Effects of <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis> (Crustacea,Cladocera) on the abundance,morphology, and prey community of <Emphasis Type="Italic">Leptodora kindtii</Emphasis> (Crustacea,Cladocera)

We hypothesized that native Leptodora kindtii would be shorter and have smaller feeding baskets in central Ontario lakes with greater abundances of small-bodied zooplankton prey, and that differences in zooplankton size among lakes could be attributed to the invasive cladoceran Bythotrephes longimanus. We evaluated these conjectures by comparing size metrics of Leptodora and the size of their preferred cladoceran prey in lakes invaded or not by Bythotrephes. Leptodora was less abundant in invaded lakes, but were smaller bodied with smaller feeding baskets only in lakes with long invasion histories. Small cladoceran abundance was greater in non-invaded lakes and was directly related to Leptodora abundance although not to Leptodora size. Mean Leptodora body size declined with increasing abundance of Bythotrephes. We evaluated three possible explanations for these patterns in Leptodora—(a) competition with Bythotrephes for zooplankton prey, (b) direct predation by Bythotrephes, and (c) size-selective predation by fish. While we were unable to unequivocally distinguish among these hypotheses, our observations are most consistent with predation by Bythotrephes changing zooplankton community composition and size structure in a manner that is detrimental to Leptodora. Our results indicate that Bythotrephes invasion may trigger more complex and subtle changes in food webs than previously thought.     

Investigations on fecundity of Bythotrephes longimanus in Lake Lucerne (Switzerland) and on Niche Segregation of Leptodora kindti and Bythotrephes longimanus in Swiss lakes

In Lake Lucerne, Switzerland, the predaceous cladocerans Leptodora kindti and Bythotrephes longimanus segregate along spatial and temporal dimensions. In spring (April–May/June), Bythotrephes longimanus occurs below 0–20 m, while Leptodora is absent. In summer and early autumn (July–September/October), when Leptodora dominates during daytime in the 0–20 m depth, Bythotrephes longimanus also lives in deeper zones. Food competition and fish predation pressure may be the cause of differences in ecology of Leptodora and Bythotrephes acquired during evolution. Due to its transparency and tolerance of higher temperature, Leptodora could avoid fish predation and, therefore, competes with Bythotrephes longimanus successfully. In addition, the differences between the two species may account for the spatial and temporal niche segregation in oligotrophic Swiss Lakes. But spatial niche segregation is less important in mesotrophic lakes with high prey density than in oligotrophic lakes with low prey density. In small, eutrophic lakes importance of temporal niche segregation also decreases, and Bythotrephes is seldom or not present. The preference of Bythotrephes to live in deeper water to avoid fish predation during summer may be the cause of its difficulties to establish itself in small and eutrophic lakes with high prey densities, where the hypolimnion is missing or anoxic.In the spring, Bythotrephes exhibits r-strategy (smaller body size and a higher fecundity), the female is already fertile after the first molt. In the summer, a K-strategy prevails (larger body length and lower fecundity than in the spring), and female Bythotrephes are fertile only after the second molt. Shortage of prey (biomass of Bosmina and Daphniadecreased after June especially in the surface layers) and the maximum fish predation pressure in summer may change the life strategy of Bythotrephes: while fecundity decreases from generation to generation, body length increases. Enhanced prey densities (e.g. during mesotrophic conditions in L. Lucerne) lead to larger individuals in summer and autumn.     

Shifting invertebrate zooplanktivores: watershed-level replacement of the native <Emphasis Type="Italic">Leptodora</Emphasis> by the non-indigenous <Emphasis Type="Italic">Bythotrephes</Emphasis> in Canadian Shield lakes

The abundance of the native, pelagic macroinvertebrate predator, Leptodora kindtii, is negatively correlated with the abundance of a new invasive competitor, Bythotrephes longimanus, in a small number of Canadian Shield lakes. However, we do not yet know if Bythotrephes is replacing Leptodora on a regional scale. We determined the distribution of both species in 166 lakes in the District of Muskoka, south-central Ontario, Canada—the watershed with the longest history and largest prevalence of Bythotrephes invasions in North America. The frequency of occurrence of Leptodora was substantially reduced (twofold) in the presence of Bythotrephes. We argue that Bythotrephes is responsible for this dramatic reduction in the frequency of occurrence of Leptodora. Lakes in which both species co-occurred could not be distinguished from invaded lakes without Leptodora, suggesting a pattern of species replacement at a watershed level. We believe this is the first account of the widespread replacement of a native, pelagic macroinvertebrate predator by Bythotrephes in North America, and it does not bode well for Leptodora given the rapid, ongoing spread of Bythotrephes.     

Foraging behavior of the predaceous cladoceran, Leptodora kindti, and escape responses of their prey

Using silhouette video photography we have made the first quantitativeobservations of foraging behavior in Leptodora kindti, a predaceouscladoceran (Haplopoda). Leptodora swims with a mean velocityof 13.4?4.0 mm s^–1 and initiates an attack only upon directcontact with potential prey. The attack sequence is as follows:Leptodora swims randomly through the water column with all fivepairs of thorac appendages spread to form a ‘feeding basket’and, seemingly by chance, encounters prey. Shortly after preymake contact with any part of Leptodora's body (usually ventral),the abdomen is rapidly pulled forward, clamping itself underthe feeding basket so that the telson closes it at the posteriorend. The duration of this movement is always the same and weconclude that it is an indiscriminate reflex. If the prey isencountered anywhere but a short distance directly in frontand slightly below the Leptodora, it is not captured. The speedof copepod escape responses effectively allows them to avoidcontact with the predator. Daphnia's escape response, particularlythat of juveniles, is slower and leaves them far more susceptibleto Leptodora predation.     

The ecological niches of <Emphasis Type="Italic">Bythotrephes</Emphasis> and <Emphasis Type="Italic">Leptodora:</Emphasis> lessons for predicting long-term effects of invasion

We here exploit two large datasets on zooplankton in Norwegian lakes, spanning a wide range of geographical, physical, chemical and biological properties, to assess the ecological niches and habitats of Bythotrephes longimanus and Leptodora kindtii. The species overlapped geographically, yet co-occurred only in a limited number of lakes. Bythotrephes inhabited virtually all types of lakes, except alpine localities and productive lakes dominated by cyprinid communities where the hyaline Leptodora was most abundant. The zooplankton communities also differed in Bythotrephes and Leptodora lakes, probably both reflecting different predatory regimes, but also water quality and other lake-specific properties. We found no evidence for species being excluded by the presence of Bythotrephes, rather the diversity in general was higher in lakes with these predators present compared with those without. We found, however, a very close association between Bythotrephes and Daphnia galeata and to some extent also between Bythotrephes and D. longispina, suggesting that these species also may benefit from Bythotrephes invasion. Both Bythotrephes and Leptodora species occur naturally in this region, and knowledge about the ecological preferences and the zooplankton community composition in Bythotrephes—and Leptodora lakes will provide valuable information about the long-term effects of Bythotrephes invasion and potential interaction with of Leptodora as top invertebrate predator.     

Food limitation impacts life history of the predatory cladoceran Bythotrephes longimanus, an invader to North America

North American invasions of the predatory cladoceran Bythotrephes longimanus have resulted in declines in native zooplankton abundance, species richness, and diversity. In the field, population maxima of Bythotrephes are positively correlated to those of their zooplankton prey. To test the hypothesis that increased prey availability enhances Bythotrephes fitness, we reared Bythotrephes in the laboratory on three mixed-species prey densities (equivalent to 15, 30, and 45 prey organisms day^?1; designated “low,” “medium,” and “high” food treatments, respectively) over 22 days at 21°C. Bythotrephes consumed the daily equivalent of 9, 14, and 22 prey organisms at the low, medium, and high food densities. Smaller, slower prey were most often selected. Indeed, with increasing prey density, Bythotrephes’ predation rates increased, resulting in significantly higher population growth rates, net reproductive rates, growth, and first brood clutch and offspring sizes; significantly faster generation times; and shorter maximum life spans. We propose that the positive relationship between Bythotrephes population maxima and prey seen in the field is largely due to increased predation rates by Bythotrephes when prey abundance is high and the fitness benefits that ensue. Our findings may be useful for Bythotrephes risk and impact assessments.     

Increased abundance of the non-indigenous zooplanktivore, <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis>, is strongly correlated with greater spring prey availability in Canadian Shield lakes

The non-indigenous zooplanktivore, Bythotrephes longimanus, is a large Palaearctic cladoceran that is spreading rapidly in the Great Lakes watershed in North America. As a voracious predator, Bythotrephes can reduce herbivorous cladoceran abundance and diversity; however, the variables that affect its abundance are not well understood. To determine what bottom-up factors are associated with the abundance and seasonal dynamics of established Bythotrephes populations, two Bythotrephes datasets from lakes in south-central Ontario, Canada, were analysed using multiple regression and multivariate analyses: a multi-lake dataset of nine lakes sampled in 2003 and a multi-year dataset of one of these lakes, Harp Lake, sampled from 1994–1998 and 2001–2004. Bottom-up variables tested were Secchi disk depth, epilimnetic temperature, cladoceran (prey) density, total phosphorus, dissolved organic carbon and Chlorophyll a, as well as maximum depth for the multi-lake dataset. In both analyses and datasets, springtime abundance of herbivorous cladocerans was consistently found to be a significant factor associated with Bythotrephes (June–September) abundance; Bythotrephes annual abundance was significantly and positively associated with mean May and June prey abundance, along with mean Secchi disk depth for the multi-lake dataset, and groups of lakes or years with similar Bythotrephes seasonal abundance patterns were predicted by June prey abundance. Additionally, prey availability was the dominant contributor towards changes in weekly Bythotrephes birth rates calculated for two of the study lakes. Our study suggests that prey availability influences Bythotrephes abundance, which provides evidence that Bythotrephes establishment success is affected by the abundance of its prey.     

Interpreting Geographic Variation in Life-History Traits

The geographic variation in the length of the larval periodand the size at metamorphosis of the wood frog,Rana sylvatica,is examined for populations in the tundra of Canada, the mountainsof Virginia, and the lowlands of Maryland. We argue that theobserved differences in developmental plasticity, heriisbilitiesand genetic covariances of traits among localities result fromdifferential selection pressures in each environment, and arerelated to the physiological constraints inherent in developmentand to the degree of compromise between the timing and sizeat metamorphosis allowed in each environment. In Maryland populationsfitness has been maximized by evolutionary changes in size alone;body size in this population is canalized, has low heritabilityand is highly correlated with juvenile survival relative todevelopmental time. In Canada, minimum developmental time yieldsmaximum fitness; the length of the larval period in this populationis canalized and genetically monomorphic relative to body size.In contrast, fitness in the Virginia populations has been determinedby correlated and pleiotropic effects of genes on both developmentaltime and larval body size, and both traits are equally canalized,affect juvenile survivorship equally and display moderate heritabilities.These results stress the importance of interpreting variationin life-history traits relative to constraints inherent in developmentand those imposed by the environment. Heritability and survivorshipdata support the general notion that fitness traits should havelow levels of additive genetic variation, but also suggest thatantagonistic pleiotropy may act to preserve genetic variationin fitness traits under simultaneous selection, and cautionagainst inferring evolutionary importance of individual traitswithout considering the possible presence of pleiotropy.     

No evidence of Wolbachia among Great Lakes area populations of Daphnia pulex (Crustacea: Cladocera)

Wolbachia endobacteria are commonly associated with a varietyof arthropod species as hosts and induce known changes in theirhosts’ life-history traits. Despite exhibiting severalWolbachia-like life-history traits, and despite being a commonmodel organism, the zooplankton Daphnia pulex has not been formallytested for infection with Wolbachia. Among 203 isolates exhibitinga range of life-history phenotypes, we found no evidence ofWolbachia. This leaves the genes of D. pulex as the most likelycause of its own life-history traits.     

The invasive predator <Emphasis Type="Italic">Bythotrephes</Emphasis> induces changes in the vertical distribution of native copepods in Lake Michigan

Invasive predators can have large negative effects on native prey populations. The susceptibility of native prey to invasive predators may depend on their ability to respond behaviorally to the presence of these non-native predators. In a field survey conducted in Lake Michigan over several years, we found that high densities of the invasive predatory cladoceran Bythotrephes were correlated with lower vertical distributions of some species and age classes of native copepods; moving from inhabiting primarily the epiliminion at low Bythotrephes density to primarily the hypolimnion at high Bythotrephes density. Five groups showed this pattern; diaptomid copepodites, adult cyclopoids, Diacyclops thomasi, and the adult diaptomids Leptodiaptomus ashlandi and L. minutus. In contrast, Bythotrephes density was not correlated with the vertical distribution of copepod nauplii and adult L. sicilis. Laboratory experiments suggest that the changes in the vertical distribution in the field at high Bythothrephes are due to an inducible, plastic response to predation threat from Bythotrephes signaled by water-borne cues. Species that were lower in the field at high Bythotrephes densities responded behaviorally to water-borne cues from Bythotrephes by moving to lower levels of experimental water columns. These species included D. thomasi and L. minutus, with L. ashlandi displaying a non-significant trend in the same direction. In contrast, L. sicilis, which was not correlated with Bythotrephes density in the field, was unaffected by the water-borne cues. Differences in vertical distribution shifts among these native copepod species and life-history stages are likely due to species-specific differences in spatial overlap with Bythotrephes and their relative ability to migrate large distances or employ alternative avoidance strategies. The varied responses exhibited among the copepod groups likely alter their interactions with each other, their resources and other predators, thus revealing the complex effects Bythotrephes can have on invaded communities.     

Seasonal size variation in the predatory cladoceran Bythotrephes cederstroemii in Lake Michigan

• 1 Dry weight, body length and spine length were measured for the exotic cladoceran Bythotrephes cederstroemii collected from offshore and inshore stations in southeastern Lake Michigan. Average dry weight of each developmental stage exhibited seasonal variation by a factor of more than 5.
• 2 Mean dry weight of Bythotrephes was closely correlated with water temperature. Contrary to the inverse relationship between water temperature and body size frequently observed for other invertebrates, the dry weight of Bythotrephes increased at higher ambient temperatures.
• 3 No significant correlation was observed between abundances of major zooplankton taxa and the dry weight of Bythotrephes. An indirect effect of temperature on prey consumption may cause seasonal variation in dry weight of Bythotrephes in Lake Michigan.
• 4 Distances between adjacent pairs of barbs, added to the caudal spine with each moult, are significantly shorter in Bythotrephes which produce resting eggs. Less material investment in the exoskeleton of sexually reproducing females was observed in favour of growth and reproduction.

     

The caudal appendage of the cladoceran Bythotrephes cederstroemi as defense against young fish

Rainbow trout (Onchorhynchus mykiss), 40–80 mm in length,were fed the cladoceran Bythotrephes cederstroemi with and withoutits caudal appendage (spined or despined) to determine if theappendage makes ingestion of the zooplankter difficult for smallfish. Handling time was measured as the period of opercularand buccal activity as fish ingested prey. In 980 observationsfor 13 trout, fish spent 8 s longer or 800% more time handlingspined Bythotrephes than despined. Fish ultimately rejectedsignificantly more spined prey than despined prey. Average handlingtimes of spined prey were negatively correlated with fork lengthand smaller fish rejected spined prey more often. All fish handleddespined prey equally well. Eight of 13 fish changed their handlingefficiency over time. However, only two fish decreased handlingtime on spined prey, while two fish increased handling timeon spined prey. Four fish decreased their handling time on despineditems. Bythotrephes' caudal appendage increases the likelihoodthat it will be rejected after capture by fish and decreasesfish predation rate by increasing handling time. This is newevidence of a freshwater zooplankter having a structural defenseeffective against young fish. Present address: Department of Biological Sciences, Lake SuperiorEcosystems Research Center, Michigan Technological University,Houghton, Ml 49931, USA     

Food and feeding behaviour of a planktivorous cyprinid, Pelecus cultratus (L.), in a shallow eutrophic lake, Neusiedler See (Austria)

Pelecus cultratus (razor fish), a cyprinid fish, has become a dominant species in Neusiedler See. Gut content analyses of 400 specimens collected in 1989 and 1990 showed that Pelecus fed mainly on large zooplankton (Diaphanosoma, Leptodora and Arctodiaptomus), although their diet also included Insecta (larvae, pupae and adults) and Arachnida (spiders), occasionally small fishes. Comparison of the relative abundance of the zooplankton species in the stomach to the lake indicated that Pelecus strongly selected cladocerans over copepods, and fed mostly on large-sized individuals of Diaphanosoma, Leptodora and Arctodiaptomus. The fish showed a significant positive selectivity only for individuals of Diaphanosoma > 1.0 mm and Leptodora > 4.0 mm. In contrast, selectivity increased continuously in relation to the diameter of the compound eye of both prey species. This suggested that prey visibility was a key factor in determining the prey selectivity by Pelecus. It also seems likely that the persistence of the Leptodora population in Neusiedler See can be attributed to negligible predation pressure on the smaller sized individuals of this species.     

Reproductive output,costs of reproduction,and ecology of the smooth snake,Coronella austriaca,in the eastern Italian Alps

A 5-year mark-recapture study of smooth snakes (Coronella austriaca) in the Carnic Alps (1100 m above sea level) of north-eastern Italy provided extensive information on the biology and life-history of these small viviparous snakes. Offspring were relatively large (mean=15 cm total length, 2.9 g) when they were born in late summer, and females grew to maturity (44 cm, 50 g) in approximately 4 years. Larger neonates retained their size advantage for at least 12 months, but did not have a higher probability of survival. Although sexual size dimorphism (at birth and at mean adult body sizes) was minor, the sexes differed significantly in several respects. Females grew faster than males during juvenile life, and adult females diverged in dietary habits from the rest of the population. Whereas juveniles (of both sexes) and adult males fed primarily on lizards, larger females shifted to feeding less frequently, but taking larger prey (mammals and snakes). Reproductive output increased strongly with maternal body size: larger females reproduced more frequently, produced larger litters of larger neonates, had higher relative clutch masses (RCMs), and had a lower proportion of stillborn off-spring. Most females produced a litter every 2nd or 3rd year. We did not detect significant year-to-year variation in reproductive traits over the 5 years of our study. Females were consistent from one litter to the next in several traits (e.g., litter sizes, offspring sizes and shapes, proportions of stillborn neonates, RCMs), but this consistency was due to differences in body size among females rather than to size-independent maternal effects. Overall litter sex ratios averaged 50/50, but sex ratios tended to be more male-biased in litters that were unusually large relative to maternal body size, and in litters containing a high proportion of stillborn offspring. Costs of reproduction appear to be high in this population, in terms of both energy allocation and risk. Reproduction reduced growth rates, and females that recovered condition more quickly in the year after reproduction were able to reproduce again after a briefer delay. Mortality was highest in reproducing females with high RCMs, and in females that were very emaciated after parturition. The marked increase in reproductive output with increasing maternal body size in C. austriaca may reflect a reduction in costs as females grow larger, and the dietary shift to larger prey may enhance the rate that females can accumulate energy for reproduction.     

Adult body size = <Emphasis Type="Italic">f</Emphasis> (initial size + growth rate × age): explaining the proximate cause of Bergman’s cline in a toad along altitudinal gradients

Animals that exhibit indeterminate growth obey such a functional relationship: adult body size = f (initial size + growth rate × age). Using this framework, we investigated how and why body sizes of a toad species (Bufo andrewsi) covaried across six altitudes (760–2,100 m) in western China. Towards high altitudes, toads tended to produce large eggs, attain large sizes at metamorphism and have great average age, but grow slowly. This indicated that the former three variables contributed more to the observed altitudinal increase in body size than did the last one. The altitudinal variation in these life-history traits should be adaptive to increased climate harshness and decreased predation risks at higher altitudes. We suggest that the relative significance of responses of these size-related parameters to local environments may provide critical cues to explaining considerable variability in geographic size pattern among ectothermic vertebrates.     

Genetic and phenotypic relationships between immune defense,melanism and life-history traits at different temperatures and sexes in Tenebrio molitor

Insect cuticle melanism is linked to a number of life-history traits, and a positive relationship is hypothesized between melanism and the strength of immune defense. In this study, the phenotypic and genetic relationships between cuticular melanization, innate immune defense, individual development time and body size were studied in the mealworm beetle (Tenebrio molitor) using three different temperatures with a half-sib breeding design. Both innate immune defense and cuticle darkness were higher in females than males, and a positive correlation between the traits was found at the lowest temperature. The effect of temperature on all the measured traits was strong, with encapsulation ability and development time decreasing and cuticle darkness increasing with a rise in temperature, and body size showing a curved response. The analysis showed a highly integrated system sensitive to environmental change involving physiological, morphological and life-history traits.     

The influence of zooplankton defenses on prey selection by young Chaoborus larvae

Prey exhibiting a variety of morphological and behavioral antipredatordefenses were examined for susceptibility to first through thirdinstar Chaoborus americanus larvae. Synchaeta sp. was highlyvulnerable to these predators due to high encounter rates, itsrelatively soft cuticle, and lack of a precontact defense. Keratellatestudo exhibited postcontact defenses and was more susceptibleto predators than were prey of comparable size that exhibitedprecontact escape behaviors (Hexarthra mira and Diaptomus birgeinauplii). Ontogenetic changes were observed in the escape responseof D.birgei and the foraging success of C.americanus larvae.Chaoborus was observed to prey on the protists Bursaria andVolvox. Precontact defenses were more effective than postcontactdefenses in the prey species examined; however, the relativeeffectiveness of prey defenses cannot be generalized by preygenus nor by the defensive tactics employed. Present address: Department of Entomology, University of California,Riverside, CA 92521, USA     

Indirect food web effects of <Emphasis Type="Italic">Bythotrephes</Emphasis> invasion: responses by the rotifer <Emphasis Type="Italic">Conochilus</Emphasis> in Harp Lake,Canada

As a recent invader of North American lakes, Bythotrephes longimanus has induced large changes in crustacean zooplankton communities through direct predation effects. Here we demonstrate that Bythotrephes can also have indirect food web effects, specifically on rotifer fauna. In historical time series data, the densities of the colonial rotifer Conochilus unicornis significantly increased after Bythotrephes invasion in Harp Lake, Ontario. No such changes were observed in a non-invaded reference lake, the nearby Red Chalk Lake. Evidence for two mechanisms explaining the Conochilus increase was examined based on changes to the crustacean zooplankton community over time. Rapid and severe declines in several herbivorous species of cladoceran zooplankton after Bythotrephes detection indicated a decrease in exploitative competition pressure on Conochilus. Secondly, a later and significant decline to virtual extinction of native invertebrate predators (Mesocyclops and Leptodora) could account for the observed Conochilus increase which also began 1–2 years after invasion by Bythotrephes. Ultimately, it appears that both reduced competition followed by a loss of native invertebrate predators were necessary to lead to the large Conochilus densities observed following invader establishment. From this analysis of long-term community data, it appears that Bythotrephes has important indirect, as well as direct, food web effects in newly invaded North American lakes with implications for trophic relationships.     

FACTORS AFFECTING PREY CHOICE AND FEEDING TECHNIQUE IN THE CARNIVOROUS SNAIL EUGLANDINA ROSEA FERUSSAC

Euglandina feeds on other gastropods either by quickly suckingthe contents from the shell, or by swallowing the prey whole,which can be more time consuming. When Euglandina were offeredprey species of various sizes, they preferentially consumedthe smaller individuals, and often swallowed these whole. Handlingtime increased with decreasing density since swallowing preywhole became more frequent at the lower densities. The relationship between prey size, predator size, handlingtime and feeding technique was examined. For the two feedingtechniques observed, handling time increased exponentially withthe size of the prey and decreased exponentially with the sizeof the predator. Predators of intermediate size tended to swallowprey whole more frequently than did the smallest and largest. These data are interpreted in terms of the animal behaving insuch a way as to balance its rate of intake of both organicmatter, which resides mainly in the soft parts, and of calcium,which resides mainly in the shell. (Received 11 July 1988; accepted 31 October 1988)     

The presence of fish induces the quick release of offspring by Daphnia

Daphnia exposed to the presence of fish mature at smaller size and are quicker to release fewer and smaller neonates than females not exposed to this factor. Releasing neonates requires intensive movement, rendering the female very susceptible to fish predation. A reduction of this time may thereby be adaptive in the presence of fish. Since there is no correlation between the time needed to release a whole clutch and other life-history traits that show a shift in the presence of fish kairomone, the shortening of the time needed to release the young can itself be an adaptation, and not merely a consequence of change in other life-history traits.