Length Of Larval Period Research Articles

Optimal level of dietary protein for Rana perezi Seoane larvae

. The objective of this study is to determine the optimal protein level for best growth and development of Rana perezi Seoane larvae. Performance of four diets containing 28%, 33·5%, 39% and 45·5% protein was compared. Tadpoles provided diets with 39% and 45·5% gross protein achieved the best results with regard to growth (weight gain, growth rate, feed conversion, etc.) and development (length of larval period, percentage of metamorphosis, etc.). Considering that protein is the most expensive diet component, it is estimated that the optimum quantity of protein for R. perezi larvae is about 39%.

Does the Spatial Scale of Experimentation Matter? A Test with Tadpoles and Dragonflies

We report the results of a study designed to examine whether the spatial scale of experimentation in artificial ponds (cattle watering tanks) influences the results of manipulative studies of competitive and predatory interactions. The experiment consisted of two initial densities of Rana utricularia (Ranidae) tadpoles (0.035 and 0.14 animals/L) and the absence of presence (0.005 animals/L) of predatory Tramea lacerata (Odonata, Libellulidae) naiads in each of two water depths in cattle tanks of equal dimensions. Depth of tank did not significantly affect survival rate of tadpoles either as a main effect or as part of any interactive effect. At low tadpole densities they increased them significantly. Length of larval period increased significantly with increases in the density of surviving tadpoles at similar rates at both water depths, but deeper tanks produced significantly longer larval periods for the same tadpole density. Size at metamorphosis decreased significantly with increases in the density of surviving tadpoles, but it decreased at a significantly faster rate in the shallower tanks. The results indicate that experiments at smaller spatial scales do no necessarily distort the numerical dynamics in the system but that they can overestimate the importance of density variation in producing variation in some phenotypic characters.

The Interaction of Predation, Competition, and Habitat Complexity in Structuring an Amphibian Community

We examined effects of predation and competition on population dynamics and community structure in an aquatic system in central Arizona by measuring how refugia for tadpoles (Hyla eximia) influence predation by larval salamanders (Ambystoma tigrinum). Using field enclosures with three densities of A. tigrinum and two of H. eximia and two degrees of habitat complexity, we measured the effects of predation, competition, and habitat complexity on four components of fitness in larval amphibians: mass at metamorphosis, growth rate, length of larval period, and survival. Hyla eximia had low survival where salamanders were present. Tadpoles in treatments with two or four salamanders had one-fifth the survival of tadpoles in treatments with no salamanders. We found some evidence of density-dependent effects in Ambystoma. Mean salamander mass was negatively affected by salamander density. Density-dependent effects in H. eximia were weak. Habitat complexity alone did not affect any response variable but interacted with salamander density and tadpole density in a complex, nonadditive fashion. Many studies that have examined the relative importance of predation and competition in structuring amphibian communities have ignored habitat complexity. Our experiment, which examined different degrees of habitat complexity, suggests that the relationship between predation and competition may be nonadditive depending on the degree of habitat complexity.

Effects of Egg Size and Density on Metamorphic Traits in Tadpoles of the Natterjack Toad (Bufo calamita)

Effects of Egg Size and Density on Metamorphic Traits in Tadpoles of the Natterjack Toad (Bufo calamita)

GENETIC CORRELATIONS AMONG MORPHOMETRIC TRAITS AND RATES OF GROWTH AND DIFFERENTIATION IN THE GREEN TREE FROG, HYLA CINEREA.

It is often proposed that the morphometric shape of animals often evolves as a correlated response to selection on life-history traits such as whole-body growth and differentiation rates. However, there exists little empirical information on whether selection on rates of growth or differentiation in animals could generate correlated response in morphometric shape beyond that owing to the correlation between these rates and body size. In this study genetic correlations were estimated among growth rate, differentiation rate, and body-size-adjusted head width in the green tree frog, Hyla cinerea. Head width was adjusted for size by using the residuals from log-log regressions of head width on snout-vent length. Size-adjusted head width at metamorphosis was positively genetically correlated with larval period length. Thus, size-independent shape might evolve as a correlated response to selection on a larval life-history trait. Larval growth rate was not significantly genetically correlated with size-adjusted head width. An additional morphometric trait, size-adjusted tibiofibula length, had a nonnormal distribution of breeding values, and so was not included in the analysis of genetic correlations (offspring from one sire had unusually short legs). This result is interesting because, although using genetic covariance matrices to predict long-term multivariate response to selection depends on the assumption that all loci follow a multivariate Gaussian distribution of allelic effects, few data are available on the distribution of breeding values for traits in wild populations. Size at metamorphosis was positively genetically correlated with larval period and larval growth rate. Quickly growing larvae that delay metamorphosis therefore emerge at a large size. The genetic correlation between larval growth rate and juvenile (postmetamorphic) growth rate was near zero. Growth rate may therefore be an example of a fitness-related trait that is free to evolve in one stage of a complex life cycle without pleiotropic constraints on the same trait expressed in the other stage.

Comparing bivariate reaction norms among species: time and size at metamorphosis in three species of Hyla (Anura: Hylidae).

Univariate and bivariate methods for comparing norms of reaction among species are discussed and illustrated with an example using North American hylid treefrogs. Norms of reaction for size at metamorphosis (SM) and length of larval period (LP) were compared among treefrog species raised at different food levels (Hyla cinerea vs H. gratiosa) and at different temperatures (H. cinerea vs H. gratiosa vs H. squirella). Hyla cinerea and Hyla gratiosa show parallel norms of reaction across food levels and temperatures. Across temperatures, H. squirella shows a much smaller change in SM relative to change in LP than do H. cinerea and H. gratiosa. This difference in shape of reaction norms may reflect different histories of selection resulting from these species' use of different larval habitats.

The Effect of Ontogeny on Interspecific Interactions in Larval Amphibians

We used 3 x 2 x 2 factorial experiments in a temporary pond to assess the relative importance of predator density (0, 2, or 4 salamander larvae), prey density (20 or 40 tadpoles), and time of prey introduction (early or late) on interactions between predaceous larval salamanders (Ambystoma tigrinum) and their anuran prey (Pseudacris triseriata). We examined how these factors and their interactions affected mass at metamorphosis, length of larval period, growth rate, and survival. These experiments demonstrated the effect of body size on predator—prey interactions by manipulating time of introduction of size—structured prey in a system with a gape—limited predator. Microhabitat differences in primary productivity, sources of predation other than salamanders, and abiotic factors were important determinants of community structure. High levels of salamander predation negatively affected tadpole survival, but affected no other response variable. We detected no density—dependent effects in either taxon. Tadpoles in the early treatment had longer larval periods and lower survival rates than those in the late treatment. Interactions of factors, including time of introduction, were common. Biotic and abiotic factors differentially affecting growth of predator or prey appear to be responsible for these differences. Priority effects in arrival to breeding sites, or climatic events affecting the growth rate of predator, prey, or both, play a role in determining community structure. Interactions of factors complicate the predator—prey relationship. In our experiment, the effects of time of introduction are highly correlated with body size in the two temporal treatments. Our results indicate that more realistic models of predator—prey systems, or population dynamics, should incorporate size structure of predator and prey.

Reproductive biology of the fire-bellied toads, Bombina bombina and B. variegata (Anura: Discoglossidae): egg size, clutch size and larval period length differences

Clutch size, egg size, larval size and duration of larval period were estimated for samples of Bombina bombina and B. variegata from several populations. Bombina variegata lays fewer eggs per batch (mean 17.4) than B. bombina (mean 32.5). The volume of B. variegata eggs is 2.4 times greater and the dry weight 2.3 times greater than that of B. bombina. As a consequence, the volume of B. variegata hatchlings and their dry weight are respectively 1.9 times and 2.5 times greater than that of B. bombina. Analysis of variance showed that variation of egg size in B. variegata is significantly greater than in B. bombina. When reared under laboratory conditions freshly metamorphosed individuals of both species showed no difference in body size or weight. Although growth rates are equal, tadpoles of B. variegata reached metamorphosis 14 days earlier than B. bombina tadpoles because of differences in size of hatchlings. Differences in reproductive parameters found for the two European Bombina species can be interpreted as adaptations to contrasting environments in which they breed.

Populational Changes of the Enteric Protozoans Opalina spp. and Nyctotherus cordiformis During the Ontogeny of Anuran Tadpoles

ABSTRACT. We studied ontogenetic population changes of Opalina and Nyctotherus cordiformis in eight species of tadpoles from 10 sites in east‐central Mississippi. Most tadpoles acquired Opalina early in development, while the acquisition of N. cordiformis was variable. Developmental stage, species and collection site explained significant amounts of the variation in Opalina density of tadpoles (F= 11.6; df = 27, 235; P < 0.0001) and metamorphs (F= 7.31; df = 24, 84; P < 0.0001). Relationships between Opalina density and host stage showed either (1) a gradual decrease or (2) a gradual increase throughout host ontogeny. Opalina densities declined during metamorphosis. Density variations of N. cordiformis were explained by host species of tadpoles (F= 9.30; df = 7, 142; P < 0.0001) and by host species and stage of metamorphs (F= 5.85; df = 8, 62; P < 0.0001).The length of larval period, habitat duration and generation time of the protozoans are suggested as major modifiers of the protozoan densities. Hosts with long larval periods show a decreasing population density and hosts with short developmental periods show a pattern of increasing density. Neither pattern was detected in tadpoles from temporary sites. Metamorphic declines in protozoan density, but not necessarily the loss of protozoans, reflect metamorphic alterations of the gut common to all hosts.

Factors Affecting Population Fluctuations in Larval and Adult Stages of the Wood Frog (Rana Sylvatica)

During a 7—yr population on the wood frog, Rana sylvatica, the breeding population size fluctuated by a factor of 10 and juvenile production by a factor of 100. Variation in the adult population among years was largely due to variation in juvenile recruitment. Annual net replacement rates (Ro) varied from 0.009 to 7.49. Survivorship curves (calculated using the number of eggs deposited as the initial point) showed that most variation in the proportion of individuals surviving to adulthood was due to variation on larval survival; juvenile and adult survival was relatively constant among years. Male and female survival did not differ. Because females matured a year later than males, on average 2.3 times as many males as females from a given clutch survived to breed. This difference accounted for the observed male—biased sex ratio in breeding choruses. Premetamorphic survival and size at metamorphosis were negatively correlated with the number of eggs deposited. Length of larval period was positively correlated with number of eggs deposited. Survival was higher among juveniles that metamorphosed early and were large at metamorphosis. Larger juveniles matured earlier and were also larger as adults. The population appeared to be regulated through density—dependent factors affecting larval survival, larval size, and time of metamorphosis. Adult population size also negatively affected total clutch volume. Mean monthly rainfall positively affected adult survival.

Interactions between Larval Salamanders: A Case of Competitive Equality

We used a replicated field experiment to determine the relative intensity of intraspecific and interspecific competition between larvae of Siren intermedia and Notophthalmus viridescens. These two salamanders possess markedly different morphologies and life histories and belong to distantly related salamander families, but feed on similar prey. Each species was raised in artificial ponds, alone at either low density (3 larvae per pond, 1.95 m-2) or high density (6 larvae per pond, 3.90 m-2) or together in a 3:3 ratio for a combined density of 6 larvae per pond. These 5 treatments were each replicated 4 times in a total of 20 ponds. The experimental design allowed us to explicitly compare the relative intensities of intra- and interspecific competition under realistic conditions. Competition affected different components of fitness in each species. Both intraspecific and interspecific competition caused a decline in survival of Notophthalmus, but had no detectable effect on body size or length of larval period. Negative densitydependent mortality may regulate Notophthalmus populations in the larval stage of their complex life cycle. Intraspecific competition caused a decline in both the survival and growth of larval Siren, but interspecific competition caused only growth to decline. Decreased growth rates may increase the vulnerability of larval Siren to desiccation and to predation. We found no evidence to reject the null hypothesis that larvae of these two species are equal competitors at the densities we employed. Our results indicate that, despite dissimilarities in life history and morphology, and a distant phylogenetic relationship, larval Siren and larval Notophthalmus compete as equals. J.E. Fauth, W.J. Resetarits, Jr. and H.M. Wilbur, Dept of Zoology, Duke Univ., Durham, NC 27706, USA.

Effects of Larval Density in Ambystoma Opacum: An Experiment Large‐Scale Field Enclosures

This experiment was designed to measure the effects of larval density on larval traits in the salamander Ambystoma opacum, and to ascertain whether previous studies conducted at smaller spatial scales or higher densities produced artifactual results. Density effects on larval growth, body size at metamorphosis, length of larval period, and survival to metamorphosis were studied in A. opacum in large—scale (41 m2 and 23 m2) field enclosures in two temporary ponds. Each enclosure contained indigenous populations of prey (zooplankton and insects) and predators, as well as the range of microhabitats present in these natural ponds. Initial larval densities were chosen to represent high and low levels of naturally occurring mean densities. Larvae at high density grew more slowly, exhibited longer larval periods, were smaller at metamorphosis, and had reduced survival compared with those raised at the low density. At one site where experiments were conducted during three years, larval quality and quantity were enhanced in the year of longest hydroperiod. When ponds dried early, larvae metamorphosed sooner, were smaller, and exhibited lower survival. Slower growth at high densities probably increased mortality through an increased probability of desiccation in the early—drying years, and increased susceptibility to predation in all years. Larval survival in the enclosures was not elevated compared to natural ponds. as may occur in predator—free small enclosures and artificial ponds. These results suggest that, in natural ponds, the importance of intraspecific competition is dependent upon hydroperiod, and the intensity of competition influences predation risk. Thus, both density—dependent and density—independent factors affect body size and recruitment of larval A. opacum into the adult population. The use of large—scale field enclosures has advantages and disadvantages: it allows the examination of density—dependent process under natural conditions and provides high statistical power because of low variability in larval traits; however, experimental designs must be simple and underlying mechanisms are difficult to identify.

Life History Correlates of a Color Polymorphism in the Ornate Chorus Frog, Pseudacris ornata

The ornate chorus frog, Pseudacris ornata, is a small hylid from the coastal plain of the United States. The background color of most individuals is brown or gray, but those bearing a rare, dominant allele are bright green. Explanations for the persistance of such polymorphisms include selection on color itself and selection on traits genetically correlated with color. There is evidence that length of larval period is correlated with dorsal color or pattern in other frog species. However, individually raised P. ornata bearing the green allele do not differ from those not bearing the allele in length of larval period or in three other fitness-related traits: larval growth rate, size at metamorphosis, and juvenile growth rate. MANY species of frogs exhibit conspicuous polymorphisms for back pattern or color

Interannual variability in density, timing and survival of Alaskan red king crab Paralithodes camtschatica larvae

Larvae of the red king crab Paralithodes canltschatica were sampled with 5 replicate tows of a 1 0 m2 N I 0 net from March to July, every 2 wk in 1985 and weekly in 1986 through 1988, in Auke Bay, Alaska Interannual variation exlsted in hme of hatch, length of larval period, larval density, synchrony w t h the spring phytoplankton bloom, and survival Hatching occurred earher, and the penod larvae were present was longer, in years with warmer water temperatures. Length of the larval penod ranged from 56 d (1985) to 113 d (1988). Maximum density of stage 1 zoeae varied from 10 (1988) to 45 per 100 m3 (1987). Larval survival from stage 1 zoea to glaucothoe, estimated by instar analysis, ranged from 0.7 (1986) to 3 % (1988), and survival from stage 1 to stage 4 zoeae ranged from 12.6 % in 1986 to 14 9 ' ' # I in 1987. Hatching coincided with the sprlng phytoplankton bloom in 1985 and 1987, but not in 1986 or 1988. Survival was not directly related to temporal synchrony of king crab larvae with the spring phytoplankton bloom.

Interactive Effects of Dragonfly Predation in Expermental Pond Communities

One goal of ecology is to generate dynamic models that predict the species composition of complex natural communities. The general applicability of such models will depend on how often unpredictable higher order interactions occur between competitors and predators. I experimentally tested for interactions in a two predator—multiple prey community by manipulating the presence or absence of two species of predatory dragonfly larvae (Anax junius and Tramea carolina) in replicated outdoor artificial ponds containing identical number of four species of anuran tadpoles and a diverse zooplankton assemblage. The impact of predation on prey communities was measured by the relative abundances and larval performances (size at metamorphosis, length of larval period, and growth rate) of the anurans, and by the densities of zooplankton sampled at 10—d intervals. Zooplankton reached high densities in the experimental ponds (up to 2700 animals/L). Dragonfly predation had a marginally significant impact on zooplankton species composition, and the effects on zooplankton size structure were not significant. Predation by Anax and Tramea profoundly modified the species composition of anurans: the relative abundance of toads (Bufo americanus) increased in the presence of predators, as two hylid species (Hyla crucifer and Pseudacris triseriata) decreased. The interaction between Anax and Tramea on anuran species composition was additive. The performance of surviving larvae of two anurans (Bufo and Pseudacris) was significantly improved by predation (growth rate increased and larval period shortened), whereas the performance of two others (Hyla and Rana utricularia) showed no response to dragonflies. The absence of strong interactions between the predatory odonates implies that in some cases it may be valid to build models of complex ecological communities by additively combining results from more simple subsets of species.

HABITAT DURATION, LENGTH OF LARVAL PERIOD, AND THE EVOLUTION OF A COMPLEX LIFE CYCLE OF A SALAMANDER, AMBYSTOMA TEXANUM.

In many organisms, genotypic selection may be a less effective means of adapting to unpredictable environments than is selection for phenotypic plasticity. To determine whether genotypic selection is important in the evolution of complex life cycles of amphibians that breed in seasonally ephemeral habitats, we examined whether mortality risk from habitat drying in natural populations of small-mouthed salamanders (Ambystoma texanum) corresponded to length of larval period when larvae from the same populations were grown in a common laboratory environment. Comparisons were made at two levels of organization within the species: 1) among geographic races that are under strongly divergent selection regimes associated with the use of pond and stream habitats and 2) among populations within races that use the same types of breeding habitats. Morphological evidence indicates that stream-breeding A. texanum evolved from pond-breeding populations that recently colonized streams. Larvae in streams incur heavy mortality from stream drying, so the upper bound on length of larval period is currently set by the seasonal duration of breeding sites. We hypothesized that selection would reduce length of larval period of pond-breeders that colonize streams if their larval periods are inherently longer than those of stream-breeders. The results of laboratory experiments support this hypothesis. When grown individually in a common environment, larvae from stream populations had significantly shorter larval periods than larvae from pond populations. Within races, however, length of larval period did not correlate significantly with seasonal duration of breeding sites. When males of both races were crossed to a single pond female, offspring of stream males had significantly shorter larval periods than offspring of pond males. Collectively, these data suggest that differences in complex life cycles among pond and stream-breeders are due to genotypic selection related to mortality from habitat drying. Stream larvae in the common-environment experiment were significantly smaller at metamorphosis than pond larvae. Yet, the evolution of metamorphic size cannot be explained readily by direct selection: there are no intuitively obvious advantages of being relatively small at metamorphosis in streams. A positive phenotypic correlation was observed between size at metamorphosis and length of larval period in most laboratory populations. A positive additive genetic correlation between these traits was demonstrated recently in another amphibian. Thus, we suspect that metamorphic size of stream-breeders evolved indirectly as a consequence of selection to shorten length of larval period.

Habitat Duration, Length of Larval Period, and the Evolution of a Complex Life Cycle of a Salamander, Ambystoma texanum

Habitat Duration, Length of Larval Period, and the Evolution of a Complex Life Cycle of a Salamander, Ambystoma texanum

Regulation of Structure in Complex Systems: Experimental Temporary Pond Communities

Interactions among competition, predation, and disturbance in determining the abundances of four species of anurans were studied in a factoral experiment using 36 replicated experimental ponds. Hatchlings of the four species (Rana utricularia, Scaphiopus holbrooki, Bufo americanus, and Hyla chrysoscelis) were introduced at the same relative abundances at a low or high initial density. Competition determined survival, body size at metamorphosis, and length of larval period in high—density communities, which were food—limited. The predatory salamander Notophthalmus viridescens did not alter either the total number of metamorphs or their combined biomass in the high—density communities, but the relative abundance of metamorphs was shifted as Scaphiopus holbrooki, the competitive dominant, was selectively eaten. In low—density communities, predation decreased survival and biomass production of tadpoles, often to zero, in all species. For each of the four combinations of tadpole density and presence or absence of predators, ponds were drained at three rates to simulate different drying regimes. Competition slowed growth and thus increased the risk of desiccation in high—density populations in drying ponds. Predation ameliorated the effects of competition, allowing survivors to grow rapidly enough to metamorphose before ponds dried. Survival of tadpoles to metamorphosis, body size at metamorphosis, and the timing of metamorphosis were determined in a complex way by interactions among all of the treatment effects and the life history characteristics of the four species studied. Scaphiopus was the species least sensitive to tadpole density and was the competitive dominant in ponds without newts. It has a rapid growth rate and metamorphosed soon enough to escape desiccation. It suffered the greatest risk of predation and was eliminated from even some of the high—density communities. Rana was most successful in low—density communities without predators. No Rana survived in rapidly drying (50—d) ponds because of insufficient time to obtain a minimum size for metamorphosis. Rana were also eliminated from most populations exposed to predation. The effect of competition on Bufo in high—density populations, few or no survivors, was reversed by predation as newts selectively fed on Scaphiopus and Rana. This result was most striking in the tanks that dried most rapidly. Hyla did very poorly in all slowly drying (100—d) tanks compared with controls because of intense competition. It had moderate success in high—density communities where newts had removed most competitors. These results show that biological and environmental factors interact to determine the structure of anuran communities. Neither competition no predation is the single unifying force, but rather they interact to determine the different consequences of the date of drying of a pond to the success of each species.

A QUANTITATIVE-GENETIC ANALYSIS OF LARVAL LIFE-HISTORY TRAITS IN HYLA CRUCIFER.

We used a half-sib design to examine the genetic components of phenotypic variance in several life-history traits in Hyla crucifer. Egg viability, hatchling size, larval growth rate, length of larval period, and size at metamorphosis play critical roles in determining survivorship and are subject to persistent selection. Egg viability varied among families considerably, with most embryo mortality occurring between gastrulation and neurulation. Hatchling size was the only trait in which maternal effects were influential. Dominance genetic variance played the predominant role in determining phenotypic variance in hatchling size, growth rate, and length of larval period, accounting for, respectively, 70, 63, and 47% of the total variance. Size at metamorphosis displayed little dominance genetic variance and, unlike the other traits, displayed a high heritability. All additive genetic correlations between traits were positive. The directions of environmental correlations were the same as the directions of changes that have been induced in previous experimental work. The correlations due to dominance effects described a principal axis that independent ecological studies indicate to be directly correlated with fitness. These results agree with theoretical expectations for traits under consistent directional selection.

Anuran Size at Metamorphosis: Experimental Test of a Model Based on Intraspecific Competition

Size at metamorphosis and length of larval period are often, but not invariably, correlated within cohorts of larval anurans. Collins (1979) proposed intraspecific competition as one causative agent for such a correlation in nature, and this hypothesis was supported by observational data. This study reports the results of an experimental test of that hypothesis. Isolated larvae were reared at one of two food levels (high or low) and, during the period in which metamorphosis occurred, were maintained at either constant per capita food levels, or at increasing per capita levels designed to mimic effects of continual release from scramble—type, exploitative competition. Low food levels decreased average larval growth and average size at metamorphosis and increased average length of larval period. A reduced level of food had no effect on the relative variation in length of larval period. Only competitive release affected the relative variation in size at metamorphosis. All four treatments displayed a positive correlation between size at metamorphosis and length of larval period, but only in the combination of low food levels and competitive release was the correlation more than barely significant. Size at metamorphosis displayed a positive allometric relationship to length of larval period in the low food—release treatment, and a negative allometric relationship in the other three treatments. These results support Collins' (1979) model for intraspecific competition as one causative mechanism for such relationships. Certain aspects of these results support each of two distinct models for the timing of amphibian metamorphosis, but the data as a whole do not fit well with either model.