are reproductively isolated. While the large morphotype corresponds to that of P. poppei, the taxonomic affinities of the small morph are uncertain.     

Predator-specific inducible defenses in the rotifer Keratella tropica

1. A Patagonian strain of Keratella tropica has very different induced morphological responses to two predators – the carnivorous rotifer Asplanchna brightwelli and the interference competitor Daphnia pulex. Asplanchna induces the most exuberant morph. Compared to the basic morph, it has a fourfold longer right posterolateral spine (up to c. 115 μm), greatly elongated anterolateral and anterosubmedian spines, and no left posterolateral spine. Transitional morphs have an incompletely developed right posterolateral spine and a reduced left posterolateral spine. Daphnia induces moderate development of both posterior spines but no elongation of any anterior spines. Induction of these morphs by Asplanchna and Daphnia is mediated by kairomones. 2. The Asplanchna‐induced morph is much better defended against large (0.9 mm) Asplanchna than either the basic or Daphnia‐induced morph. The long right posterolateral spine usually prevents capture or ingestion. The Asplanchna‐ and Daphnia‐induced morphs are similarly susceptible to interference from large (3 mm) D. pulex. 3. Life‐table experiments with cohorts of the basic and Asplanchna‐induced morphs at 5 × 10³ and 2 × 10⁴ cells of Cryptomoas erosa per millilitre indicate little or no cost of the induced defense. Lifetime fecundity (13–15 offspring per female) did not differ significantly between morphs. The mean intrinsic rate of natural increase (r_m day⁻¹) of the induced morph was very slightly but significantly lower than that of the basic morph at the lower food concentration (0.46 versus 0.48) but no different from it at the higher food concentration (0.53 versus 0.54). However, spine development may involve undetermined allocation costs and environmental costs relating to interactions with other organisms. 4. It is not clear why K. tropica has separate induced responses to Asplanchna and Daphnia. Moderate spine development probably reduces damage or ingestion by small (<1.5 mm) daphniids, as in other species of Keratella, but further development may confer no protection against larger ones. Thus, the ratio of benefit to cost with daphniids (and other cladocerans) may be highest for intermediate spine development. In contrast, much greater spine development seems necessary for effective defense against Asplanchna. The more moderate response to Daphnia also may reflect less likely spatial and temporal overlap.     

Differential visual predation on morphs of Timema cristinae (Phasmatodeae:Timemidae) and its consequences for host range

Timema cristinae is a herbivorous insect that exhibits polymorphism for body coloration (green, red and grey morphs) and for pattern (striped, expressed only in the green morph, and unstriped). The striped green morph is associated with ceanothus ( Ceanothus spinosus ) and the unstriped green morph is associated with chamise ( Adenostoma fasciculatum ). This study examines the relative vulnerabilities to predation of the different pattern and colour morphs on their natural backgrounds. The vulnerabilities of the striped and unstriped morphs on their two food plants were tested using uncaged wild birds (Scrub Jays) and captive western fence lizards. Strong differential predation was observed suggesting that each morph is most cryptic on the food plant on which it is most common. Furthermore, in a mark-recapture experiment in a patch of ceanothus the unstriped and red morphs were recaptured in higher proportion than the other morphs. The vulnerabilities of the grey and green morphs on the ground and foliage were tested using lizards. The grey morph was more vulnerable on the plants than the green morph, but the inverse was observed on the ground (where they drop after a disturbance). This may be why the grey morph is not associated with specific food plants. The striped and colour polymorphisms in T. cristinae appear to be an evolutionary consequence of differential predation on different backgrounds. The implications of differential predation to food-plant utilization are discussed.     

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

Field data from seven alpine lakes in Serra da Estrela. Portugal.show that reproduction in Daphnia may be as efficiently controlledby fish predation and copepod predation on eggs in brood cavitiesas it is by food limitation. Body length and clutch size estimatesin Daphnia pulicaria revealed high inter- and intra-populationvariability in maturation size (body size at first reproduction).and in number of eggs per clutch. Daphnia at first maturationin lakes stocked with rainbow trout were half the size of thosefound in fishless lakes (body length of 0.86–0.95 and1.55–1.81 mm. respectively). The mean number of eggs perclutch was reduced to a similar degree by food limitation, predationby fish and copepod predation on eggs in brood cavities, butthe 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 selectiveremoval of individuals with maximum clutches, so variation decreased.Copepod predation caused removal of eggs from brood cavitiesof randomly infested females, so that variation increased, particularlyat a high food level when non-infested females carried largeclutches of eggs.     

The effect of a seasonal time constraint on development time, body size, condition, and morph determination in the horned beetle Allomyrina dichotoma L. (Coleoptera: Scarabaeidae)

Abstract.  1. In horned beetles selection favours males that adjust their investment in horn development in relation to cues that predict adult body size. Here it is shown that in the Japanese horned beetle, Allomyrina dichotoma . There is a significant discontinuity in the horn length body size allometry. This can be described as a linear relationship that is shifted towards an increased horn length to body length ratio in males with horns longer than 16 mm.
2. Larval nutrition explains morph determination in A. dichotoma . However, unlike other species, variation in larval nutrition was the result of a seasonal time constraint that limits the time available for feeding prior to the onset of winter diapause.
3. Even when eggs were reared with an ad libitum food supply, minor morphs were still observed. Individuals that were oviposited later in the season had less time to feed, shorter development times, eclosed as smaller individuals and, in the case of males, were more likely to be hornless. Major morphs, minor morphs, and females all reduced their body size in response to seasonal time constraints in the same way. However, males that were laid later in the season had faster development times than females laid at the same time, but showed no reduction in their size relative to females, suggesting seasonal time constraints increase growth rates in males but not in females.
4. No evidence was found that seasonal time constraints resulted in a reduction of size-corrected fat reserves at eclosion, or that minor morphs gained any developmental advantage by reducing investment in horn length.     

Extreme cyclomorphosis in Daphnia lumholtzi

1. Daphnia lumholtzi, not previously reported in North America, was found in a small reservoir in East Texas in January, 1991, This species possesses extremely long spines and large fornices; an allometric study was performed to detect any temporal differences in specific growth rates of the spines relative to the body. 2. In nature, mature females attained 1.8mm body length, excluding spines, but when the head and tail spines are included, the total length reached a maximum of 5.6mm. 3. Differences in the growth patterns of the head spine and the tail spine relative to the body existed for D. lumholtzi from January to March 1991. Both the head and the tail spines grew at a faster rate than the body during all 3 months although the rates varied between them. The results contradict the invertebrate predation hypothesis (Dodson, 1974) in that D. lumholtzi's head and tail spines continue to grow during adulthood instead of stopping after the juvenile instars. 4. The head spines grew at a constant allometric rate over time while the tail spine grew faster as the temperature increased. Both varied significantly in length over the 3 months, with animals having the shortest spines in February and the longest in March.     

Reproductive success and risk of predation in normal and melanistic colour morphs of the adder, Vipera berus

In a population of adders (Vipera berus) in Southwest Sweden, melanistic males were heavier than normal coloured males of the same length. Victory in male-male sexual combats was positively related to size. Higher risk of predation in the black morph was inferred from experiments showing a high predator attack rate on models of the black morph. Even the bright colour in newly moulted basking males of the normal morph gives cryptic protection. In females, melanism probably also affects body size and risk of predation by visually searching predators. The thermoregulatory influence of black colour, the reproductive success and the maintenance of two colour morphs in the population are discussed.     

UV radiation affects antipredatory defense traits in Daphnia pulex

In aquatic environments, prey perceive predator threats by chemical cues called kairomones, which can induce changes in their morphology, life histories, and behavior. Predator‐induced defenses have allowed for prey, such as Daphnia pulex, to avert capture by common invertebrate predators, such as Chaoborus sp. larvae. However, the influence of additional stressors, such as ultraviolet radiation (UVR), on the Daphnia–Chaoborus interaction is not settled as UVR may for instance deactivate the kairomone. In laboratory experiments, we investigated the combined effect of kairomones and UVR at ecologically relevant levels on induced morphological defenses of two D. pulex clones. We found that kairomones were not deactivated by UVR exposure. Instead, UVR exposure suppressed induced morphological defense traits of D. pulex juveniles under predation threat by generally decreasing the number of neckteeth and especially by decreasing the size of the pedestal beneath the neckteeth. UVR exposure also decreased the body length, body width, and tail spine length of juveniles, likely additionally increasing the vulnerability to Chaoborus predation. Our results suggest potential detrimental effects on fitness and survival of D. pulex subject to UVR stress, with consequences on community composition and food web structure in clear and shallow water bodies.     

Comparative life history and parasitism of a new colour morph of the walnut aphid in California

1 The walnut aphid Chromaphis juglandicola is a yellow aphid. In 2003, however, a white colour morph was discovered in the Sacramento Valley of California. The colour dimorphism occurs between clone lines and, when white morphs are present, they occur in mixed colour morph colonies on the underside of walnut leaves. 2 Laboratory experiments were undertaken to evaluate the thermal requirements for development, adult longevity and progeny production of the two colour morphs. Host instar preference of Trioxys pallidus, a parasitoid responsible for the successful biological control of the walnut aphid in California, was examined separately for each colour morph, and host colour preference was investigated for the preferred instar. 3 No differences in thermal requirements for development, adult size or mean longevity were detected between yellow and white colour morphs. A small difference in early reproduction was detected: white colour morphs produced more progeny on each of the two first days of adult reproduction than yellow colour morphs. 4 Trioxys pallidus showed a slight preference for the fourth instar of the yellow morph over the second‐ and third‐, but equal preference for second, third and fourth instars of the white morph. When offered equal numbers of fourth instars of the two colour morphs, T. pallidus did not show any colour preference. 5 The differences in early aphid reproduction and host instar preference by T. pallidus were combined in a stage‐structured matrix model. Model analysis showed a greater potential for population growth of the white morph over the yellow morph, with early reproduction having a greater influence than host instar preference.