Effects of head shape variation on growth, metamorphosis and survivorship in larval salamanders (Hynobius retardatus)

The effects of head shape variation on growth and metamorphosis in larval salamander (Hynobius retardatus) were examined by a laboratory experiment and a field experiment. In the laboratory experiment, each larva was fed equal amounts and was prevented from accessing others in both the solitary and group treatments, although chemical cues could be transmitted through water in the group treatment. The relative head width of larvae became larger in the group treatment during the early periods but having a large head width did not finally influence growth rate and days for metamorphosis. In the field experiment, larvae were allowed to contact each other directly in two density conditions. The enlarged relative head width was linked to high growth rate in the high-density treatment but not in the low-density treatment. The larval body size distribution in the high-density condition tended to be smaller, and there was a small proportion of large-sized individuals with a broad head width. Moreover, the small number of large larvae metamorphosed much earlier than the others. The mortality of larvae in high-density conditions was much higher than that in the low-density treatments. This would be a consequence of cannibalism in the high-density condition. From the experimental results obtained, it is argued that for the larvae of H. retardatus having a large head is an adaptive tactic that maximizes fitness, particularly in temporary ponds with an unpredictable environment and in crowded conditions.     

Spatial variation in abiotic and biotic factors in a floodplain determine anuran body size and growth rate at metamorphosis

Body size at metamorphosis is a critical trait in the life history of amphibians. Despite the wide-spread use of amphibians as experimental model organisms, there is a limited understanding of how multiple abiotic and biotic factors affect the variation in metamorphic traits under natural conditions. The aim of our study was to quantify the effects of abiotic and biotic factors on spatial variation in the body size of tadpoles and size at metamorphosis of the European common toad (Bufo b. spinosus). Our study population was distributed over the riverbed (active tract) and the fringing riparian forest of a natural floodplain. The riverbed had warm ponds with variable hydroperiod and few predators, whereas the forest had ponds with the opposite characteristics. Spatial variation in body size at metamorphosis was governed by the interactive effects of abiotic and biotic factors. The particular form of the interaction between water temperature and intraspecific tadpole density suggests that abiotic factors laid the foundation for biotic factors: intraspecific density decreased growth only at high temperature. Predation and intraspecific density jointly reduced metamorphic size. Interspecific density had a negligible affect on body size at metamorphosis, suggesting weak inter-anuran interactions in the larval stage. Population density at metamorphosis was about one to two orders of magnitudes higher in the riverbed ponds than in the forest ponds, mainly because of lower tadpole mortality. Based on our results, we conclude that ponds in the riverbed appear to play a pivotal role for the population because tadpole growth and survival is best in this habitat.     

Temperature-dependent variation in Anopheles merus larval head capsule width and adult wing length: implications for anopheline taxonomy

Abstract. Seasonal variations in mosquito larval head capsule width and adult female wing length were investigated in a field population of Anopheles merus Donitz at Nceswana Lake, Ophansi, within the endemic malaria area of Natal, South Africa. An inverse relationship was detected between each of these morphological characters and seasonal fluctuations in air/water temperatures. Mean head capsule width in all larval instars decreased by 4.8-7.9% in summer, while mean wing length decreased by 19.6%. These changes are discussed in relation to the annual range in mean air temperature in southern Africa and the distribution of An. merus. Implications for the use of such morphological characteristics in existing taxonomic keys are discussed.     

Effects of induced variation in anuran larval development on postmetamorphic energy reserves and locomotion

Anuran larvae exhibit high levels of phenotypic plasticity in growth and developmental rates in response to variation in temperature and food availability. We tested the hypothesis that alteration of developmental pathways during the aquatic larval stage should affect the postmetamorphic performance of the Iberian painted frog (Discoglossus galganoi). We exposed tadpoles to different temperatures and food types (animal- vs. plant-based diets) to induce variation in the length of the larval period and body size at metamorphosis. In this species, larval period varied with temperature but was unaffected by diet composition. In contrast, size at metamorphosis was shaped by the interaction between food quality and temperature; tadpoles fed on an animal-based diet became bulkier metamorphs than those fed on plant-based food at high (22°C) but not at low (12°C) temperature. Body condition of newly metamorphosed frogs was unrelated to the temperature or food type experienced during the premetamorphic stage. Frogs maintained at high temperature during the larval period showed reduced jumping ability, especially when fed on the plant-based diet. However, when considering size-independent jumping ability, cold-reared individuals exhibited the lowest performance, and herbivores reared at 17°C the highest. Cold-reared (12°C) frogs accumulated larger amounts of energy reserves than individuals raised at 17°C or 22°C. This was still the case after correction for differences in body mass, thus indicating some size-independent effect of developmental temperature. Despite the higher lipid content of the carnivorous diet, the differences in energy reserves between herbivores and carnivores were relatively weak and associated with differences in body size. These results suggest that the consequences of environmental variation in the larval habitat can extend to the terrestrial phase and influence juvenile growth and survival.     

Regional specificity of anuran larval skin during metamorphosis: dermal specificity in development and histolysis of recombined skin grafts

Summary Skins from back and tail were dissected from tadpoles of Rana japonica prior to resorption of the tail and separated into epidermis and dermis by treatment with neutral protease. Homotypically and heterotypically recombined skins were constructed from the separated epidermis and dermis and transplanted into the tail of the original tadpole. Skin grafts using dermis from tail region degenerated simultaneously with resorption of the tail. However, skin grafts containing dermis from back region survived on the posterior part of the juvenile frog beyond metamorphosis. Furthermore, all epidermis underlaid with dermis from back region formed secretory glands and became flattened epithelia characteristic of adult back skin, regardless of region from which the epidermis came. Even when epidermis isolated from tail skin was cultured inside a back skin graft, the tail epidermis survived forming an epithelial cyst and developed secretory glands. These results suggest that regional specificities of anuran larval skin, i.e., development of back skin and even histolysis of tail skin, are determined by regionally specific dermis. The results also suggest that some of epidermal cells of tail skin are able to differentiate into epithelial cells similar to back skin of the adult under the influence of back dermis.     

Effects of preservation on pigmentation and length measurements in larval lampreys

The effects of two methods of preservation (fixation and storage in 10% formalin, and fixation in 10% formalin followed by storage in 95% alcohol) on pigmentation and morphometric features used for identification of larval Ichthyomyzon lampreys were analysed. Both short‐term (3 weeks) and long‐term (6 months) studies were conducted using digital analysis of images of fresh and preserved lampreys. Six standard morphometric lengths and 10 areas of pigmentation were analysed. All measurements were significantly affected by preservation. Preservative type affected pigmentation and morphometric characteristics differently, and characters were affected to different degrees. Multiple measurements over time showed that almost all changes occurred within 3 weeks of preservation. Regression equations allowed for accurate correction of preservation effects on morphometric measurements, but the effects on pigmentation levels were less predictable. Effects of preservation on larval lampreys need to be considered when comparing fresh and preserved specimens because they influence critical identification features.     

Interspecific variation in larval anuran anti-parasite behavior: a test of the adaptive plasticity hypothesis

Identifying drivers of interspecific differences in trait plasticity is a major goal in ecology and evolution. For instance, understanding why species invest in constitutive or induced defenses against pathogens is critical for developing accurate models of host-parasite interactions. The adaptive plasticity hypothesis (APH) suggests that, due to costs associated with plasticity, species with greater heterogeneity in their association with an enemy should be more likely to exhibit a plastic (i.e., induced) defense. Here, I tested whether the APH can explain variation among eight co-occurring anuran species in a plastic defense (change in activity level after parasite exposure) against a common trematode parasite (Digenea: Echinostomatidae). The species examined vary in life history, habitat use and phenology—traits that influence the frequency of encounters with parasites in natural ponds. Laboratory experiments were used to measure the proportional change in species’ activity in response to parasites and infection levels, and experimental results were then coupled to data from a field survey. Consistent with the APH, the activity change was greatest for species that vary most in their association with parasites, even when accounting for species phylogeny. Habitat use may thus have influenced the evolution of parasite avoidance, comparable to a similar pattern in species’ defenses against predators. Infection levels, however, correlated with species’ baseline activity levels rather than the change in activity post-exposure. General activity levels may thus contribute more strongly to species-level differences in infection rates than plasticity. Overall, these findings suggest that the APH and consideration of behavior generally enhances understanding of interspecific variation in defenses and susceptibility to parasitism, with implications for community-scale interactions and amphibian conservation.     

Plasticity in newt metamorphosis: the effect of predation at embryonic and larval stages

1. Some organisms under variable predator pressure show induced antipredator defences, whose development incurs costs and may be associated with changes to later performance. This may be of especial relevance to animals with complex life histories involving metamorphosis. 2. This study examines the effect of predation environment, experienced both during embryonic and larval stages, on palmate newt (Triturus helveticus) metamorphosis. Newt eggs were raised until hatching with or without exposure to chemical cues from brown trout (Salmo trutta), and larval development was monitored in the presence or absence of the cues. 3. Exposure to predator cues during the embryonic stage resulted in higher growth rates at the larval stage, reduced time to metamorphosis and size at metamorphosis. Metamorphs also had narrower heads and shorter forelimbs than those from predator‐free treatments. In contrast, exposure to predator cues during the larval stage did not affect metamorph characteristics. 4. These results indicate that developing embryos are sensitive to predator chemical cues and that the responses can extend to later stages. Reversion of induced defences when predation risk ceased was not detected. We discuss the possible adaptive significance of these responses.     

Development of contractile and energetic capacity in anuran hindlimb muscle during metamorphosis

Anuran larvae undergo water-to-land transition during late metamorphosis. We investigated the development of the iliofibularis muscle in bullfrog tadpoles (Rana catesbeiana) between Gosner's stage 37 and stage 46 (the last stage). The tadpoles began staying in shallow water at least as early as stage 37, kicking from stage 39, active hindlimb swimming from stage 41, and emerging onto shore from stage 42. For control tadpoles kept in water throughout metamorphosis, muscle mass and length increased two- to threefold between stages 37 and 46, with rapid increases at stage 40. Large, steady increases were found in femur mass, tetanic tension, contraction rate, and power between stages 37 and 46. Concentrations of ATP and creatine phosphate and rates of the phosphagen depletion and the activity of creatine kinase increased significantly, mainly after stage 43. Shortening velocity, tetanic rise time, and half-relaxation time varied little. Energy charge (the amount of metabolically available energy stored in the adenine nucleotide pool) remained unchanged until stage 43 but decreased at stage 46. Compared with the control, experimental tadpoles that were allowed access to both water and land exhibited 1.2- to 1.8-fold greater increases in femur mass, tetanic tension, power, phosphagen depletion rates, and creatine kinase activities at late metamorphic stages but no significant differences for other parameters measured. In sum, most hindlimb development proceeds on the basis of the increasingly active use of limbs for locomotion in water. The further increases in tension, mechanical power, and "chemical power" on emergence would be advantageous for terrestrial antigravity performance.     

Mass,length and growth rate in single cells

A heuristic model for the time course of elongation and mass increase of a population of single cells with generation times different from the mean is considered. The fundamental assumption is that the dynamics governing the fluctuations around the mean are the same as those describing the mean itself. Cell length is assumed to depend on cell mass by a $\text{1}\text{3}$ power law. The final cell length and mass are obtained by demanding that a daughter cell's initial length and mass obey the same equation as those of the mother cell when the mother cell was born. An exponential time course is assumed for simplicity, although most of the results depend only on the initial and final values, not the actual time behavior. The aspect ratio (length/radius) is found to be a constant for all cells at birth, and twice that value at maturity. Thus the individual generation time is a doubling time for the aspect ratio, even though neither mass nor length double in general. Oscillations in ancestor/progeny generation times are derived, and their stability considered. The model yields a non-hereditary determinant of individual generation time. The well-known skewness in the distribution of individual generation times and the “β-plots” of the transition probability model find a natural explanation in the inequality of cell division.     

Bone growth in length and width: the Yin and Yang of bone stability

Bone growth in length is primarily achieved through the action of chondrocytes in the proliferative and hypertrophic zones of the growth plate. Longitudinal growth is controlled by systemic, local paracrine and local mechanical factors. With regard to the latter, a feedback mechanism must exist which ensures that bone growth proceeds in the direction of the predominant mechanical forces. How this works is unknown at present. Bone growth in length is detrimental to bone stability, but this effect is counteracted by concomitant bone growth in width. This occurs through periosteal apposition, which is the responsibility of periosteal osteoblasts. The action of these cells is mainly controlled by local factors, with modulation by systemic agents. According to the mechanostat theory, periosteal apposition is regulated by mechanical requirements. An alternative model, called sizostat hypothesis, maintains that a master gene or set of genes regulate bone growth in width to reach a pre-programmed size, independent of mechanical requirements. The virtues of these two hypotheses have been the subject of much discussion, but experimental data are scarce. Future research will have to address the question how periosteal bone cells manage to integrate mechanical, hormonal and other input to shape bones that are as strong as they need to be.     

Effects of food abundance on genetic and maternal variation in the growth rate of juvenile red squirrels

Sources of variation in growth in body mass were assessed in natural and experimental conditions of high and low food abundance using reciprocal cross-fostering techniques and long-term data (1987-2002) for a population of North American red squirrels (Tamiasciurus hudsonicus). Growth rates were significantly higher in naturally good and food supplemented conditions, than in poor conditions. Mother-offspring resemblance was higher in poor conditions as a result of large increases in both the direct genetic variance and direct-maternal genetic covariance and a smaller increase in the coefficient of maternal variation. Furthermore, the genetic correlation across environments was significantly less than one indicating that sources of heritable variation differed between the two environments. These results are consistent with the hypothesis that selection has eroded heritable variation for growth more in good conditions and indicate the potential for independent adaptation of growth rates in good and poor conditions.     

Effects of imaging conditions on mitochondrial transport and length in larval motor axons of Drosophila

The distribution of mitochondria is sensitive to physiological stresses and changes in metabolic demands. Consequently, it is important to carefully define the conditions facilitating live imaging of mitochondrial transport in dissected animal preparations. In this study, we examined Schneider's and the haemolymph-like solutions HL3 and HL6 for their suitability to image mitochondrial transport in motor axons of dissected Drosophila melanogaster larvae. Overall, mitochondrial transport kinetics in larval motor axons appeared similar among all three solutions. Unexpectedly, HL3 solution selectively increased the length of mitochondria in the context of the net-direction of transport. We also found that mitochondrial transport is sensitive to the extracellular Ca(2+) but not glutamate concentration. High concentrations of extracellular glutamate affected only the ratio between motile and stationary mitochondria. Our study offers a valuable overview of mitochondrial transport kinetics in larval motor axons of Drosophila under various conditions, guiding future studies genetically dissecting mechanisms of mitochondrial transport.     

Influence of temperature on developmental rate,wing length,and larval head capsule size of pestiferous midge Chironomus crassicaudatus (Diptera: Chironomidae)

Larvae of Chironomus crassicaudatus Malloch were reared individually at nine constant temperatures from 12.5 to 32.5 degrees C (2.5 degrees C increments) for 120 d. Duration of immature stages (egg, four instars, and pupa), head capsule width of fourth instars, and wing length were recorded. Some adults emerged at all temperatures, except at 12.5 degrees C where individuals developed to fourth instars during the experiment. Sharpe and DeMichele's four-parameter model with high-temperature inhibition described the temperature-dependent developmental rates. The slowest development was observed at 15 degrees C, with developmental rate peaking between 25 and 27.5 degrees C. Developmental rate increased rapidly with increasing temperature up to 20 degrees C, slowed between 20 and 27.5 degrees C, and decreased at temperatures >27.5 degrees C. No developmental inhibition at high temperatures was observed in eggs. The most apparent high-temperature inhibition of development was recorded in fourth instars, which comprised the largest proportion of developmental time. Males developed faster than females, but females had wider larval head capsules and longer wings than males. Adult size was negatively related with temperature in both sexes, but this relationship was steeper in males than in females. Larval size peaked at 20 degrees C, whereas the head capsule width was reduced at temperatures higher and lower than 20 degrees C.