Combined effect of the insecticide carbaryl and the Chaoborus kairomone on helmet development in Daphnia ambigua

The cladoceran Daphnia ambigua was exposed to both the insecticide carbaryl and the kairomone released from the predator Chaoborus simultaneously, and its morphological changes were analyzed. Daphnia developed helmets in response to the kairomone, but not in response to carbaryl at low (sublethal) concentrations (1–3 µg 1^–1). However, the carbaryl enhanced the development of high helmets and prolonged the maintenance period of the helmets over instars in the presence of the kairomone. These results suggest that sublethal concentrations of the insecticide alter predator-prey interactions by inducing helmet formation in Daphnia, which may reduce vulnerability of the Daphnia to predation.     

Complex effects of a kairomone of Chaoborus and an insecticide on Daphnia pulex

Daphnia pulex were reared in Chaoborus-conditioned water containingthe insecticide carbaryl, and their life history parametersand morphologies were investigated. The insecticide inhibitedthe animals' growth and reproduction and delayed their maturationtime more intensely in the chaoborus-conditioned water thanin the control Chaoborus-free water, indicating that a kairomoneof Chaoborus made the Daphnia more sensitive to the insecticide.The Chaoborus conditioned water induced neckteeth formationof D.pulex in instars 1–2 and elongated the intermoultingperiod of juveniles. The moulting to the spined morphs and elongationin duration of juvenile stages seemed to increase the risk ofdamage from the insecticide. The potential population growthrate of D.pulex in treatments was estimated as a possible fitnessindicator of the animals. It was reduced synergistically bythe kairomone of Chaoborus and the insecticide. Some individualskept neckteeth until the third or fourth instar stage when theywere exposed to sublethal concentrations of the insecticidein the Chaoborus-conditioned water. This was considered as aresult of synergistic effects of both the kairomone and theinsecticide. Insecticides may be a factor inducing further developmentof protuberant structures in cyclomorphic Daphnia in naturalwater bodies.     

Multiple predator defence strategies in Daphnia pulex and their relation to native habitat

Daphnia may respond with an array of anti-predator defences(behavioural, morphological and life history) to a chemicalcue (kairomone) exuded by its predators: fish and Chaoborus.Given the wide array of potential responses, it is an interestingquestion whether anti-predator defences are coupled or independentof each other. Since anti-predator responses are costly andeven possessing the genetic information to respond to a certainpredator might involve a cost, clones may only react to predatorsthey co-occur with in nature. In this study, we provide evidencefor an uncoupling of responses by Daphnia pulex in several anti-predatordefences against Chaoborus. We were unable to detect a correlationbetween behavioural (migration), morphological (neck-spine induction)and life history [growth rate, neonate size and size at firstreproduction (SFR)] responses. Furthermore, anti-predator responsesdid not always comply with what is commonly believed. We foundthat Daphnia clones can migrate up or down when exposed to fishor Chaoborus kairomone and that population growth rate, neonatesize and SFR can increase or decrease in response to Chaoboruskairomone. We also show patterns in anti-predator defences thatseem to relate to the habitat from which clones were derived.Daphnia clones that were collected in habitats with Chaoborusas the dominant predator tended to react strongly to Chaoboruskairomone by migrating upward and producing neck-spines. Themigration behaviour against fish kairomone in these clones wasoften an unexpected upward migration. The Daphnia clone thatco-existed with fish predators showed a downward migration inthe presence of fish as well as Chaoborus kairomone. Clonesthat had occurred with either both or no predators had mixedresponses. We sometimes found an upward migration in combinationwith smaller body size as a response to Chaoborus kairomone.This may be interpreted as a behavioural defence against Chaoborusand a life-history defence against fish. Daphnia seem not toexhibit defence behaviour against predators they do not co-occurwith. It might be costly for Daphnia to maintain genetic informationto respond to these predators and protect that information fromgenetic drift.     

Chaoborus and fish-mediated influences on Daphnia longispina population structure,dynamics and life history strategies

Summary This study examined the long term effects of predation by larvae of the midge Chaoborus and simulated fish predation on experimental Daphnia longispina populations. Chaoborus predation, relative to fish predation, led to populations composed of larger individuals as a whole, larger egg-bearing individuals, and a larger primiparous instar. Daphnia retained helmets beyond the first instar in response to the presence of Chaoborus. Both types of predation, relative to predator-free controls, reduced prey population size and rates of increase, but increased population death rates. The reduction in population size due to predation led to increased resource availability for individuals remaining in the populations and increased individual fecundity in the predation treatments. The differences noted between the Chaoborus, fish, and control treatments increased with predation intensity.     

Predator-induced phenotypic plasticity in Daphnia pulex: uncoupling morphological defenses and life history shifts

Chemical cues from a predator Chaoborus sp. induce morphological defense (neck spine) and life history shifts (later reproduction, decreased fecundity but larger juvenile size) in the waterflea Daphnia pulex. These shifts have been interpreted either as costs of defense or as separate adaptation. In order to investigate if the life history shifts can be separated from the morphological defense, Daphnia pulex individuals were exposed to chemical cues from Chaoborus at different stages of life for variable periods. The daphnids that were exposed to Chaoborus started their reproduction later than the controls, although the differences were not statistically significant. Neck spine was induced only if daphnids were exposed to Chaoborus in an early stage of their life. Numbers of eggs produced were not affected by the different treatments, but egg mortality was higher in mothers exposed to Chaoborus. With these treatments it was possible to see neck spine induction without measurable life history changes or costs. On the other hand, irrespective of neck spine presence, the Chaoborus chemical(s) had an effect on Daphnia pulex mothers.Publication no 2159. Netherlands Institute of Ecology, Centre for LimnologyPublication no 2159. Netherlands Institute of Ecology, Centre for Limnology     

Is mating of copepods associated with increased risk of predation?

In laboratory experiments, we tested whether the cyclopoid copepodCyclops vicinus is more vulnerable to predation by juvenile(young-of-the-year) roach (Rutilus rutilus) and/or larvae ofthe phantom midge Chaoborus flavicans during mating than individualadult copepods of the same species. We also tested whether C.vicinusavoided mating, displayed shorter mating times and/or loweractivity when a predator (Chaoborus) was present, or when exposedto water in which predators (Chaoborus) were previously held.Fish ate the same quantity of pairs in copulation and individualegg-bearing females but significantly fewer females withouteggs. Chaoborus ate similar quantities of pairs and femalesbut fewer males. Pairs in copulation and egg-bearing femaleswere detected earlier by fish than individual non-ovigerousfemales or males, and pairs in copulation were captured mosteasily by fish. Cyclops vicinus mated less often when Chaoboruslarvae were present than in water in which Chaoborus was previouslyheld, or in pure tap water. However, predator presence or exposureto predator-treated water had no effect on (i.e. did not reduce)mating duration. Activity of C.vicinus pairs during copulationwas significantly lower when a predator was present, or in predator-treatedwater, than in pure tap water, and activity of pairs decreasedwith increasing activity of the predator. Although our experimentswere carried out under artificial conditions which cannot betransferred to the complex conditions in the field, they suggestthat mating in copepods is dangerous and that copepods haveevolved particular strategies to reduce the risk of predationduring mating.     

Neck spine protects Daphnia pulex from predation by Chaoborus, but individuals with longer tail spine are at a greater risk

We tested the prey preference of Chaoborus sp. on Daphnia pulexwith different defensive morphologies. The protective functionof inducible morphological defences, such as neck spine andlonger tail spine, was evaluated Second instar D.pulex individualsof two clones differing in their strength of neck spine inductionwere offered as prey to both Chaoborus obscuripes and Chaoborusflavicans. We used logistic regression analysis to evaluatethe effect of morphometry on the vulnerability of Daphnia. Thepresence of a neck spine and increased total length protectedD.pulex from Chaoborus predation. However, individuals witha longer tail spine were more vulnerable to Chaoborus predationChaoborus obscuripes was able to eat daphnids with a neck spinedue to the larger gape size of this chaoborid. The smaller speciesC.flavicans almost always ate prey with no neck spine.     

Daphnia invasion: population dynamics of Daphnia assemblages in two eutrophic lakes with particular reference to the introduced alien Daphnia ambigua

The population dynamics of Daphnia species (D.cucullata, D.ambigua,D.galeata) in two small, productive lakes were studied and experimentswith field enclosures were carried out to elucidate the ecologicalniches of the introduced, alien species D.ambigua. Daphnia ambiguocoexisted temporally with the similarly sized D.cucullata, butlived in deeper water. Although both species had the same clutchsizes, similar proportions of ovigerous females, and death rates.D.cucullata was nurnerically dominant over D.ambigua. This suggeststhat the observed difference in abundance is simply a resultof the slower population growth rates of D.ambigua at lowertemperatures in greater depths. The experiments with field enclosuresshowed that when D.culcullata and D.ambigua were forced to livein the same layer of water, D.ambigua was numerically dominantover D.culullata both in the enclosures with and without fish.As the difference between the abundances of the two speciesdecreased in the enclosure with fish, D.cucullata seems to bebetter protected against fish predation than D.ambigua. In theenclosures without fish, D.ambigua and D.cucullata were numericallysurpassed by the larger species D.galeara. The present resultsimply that in some productive lakes with a high fish stock,D.mbigua invaded the niche of D.galeara and that the speciescan become an important component of the native zooplankton.The results also support the opinion that co-existing Daphniacan have very similar dynamics and niches. ¹ Present address: Zoologisches Institut, Christian-Albrechts-UniversitätKiel, Olshausenstrasse 40, D-24098 Kiel, FRG     

Sexuality in Diploid and Hexaploid Races of Mercurialis annua L

The variation in sex expression found between individuals ofMercurialis annua agg. is described, distinction being madebetween ‘regular’ and ‘irregular’ monoecism.There is shown to be a relationship between sexuality, morphology,and chromosome number. M. annua sensu stricta and M. annua var. ambigua are shown tobe cytologically distinct, the former being diploid, the latterhexaploid. It is recommended that M. annua var. ambigua be regardedas a species—M. ambigua—as originally proposed byLinnaeus fils. Past investigations into the inheritance of sex are reviewedand results of further investigations into the inheritance ofmonoecism in M. annua var. ambigua are presented. Evidence obtainedsupports the hypothesis that the allosome constitution of maleplants is XXXXXY and that of monoecious plants is XXXXXX.     

Clutch detachment in a copepod after capture by a predator

When the egg-carrying copepod, Eudiaptomus gracilis, is capturedby larvae of predatory Chaoborus species (C.obscuripes or C.flavicans)in laboratory experiments, the external egg clutch detachesin 16-29% of caw, thereby allowing the eggs to escape from ingestion.Clutch detachment may be due to active removal by the femalebefore she is eaten or the result of accidental loss by thepredator during handling. Detached clutches were significantlylarger than thase that were ingested along with the female.Clutch detachment is advantageous to the female since ingestedeggs are lost, whereas detached eggs hatch normally and maypotentially propagate the genotype. Chaoborus predation maytherefore influence the evolution of clutch size and the degreeof iteroparity in Eudiaptomus.     

Composition and seasonality of the zooplankton community of Lake Valencia, Venezuela

The composition and temporal abundance patterns of planktonicherbivores in Lake Valencia, Venezuela, were studied over a5-year period. The herbivore community was typically composedof only 9 or 10 species, of which four (Notodiapromus deeveyorus,Mesocyclops decipiens, Brachionus calyciflorus, Ceriodaphniacornuta) accounted for more than 90% of community biornass (67µg l^–1 mean dry mass). Annual average biomass variedby a factor of three over the 5-year study and this is comparableto interannual variation observed in lakes of the temperatezone. Lake Valencia has a pronounced seasonal cycle based onan annual mixing period and an annual stratification period.Herbivores were usually more abundant during the mixing seasonthan the stratification season. Proximate control of herbivoreabundance patterns could be ascribed largely to predation byChaoborus. Predation pressure was diminished during the circulationseason because the Chaoborus population was reduced by two mortalitymechanisms: catastrophic losses due to chemical changes at overturn,and increased predation by fish coincident with loss of thehypolimnetic refuge. Chaoborus fed on all herbivore speciesin Lake Valencia, but electivities for specific herbivores variedgreatly. The early developmental stages of both copepod speciesexperienced low predation pressure; Notodiaptomus nauphi escapedpredation entirely. Adult copepods, on the other hand, werehighly vulnerable to predation. A statistical analysis of temporalpatterns of abundance and production for the major herbivorespecies suggests that closely related species responded differentlyto growth control mechanisms; the two species with similar responses(M.decipiens and B.calyciflorus) also had coincident abundancepatterns. Despite statistical differences in responses to growthcontrol mechanisms, there was remarkable dietary similarityamong the species. There was no obvious dietary specializationand no statistical evidence of strong selectivity for or aversionto particular algal species. Zooplankton species compositionis very simple in Lake Valencia in contrast to the high speciesrichness of many other kinds of tropical communities. Herbivorecommunity abundance is highly seasonal in Lake Valencia; theamplitudes of abundance fluctuation exceed those of other largetropical lakes and are comparable to those in lakes of the temperatezone. Herbivore production in Lake Valencia, and in tropicallakes in general, is not necessarily higher than that in temperatelakes, but production per unit biomass probably is higher.     

Zooplankton community structure driven by vertebrate and invertebrate predators

Summary A zooplankton community was established in outdoor experimental ponds, into which a vertebrate predator (topmouth gudgeon: Pseudorasbora parva) and/or an invertebrate predator (phantom midge larva: Chaoborus flavicans) were introduced and their predation effects on the zooplankton community structure were evaluated. In the ponds which had Chaoborus but not fish, small- and medium-sized cladocerans and calanoid copepods were eliminated while rotifers became abundant. A large-sized cladoceran Daphnia longispina, whose juveniles had high helmets and long tailspines as anti-predator devices, escaped from Chaoborus predation and increased. In the ponds which had fish but not Chaoborus, the large-sized Daphnia was selectively predated by the fish while small-and medium-sized cladocerans and calanoid copepods predominated. In the ponds containing both Chaoborus and fish, the fish reduced the late instar larvae (III and IV) of Chaoborus but increased the early instar larvae (I and II). Small- and large-sized cladocerans were scarcely found. The former might have been eliminated by predation of the early instar larvae of Chaoborus, while the latter was probably predated by fish. Consequently, the medium-sized cladocerans, which may have succeeded in escaping from both types of predator, appeared abundantly. The results suggest that various combinations of vertebrate and invertebrate predators are able to drive various kinds of zooplankton community structure.     

Temporal pattern of feeding response of Chaoborus larvae to starvation

The effect of starvation on the feeding rate of larval Chaoborus(Diptera. Chaoboridae) was investigated using Daphnia roseaas prey. The starvation period varied from 12 h to 22 days.The starved Chaoborus were individually incubated with 10 Daphniaunder controlled light and temperature conditions. Observationswere made on prey mortality every 2 h for the first 12 h andonce after 24 h. Feeding rates gradually increased to a maximumbetween 7–11 days of starvation. After this period, feedingrates declined to previous low levels. Generally, feeding rateswere significantly higher during the first 2–4 h of feeding.Thereafter, feeding rates were lower and exhibited no consistentpattems with length of feeding time.     

Effects of predator-released chemicals on some life history parameters of Daphnia pulex

The effects of chemicals released by fish and Chaoborus larvae on some life history traits in Daphnia pulex were studied in an in situ enclosure experiment. The mean size of Daphnia individuals was smaller in the presence of fish-released cues. Also the minimal size of an egg bearing female in the presence of fish exudates was smaller than in the population exposed to the chemicals released by Chaoborus larvae as well as in the control population. Fish-released chemicals caused the increase in clutch size in Daphnia. There were no statistically significant differences between the studied life history parameters of the control and Chaoborus treatments. The results are discussed in reference to recent laboratory research.     

Predator-induced alterations in Daphnia morphology

The freshwater cladocerans Daphnia pulex and Daphnia schodleriprotect themselves from predation by morphological alterationsinduced in response to water-soluble chemicals released by theirrespective predators. Daphnia pulex is induced by larvae ofthe phantom midge, Chaoborus. Populations of D.pulex which areinduced are those most likely to have intense interaction withthe predator. This is true both on a broad geographic scaleas well as locally Cephalic expansion in D.schodleri is inducedby notonectids, in particular Buenoa sp. This predator preferslarger prey and consequently small instars of D.schodleri showno evidence of induction Both examples of predator-induced alterationssuggest that this type of response is costly to the prey andis manifested only in those individuals and populations mostthreatened     

Competition, predation and the relative abundances of two species of Daphnia

We investigated the factors controlling the relative abundancesof two Daphnia species, D.pulex and D.laevis, in a small Wisconsinpond. D.pulex was the dominant Daphnia species in fall 1977and summer-fall 1978; D.laevis was the only Daphnia speciespresent in summer 1979. The abundance of D.laevis was positivelycorrelated with the abundance of the notonectid, Buenoa confusa.In predation trials, notonectides exhibited a distinct preferencefor D.pulex over similarly-sized D.laevis, but Chaoborus larvaefed at similar rates on both Daphnia species. Behavioral observationsrevealed that Buenoa adults were much less efficient at capturingD.laevis than D.pulex. Quantitative results of these predationtrials were combined with estimates of predator and prey densityand distribution to evaluate the effect of predation on thedaphnid populations. The effect of predation varied throughtime and microhabitat, and only infrequently could predationaccount for total prey mortality. D.laevis was most abundantat times and in places where Buenoa predation was most intense.Competition experiments illustrated the competitive superiorityof D.pulex over D.laevis. D.pulex was able to competitivelyexclude D.laevis in long term experiments, and D.pulex's fecunditywas higher than that of D.laevis in shorter experiments. Inlong-term experiments, Chaoborus larvae at natural densitieswere able to keep both Daphnia species at low, constant levelsand neither species clearly dominated when Chaoborus was present.The relative abundances of D.pulex and D.laevis were controlledby a complex of biotic and abiotic factors. Pond depth and predatordensity determined the intensity of predation on daphnid populations.When notonectid predation was intense, D.laevis dominated; whenthe intensity of predation by notonectids was low, D.pulex dominateddue to its superior competitive abilities. At different timesselective predation or high resource levels promoted the co-existenceof these two species. ¹Current address of both authors: Department of Biological Sciences,University of California, Santa Barbara, CA 93106, USA     

Life-history responses to Chuoborus of spined and unspined Daphniu pulex

Water-borne chemicals released by the larvae of the predatoryphantom midge Chaoborus are known to induce morphological modificationsin its prey Daphnia pulex: these cladocerans develop neck spineswhich may carry several teeth. Some work has shown that thesemorphological variations enhance the prey's chances of escape.but since these neck teeth are not fixed defence reactions,they are thought to entail some form of cost, such as delayedmaturation and reduced fecundity. In this study. the relationshipbetween morphological and life-history changes in four clonesof Daphnia pulex reared in the presence and absence of Chaoborusflavicans was examined. Special emphasis was placed on the genotypiccomparison of the modifications. While all four clones showeda delay in maturation time in the presence of Chaoborus, theneck spine responses differed markedly among the genotypes:one clone never had any neck teeth, another always producedone single tooth, and two clones produced varying numbers ofteeth per spine (means 2.9 and 4. respectively). These resultsindicate that there is no general pattern of neck teeth productioncorresponding to delayed maturation. What there appears to beis genetic variability in two independent and possibly adaptiveresponses. However, the clone without neck teeth was the onlyone which showed no predator-induced reduction in fecundity.Another common morphological response to Chaoborus was thatjuveniles of all clones developed elongated tail spines.     

Zooplankton community structure in southwestern Quebec lakes: the rOles of acidity and predation

The zooplankton community structure of 22 lakes with varyingacidity and fish biomass, located southwest of the mining andsmelting region of Rouyn/Noranda, Quebec, was examined in July1987. Lakes with dominant piscivorous fish communities couldbe discriminated from non-piscivorous communities using solelypH and Chaoborus abundance, demonstrating that modificationsin the zooplankton community occurred concurrently with changesin the fUh communities. Strong discontinuities in zooplanktonspecies distributions were discerned in lakes with pH values<5.3; abundance ranges for several species could be semi-quantitativelyrelated to lake morpho-metry or chemistry. Small shallow kettlelakes with elevated heat budgets had high biomasses of smallherbivorous organisms. The presence of >5 mg m^–3 ofadvanced Chaoborus instars (III and IV) was associated withreduced microcrustacean biomass in many lakes, particularlythose with low fish biomass. There was little evidence for size-selectivepredation by fish in these oligotrophic lakes. Fish biomass/effortcould be semi-quantitatively related to the biomass of Leptodorakindtii and the ratio of adult Diaptomus nunutus to copepoditestage IV.     

The effect of ultraviolet radiation on the vertical distribution and mortality of estuarine zooplankton

The effect of UV-B radiation on the vertical distribution ofthree calanoid copepod species (Tortanus dextrilobatus, Acartiuraspp. and Acanthacartia spp.) and three larval stages of Pacificherring, Clupea pallasi (1-, 7-, and 14-day-old larvae) wasinvestigated. A series of 2 m high columnar tanks equipped withinfra-red light and video-microscopy was used to investigatethe vertical distribution of zooplankton in the presence andabsence of UV-B radiation. In the presence of UV-B radiation,T.dextrilobatus and 1-day-old C.pallasi resided about 50 cmdeeper than in the absence of UV-B, while Acartiura spp. andAcanthacartia spp. showed no (or only minimal) change in verticaldistribution. Mortality experiments were also conducted outdoorsin which each copepod species was exposed to full or reducednatural radiation levels. Only T.dextrilobatus showed an increasein mortality when exposed to full radiation. Our results showedthat T.dextrilobatus and 1-day-old C.pallasi larvae were sensitiveto UV radiation (UVR), and to reduce or eliminate UV-inducedstress, they avoided the surface of the water column when UV-Bradiation was present. Copepod species were chosen to span arange of pigmentation: T.dextrilobatus (heavily pigmented),Acartiura spp. (moderately pigmented) and Acanthacartia spp.(not pigmented). The pigmentation did not appear to play a rolein UVR tolerance of the copepods, but may be a factor determiningUV tolerance of C.pallasi.     

Morphological responses of four species of cyclomorphic Daphnia to a short-term exposure to the insecticide carbaryl

Four species of cyclomorphic Daphnia (D.pulex, D.galeata mendotae,D.retrocurva, D.lumholtzi) were exposed to the insecticide carbarylfor a short term (8–4 h) from the final embryonic stageto the first instar. Daphnia pulex formed neckteeth, and theremaining three Daphnia species developed high helmets and longtailspines. The results suggest that the development of suchprotuberant structures (anti-predator morphologies) in responseto the insecticide exposure is a general phenomenon in Daphnia,and that stimuli on the nervous system of Daphnia may inducethe morphological changes, which originally evolved as a responseto predator kairomone. Two clones of D.pulcx were examined anda clone which was more sensitive to the predator Chaobonts kairomonethan another developed more marked neckteeth in response tocarbaryl, suggesting that the sensitivity in morphological responseto the insecticide may be related to the sensitivity to thekairomone. ¹Present and permanent address: Regional Environment Division,National Institute for Environmental Studies Onogawa, Tsukuba,Ibaraki 305, Japan