Echinostome metacercariae cyst elimination in Rana clamitans (green frog) tadpoles is age-dependent

Host response to parasite exposure is an important determinant of the outcome of host-parasite interactions. Factors such as host body condition and age can strongly influence host response to parasites and infection. This study followed Echinostoma revolutum infection levels in larval green frogs (Rana clamitans) exposed at 2 different ages. Tadpoles at early developmental stages are more susceptible to the adverse effects of echinostomes. Green frog tadpoles approximately 2 wk apart in age and of the same developmental stage can exhibit dramatically different responses to echinostome infection, with the younger tadpoles having high rates of mortality and the older tadpoles experiencing no mortality. The goal of the present study was to begin to explore the mechanism underlying the striking age-dependent response of tadpoles to echinostomes. I conducted 2 controlled infection laboratory experiments in which tadpoles were preserved at 6 time points ranging from 4 hr to 1 wk postinfection (PI). Tadpoles infected at the younger age did not eliminate echinostome metacercariae. However, tadpoles that were 13 days older at the time of echinostome exposure steadily eliminated metacercariae during the hours and days PI. The absence of echinostome cyst elimination in the younger tadpoles likely contributes to their elevated, infection-induced mortality rates.     

The effects of kinship and density on growth and metamorphosis of the bronze frog (Rana temporalis) tadpoles

  Five clutches of Rana temporalis eggs collected along a stream were reared separately until formation of Gosner stage 25 tadpoles. The effect of kinship and density was then studied by rearing ten siblings in 1 (sib 1) or 5 (sib 5) l water, and mixed rearing of ten tadpoles (2 from each of the 5 clutches) in 1 (mix 1) or 5 (mix 5) l water; each group replicated five times. In all the groups tadpoles showed a sigmoid growth curve. Both kinship and density interacted to affect mean proportions of individuals reaching metamorphic climax (MC), mean body mass, and size frequency at MC (day 79). The proportion of tadpoles reaching MC was highest in sib 5 (82%) followed by mix 5 (77%), sib 1 (73%), and mix 1 (64%) groups. Crowding plus mixing significantly lowered the median developmental stage mean body mass and broadened the spectrum of developmental stages or size classes at MC. The size difference of individuals at MC was inversely related to density of rearing. Frequency of different developmental stages was comparable in sib 1, sib 5, and mix 5 groups. Interestingly, small individuals were significantly greater in number in the mix 1 group compared to sib 1. The above findings suggest that genotypic heterogeneity in conjunction with crowding retards growth rate of tadpoles in comparison with those reared in a genetically homogenous (sib) environment. Further, they suggest that the adverse effect of mixed rearing is context dependent. Received: 12 April 1999 / Received in revised form: 16 June 1999 / Accepted: 18 June 1999     

Tadpoles of the bronze frog (Rana temporalis) assess predation risk before evoking antipredator defense behavior

Predation threat-associated behavioral response was studied in Rana temporalis tadpoles to discover the importance of predators’ visual and chemical cues (kairomones and diet-derived metabolites of consumed prey) in evoking antipredator behavior. The caged predators (dragonfly larvae) fed on prey tadpoles or insects (Notonecta spp.) and water conditioned with the predators provided the threat stimuli to the tadpole prey. The predators’ visual cues were ineffective in evoking antipredator behaviors in the tadpole prey. However, exposure to caged tadpole-fed predators or water conditioned with tadpole-fed predators elicited predator avoidance behavior in the tadpoles; they stayed away from the predators, significantly reduced swimming activity (swimming time and distance traveled), and increased burst speed. Interestingly, exposure to water conditioned with starved predators did not elicit any antipredator behavior in the prey. Further, the antipredator responses of predator-experienced tadpoles were significantly greater than those exhibited by predator-na?ve tadpoles. The study shows that R. temporalis tadpoles assess predation threat based exclusively on chemical cues emanating from the predators’ dietary metabolites and that the inclusion of conspecific prey items in the diet of the predators is perceived as a threat. The study also shows that antipredator behavior in these tadpoles is innate and is enhanced during subsequent encounters with the predators.     

No apparent effects of soil inoculum on green frog (Lithobates clamitans Latreille) tadpole performance

Invasions by nonnative plant species are transforming plant communities across the globe. An important challenge for ecologists is to understand how animals will respond to these changes. One way that plant invasions could affect aquatic animals is by changing the rate at which soil communities decompose litter, which could alter the flow of energy and nutrients from plant litter to aquatic communities. In this study, we measured larval amphibian responses to soil conditioned by either introduced or native genotypes of Phragmites australis L. (common reed) in northeastern North America. We collected soil from adjacent stands of introduced and native P. australis at three sites in central New York and inoculated outdoor aquatic mesocosms with soil extracts. Mesocosms contained six Lithobates clamitans Latreille (green frog) tadpoles and either low- or high-quality native P. australis americanus litter. We found that litter decomposition differed based on soil inoculum, and we observed a significant interaction between litter quality and soil inoculum; higher-quality litter tended to decompose faster when exposed to inocula from introduced P. australis, while lower-quality litter tended to decompose faster when exposed to inocula from native P. australis americanus. Tadpoles raised with high-quality litter developed faster and achieved greater body size, but soil inocula had no apparent effect on tadpoles. Our results suggest that plant invasions may alter microbial communities, causing subtle changes in litter decomposition rates, but these changes do not appear strong enough to influence larvae of a widespread amphibian.     

NAG infection of grass frog (Rana temporaria) tadpoles]

Rana temporaria tadpoles (2250) were contaminated with three types of NAG vibrio cultures. Clinical, bacteriological and morphological examinations showed the larvae to be suffering from an acute infection during the first 2 days after the contamination. Then the vibrios persisted in the tadpole organism for a long time and were excreted in the feces.     

Purification and characterization of antimicrobial peptides from the skin of the North American green frog Rana clamitans

Ten peptides with differential growth-inhibitory activity against the gram-positive bacterium, Staphylococcus aureus, the gram-negative bacterium, Escherichia coli, and the yeast Candida albicans were isolated from an extract of the skin of a North American frog, the green frog Rana clamitans. Ranatuerin-1C (SMLSVLKNLGKVGLGLVACKINKQC), ranalexin-1Ca (FLGGLMKAFPALICAVTKKC), ranalexin-1Cb (FLGGLMKAFPAIICAVTKKC), ranatuerin-2Ca (GLFLDTLKGAAKDVAGKLLEGLKCKIAGC KP), and ranatuerin-2Cb (GLFLDTLKGLAGKLLQGLKCIKAGCKP), are members of three previously characterized families of antimicrobial peptides, first identified in the North American bullfrog Rana catesbeiana. In addition, five structurally related peptides (temporin-1Ca, -1Cb, -1Cc, -1Cd, and -1Ce), comprising 13 amino acid residues and containing a C-terminally alpha-amidated residue, belong to the temporin family first identified in the European common frog Rana temporaria. Peptides belonging to the brevinin-1, brevinin-2, esculentin-1, and esculentin-2 families, previously isolated from the skins of Asian and European Ranid frogs, were not identified in the extract. The data support the hypothesis that the distribution and amino acid sequences of the skin antimicrobial peptides are valuable tools in the identification and classification of Ranid frogs.     

Bottom‐up effects of fungicides on tadpoles of the European common frog (Rana temporaria)

Biodiversity is under pressure worldwide, with amphibians being particularly threatened. Stressors related to human activity, such as chemicals, are contributing to this decline. It remains, however, unclear whether chemicals exhibiting a fungicidal activity could indirectly affect tadpoles that depend on microbially conditioned leaf litter as food source. The indirect effect of fungicides (sum concentration of a fungicide mixture composed of azoxystrobin, carbendazim, cyprodinil, quinoxyfen, and tebuconazole: 100 µg/L) on tadpoles was assessed relative to leaf litter colonized by microbes in absence of fungicides (control) and a worst‐case scenario, that is leached leaf litter without microbial colonization. The quality of leaf litter as food for tadpoles of the European common frog (Rana temporaria) was characterized through neutral lipid fatty acid profiles and microbial sum parameters and verified by sublethal responses in tadpoles (i.e., feeding rate, feces production, growth, and fatty acid composition). Fungicides changed the nutritious quality of leaf litter likely through alterations in leaves’ neutral lipid fatty acid profiles (i.e., changes in some physiologically important highly unsaturated fatty acids reached more than 200%) in combination with a potential adsorption onto leaves during conditioning. These changes were reflected by differences in the development of tadpoles ultimately resulting in an earlier start of metamorphosis. Our data provide a first indication that fungicides potentially affect tadpole development indirectly through bottom‐up effects. This pathway is so far not addressed in fungicide environmental risk assessment and merits further attention.     

Effects of nitrate on the interactions of the tadpoles of two ranids (Rana clamitans and R. catesbeiana)

Nitrogen pollution as a result of agricultural runoff and atmospheric deposition is a major challenge to aquatic ecosystems, and is likely to increase in the future. Nitrogenous pollutants are potential stressors of amphibian larvae through their toxicological impacts on individuals; however, they may also increase primary productivity. We examined how such an anthropogenic stressor could influence the interactions between two potentially competing species of tadpoles (Rana clamitans and R. catesbeiana). In a 42 d mesocosm experiment, R. catesbeiana survival, but not final mass, was reduced by nitrate addition. Rana catesbeiana survival was lower when in competition with R. clamitans than when only experiencing intraspecific competition. For R. clamitans, survival was not affected by nitrate addition or competition type. Rana clamitans in nitrate addition mesocosms were heavier than tadpoles from no nitrate mesocosms, and were heavier in intraspecific than in interspecific mesocosms. Our results suggest that nitrate can influence the performance of amphibian larvae, and that its effects could have complex effects on aquatic ecosystems.     

Resource levels and prey state influence antipredator behavior and the strength of nonconsumptive predator effects

The risk of predation can drive trophic cascades by causing prey to engage in antipredator behavior (e.g. reduced feeding), but these behaviors can be energetically costly for prey. The effects of predation risk on prey (nonconsumptive effects, NCEs) and emergent indirect effects on basal resources should therefore depend on the ecological context (e.g. resource abundance, prey state) in which prey manage growth/predation risk tradeoffs. Despite an abundance of behavioral research and theory examining state‐dependent responses to risk, there is a lack of empirical data on state‐dependent NCEs and their impact on community‐level processes. We used a rocky intertidal food chain to test model predictions for how resources levels and prey state (age/size) shape the magnitude of NCEs. Risk cues from predatory crabs Carcinus maenas caused juvenile and sub‐adult snails Nucella lapillus to increase their use of refuge habitats and decrease their growth and per capita foraging rates on barnacles Semibalanus balanoides. Increasing resource levels (high barnacle density) and prey state (sub‐adults) enhanced the strength of NCEs. Our results support predictions that NCEs will be stronger in resource‐rich systems that enhance prey state and suggest that the demographic composition of prey populations will influence the role of NCEs in trophic cascades. Contrary to theory, however, we found that resources and prey state had little to no effect on snails in the presence of predation risk. Rather, increases in NCE strength arose because of the strong positive effects of resources and prey state on prey foraging rates in the absence of risk. Hence, a common approach to estimating NCE strength – integrating measurements of prey traits with and without predation risk into a single metric – may mask the underlying mechanisms driving variation in the strength and relative importance of NCEs in ecological communities.     

Managing predators: The influence of kangaroo rat antipredator displays on sidewinder rattlesnake hunting behavior

Upon sensing predators in their vicinity, many prey species perform antipredator displays that are thought to provide information to the predator that deters it from attacking (predator‐deterrent signals). These displays can be complex, incorporating a variety of signaling elements as well as direct physical harassment of the predator. Although the display behaviors in these communication systems are often well characterized, evidence of the efficacy of these displays in deterring predators is limited due to the challenges associated with studying free‐ranging predators. Here, we examine how the anti‐snake signals of the desert kangaroo rat (Dipodomys deserti) influence the ambush hunting behaviors of sidewinder rattlesnakes (Crotalus cerastes). We found that, although desert kangaroo rats incorporate a number of signal elements into their antipredator display, only sand kicking behavior was a significant factor in motivating sidewinder rattlesnakes to cease hunting: high rates of sand kicking led to early abandonment of ambush coils. These results indicate that anti‐snake displays of small mammals may be especially effective at mitigating the threat posed by rattlesnakes when those displays incorporate physical harassment as well as signaling.     

Echinostome infection in green frogs (Rana clamitans) is stage and age dependent

The increasing threat of emerging infectious diseases in both wildlife and humans has spurred interest in the causes of disease emergence, including the role of anthropogenic change. A prior field study of infection patterns in amphibians suggests that echinostome infection may be an emerging disease of green frogs Rana clamitans living in urbanized environments. We examined the impact of echinostome infection on green frog tadpoles at a wide range of developmental stages (Gosner stage 25–39). Echinostome infection was associated with green frog mortality rates of up to 40% in an early developmental stage, and none in later developmental stages. Tadpoles exposed to higher echinostome doses exhibited higher edema rates, a potential sign of compromised renal function. Histopathological analysis further supported the hypothesis that echinostome-induced tadpole mortality resulted from compromised renal function. Given that the timing of highest cercarial shedding can coincide with the most vulnerable stages of green frog tadpole development, echinostomes could significantly impact green frog survival in nature.     

Metamorphic changes in the vagal innervation to the alimentary tract in the green frog, Rana clamitans (Ranidae).

The alimentary tract of typical anurans shortens extensively at metamorphosis, when the microphagous tadpole transforms into a macrophagous frog. We used methylene blue vital stain and Sihler stain, in conjunction with microdissection, to map the distribution of the gastric branch of the vagus nerve in green frogs (Rana clamitans) before and after metamorphosis. After metamorphosis, terminal divisions of the gastric branch can be observed on the tunica musculosa extending as far as the pylorus. In the tadpole, the gastric branch ends abruptly on the esophagus. The premetamorphic pattern of innervation is consistent with the absence of digestive activity in the foregut before metamorphosis. Preliminary experiments, involving stimulation of the gastric branch of the vagus in the tadpoles, confirm the anatomical observation of the absence of vagal innervation before metamorphosis.     

A study on hearing in frogs (Rana pipiens and Rana clamitans)

Zusammenfassung Die relative Reizschwellkurve für die Schallempfindung bei den Fröschen Rana clamitans und R. pipiens wurde in dem Frequenzbereich von 30–15000 Hz bestimmt. Die Kurve ist dieselbe für beide Geschlechter. Versuche zeigten, daß Schälle entweder inhibierend oder stimulierend auf die Atmungsbewegungen wirken können. Diese Wirkung wird nicht von der Frequenz oder der Intensität der Schälle bestimmt, sondern von dem physiologischen Zustand des Tieres.Die Latenzzeit zwischen Reiz und Effekt ist unabhängig von der Tonfrequenz, wird aber von den Intervallen zwischen aufeinanderfolgenden Reizen beeinflußt; sie wird kürzer, wenn die Intervalle verlängert werden.Dressurversuche mit 18 Tieren, unter Anwendung von Strafung durch elektrische Schläge und einem Dressurton von 300 Hz, ergaben positive Erfolge bei zwei Individuen von R. clamitans.

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Contribution No. 67 of the Department of Biology, Hamilton College, McMaster University, Hamilton, Ontario.     

Experimental and model analyses of the effects of competition on individual size variation in wood frog (Rana sylvatica) tadpoles

1. Size variation is a ubiquitous feature of animal populations and is predicted to strongly influence species abundance and dynamics; however, the factors that determine size variation are not well understood. 2. In a mesocosm experiment, we found that the relationship between mean and variation in wood frog (Rana sylvatica) tadpole size is qualitatively different at different levels of competition created by manipulating resource supply rates or tadpole density. At low competition, relative size variation (as measured by the coefficient of variation) decreased as a function of mean size, while at high competition, relative size variation increased. Therefore, increased competition magnified differences in individual performance as measured by growth rate. 3. A model was developed to estimate the contribution of size-dependent factors (i.e. based on size alone) and size-independent factors (i.e. resulting from persistent inherent phenotypic differences other than size that affect growth) on the empirical patterns. 4. Model analysis of the low competition treatment indicated that size-dependent factors alone can describe the relationship between mean size and size variation. To fit the data, the size scaling exponent that describes the dependence of growth rate on size was determined. The estimated value, 0-83, is in the range of that derived from physiological studies. 5. At high competition, the model analysis indicated that individual differences in foraging ability, either size-based or due to inherent phenotypic differences (size-independent factors), were much more pronounced than at low competition. The model was used to quantify the changes in size-dependent or size-independent factors that underlie the effect of competition on size-variation. In contrast to results at low competition, parameters derived from physiological studies could not be used to describe the observed relationships. 6. Our experimental and model results elucidate the role of size-dependent and size-independent factors in the development of size variation, and highlight and quantify the context dependence of individual (intrapopulation) differences in competitive abilities.     

Inheritance and expression of a sex-linked enzyme in the frog,Rana clamitans

The pattern of inheritance indicates that the gene for aconitase-1 is sex linked in the frog. Rana clamitans, and that the male is the heterogametic sex. Unlike mammals, both male and female frogs carry and express two alleles for this sex-linked gene. Therefore, the sex chromosomes in these frogs and probably others behave like an autosomal pair, with one homologue carrying a male-determining element.     

Growth and development of larval green frogs (Rana clamitans) exposed to multiple doses of an insecticide

Our objective was to determine how green frogs (Rana clamitans) are affected by multiple exposures to a sublethal level of the carbamate insecticide, carbaryl, in outdoor ponds. Tadpoles were added to 1,000-l ponds at a low or high density which were exposed to carbaryl 0, 1, 2, or 3 times. Length of the larval period, mass, developmental stage, tadpole survival, and proportion metamorphosed were used to determine treatment effects. The frequency of dosing affected the proportion of green frogs that reached metamorphosis and the developmental stage of tadpoles. Generally, exposure to carbaryl increased rates of metamorphosis and development. The effect of the frequency of carbaryl exposure on development varied with the density treatment; the majority of metamorphs and the most developed tadpoles came from high-density ponds exposed to carbaryl 3 times. This interaction suggests that exposure to carbaryl later in the larval period stimulated metamorphosis, directly or indirectly, under high-density conditions. Our study indicates that exposure to a contaminant can lead to early initiation of metamorphosis and that natural biotic factors can mediate the effects of a contaminant in the environment.