Competitive ability and the coexistence of anuran larvae in freshwater rock‐pools

• 1 I examined the competitive ability of larval common frogs (Rana temporaria) and common toads (Bufo bufo) in a laboratory experiment by manipulating the number and size of Rana and the number of Bufo.
• 2 Bufo was the most active tadpole type, whereas small Rana was more active than large Rana.
• 3 Especially at high food availability, the growth rate of large Rana at metamorphosis was reduced in the presence of small Rana, whereas that of small Rana was not affected by larger conspecifics.
• 4 Both size classes of Rana were negatively affected by the presence of Bufo. Size at metamorphosis of Bufo was greater when large Rana was present. These results suggest that activity level is important in determining competitive ability.
• 5 To study competition in the field, tadpoles of Bufo were introduced to small rock‐pools that did not support Bufo naturally, but were with or without Rana.
• 6 In the experimental pools, there were no interspecific effects on the growth rate of either species. However, the growth rate of Bufo was lower in the experimental pools than in pools inhabited naturally. Futher, survival of Bufo was low in small pools. Lower food availability or physical factors associated with small pools, rather than interspecific competition, may be the reason why Bufo avoids breeding in small rock‐pools.

     

金线蛙入蛰时几项生理常数间的关系研究

本文经统计学分析处理,表明金钱蛙（Ｒａｎａｐｌａｎｃｙｉ）入蛰时的体长与体重、胴体重和体重与肝重、脂肪体重、生殖腺重之间均存在非常显著的线性相关关系。     

Effects of changing climate on aquatic habitat and connectivity for remnant populations of a wide‐ranging frog species in an arid landscape

Amphibian species persisting in isolated streams and wetlands in desert environments can be susceptible to low connectivity, genetic isolation, and climate changes. We evaluated the past (1900–1930), recent (1981–2010), and future (2071–2100) climate suitability of the arid Great Basin (USA) for the Columbia spotted frog (Rana luteiventris) and assessed whether changes in surface water may affect connectivity for remaining populations. We developed a predictive model of current climate suitability and used it to predict the historic and future distribution of suitable climates. We then modeled changes in surface water availability at each time period. Finally, we quantified connectivity among existing populations on the basis of hydrology and correlated it with interpopulation genetic distance. We found that the area of the Great Basin with suitable climate conditions has declined by approximately 49% over the last century and will likely continue to decline under future climate scenarios. Climate conditions at currently occupied locations have been relatively stable over the last century, which may explain persistence at these sites. However, future climates at these currently occupied locations are predicted to become warmer throughout the year and drier during the frog's activity period (May – September). Fall and winter precipitation may increase, but as rain instead of snow. Earlier runoff and lower summer base flows may reduce connectivity between neighboring populations, which is already limited. Many of these changes could have negative effects on remaining populations over the next 50–80 years, but milder winters, longer growing seasons, and wetter falls might positively affect survival and dispersal. Collectively, however, seasonal shifts in temperature, precipitation, and stream flow patterns could reduce habitat suitability and connectivity for frogs and possibly other aquatic species inhabiting streams in this arid region.     

黑斑蛙肾的组织结构

应用组织学和组织化学方法研究了黑斑蛙(Rana nigromaculata)肾的显微结构。结果表明,黑斑蛙的肾除具有肾单位和集合小管外,还见有淋巴样组织分散于肾实质中。在肾腹侧发现有与真骨鱼类斯坦尼斯小体相似的结构,其中聚集有较多的肥大细胞。说明黑斑蛙的肾具有多种生理功能。     

Novel Rana keratin genes and their expression during larval to adult epidermal conversion in bullfrog tadpoles

The conversion of the larval to adult epidermis during metamorphosis of tadpoles of bullfrog, Rana catesbeiana, was investigated utilizing newly cloned Rana keratin cDNAs as probes. Rana larval keratin (RLK) cDNA (rlk) was cloned using highly specific antisera against Xenopus larval keratin (XLK). Tail skin proteins of bullfrog tadpoles were separated by 2-dimensional gel electrophoresis and subjected to Western blot analysis with anti-XLK antisera. The Rana antigen detected by this method was sequenced and identified as a type II keratin. We cloned rlk from tadpole skin by PCR utilizing primers designed from these peptide sequences of RLK. RLK predicted by nucleotide sequences of rlk was a 549 amino acid -long type II keratin. Subtractive cloning between the body and the tail skin of bullfrog tadpole yielded a cDNA (rak) of Rana adult keratin (RAK). RAK was a 433 amino acid-long type I keratin. We also cloned a Rana keratin 8 (RK8) cDNA (rk8) from bullfrog tadpole epidermis. RK8 was 502 amino acid-long and homologous to cytokeratin 8. Northern blot analyses and in situ hybridization experiments showed that rlk was actively expressed through prometamorphosis in larva-specific epidermal cells called skein cells and became completely inactive at the climax stage of metamorphosis and in the adult skin. RAK mRNA was expressed in basal cells of the tadpole epidermis and germinative cells in the adult epidermis. The expression of rlk and rak was down- and up-regulated by thyroid hormone (TH), respectively. In contrast, there was no change in the expression of RK8 during spontaneous and TH-induced metamorphosis. RK8 mRNA was exclusively expressed in apical cells of the larval epidermis. These patterns of keratin gene expression indicated that the expression of keratin genes is differently regulated by TH depending on the type of larval epidermal cells. The present study demonstrated the usefulness of these genes for the study of molecular mechanism of postembryonic epidermal development and differentiation.     

Osmotic and metabolic responses to dehydration and urea-loading in a dormant,terrestrially hibernating frog

Physiological responses to dehydration in amphibians are reasonably well documented, although little work has addressed this problem in hibernating animals. We investigated osmotic and metabolic responses to experimental manipulation of hydration state in the wood frog (Rana sylvatica), a terrestrial hibernator that encounters low environmental water potential during autumn and winter. In winter-conditioned frogs, plasma osmolality varied inversely with body water content (range 69–79%, fresh mass) primarily due to increases in sodium and chloride concentrations, as well as accumulation of glucose and urea. Decreased hydration was accompanied by a marked reduction in the resting rate of oxygen consumption, which was inversely correlated with plasma osmolality and urea concentration. In a separate experiment, resting rates of oxygen consumption in fully hydrated frogs receiving injections of saline or saline containing urea did not differ initially; however, upon dehydration, metabolic rates decreased sooner in the urea-loaded frogs than in control frogs. Our findings suggest an important role for urea, acting in concert with dehydration, in the metabolic regulation and energy conservation of hibernating R. sylvatica.     

Canopy Closure,Competition, and the Endangered Dusky Gopher Frog

Abstract: A major challenge facing wildlife biologists is understanding why some species go extinct while others persist in the same habitat. To address this question, we investigated whether tree canopy closure over ponds affects growth and survival of rare and common tadpoles within ponds and mediates competitive interactions among species. We conducted 2 experiments to test whether canopy closure and competition may have contributed to the decline of the endangered dusky gopher frog (Rana sevosa), but allowed the persistence of the southern leopard frog (R. sphenocephala). We explored the response of both species to canopy closure in single-species and mixed- (1:1) species treatments of identical total tadpole density. An experiment using aquatic enclosures in temporary ponds showed that canopy closure reduced tadpole growth approximately 20% for both species. Survival of dusky gopher frog tadpoles was higher in mixed-species enclosures than in single-species enclosures. In a complementary experiment using artificial ponds, dusky gopher frogs had lower survival to metamorphosis, reduced size at metamorphosis, and produced a lower total biomass of metamorphosed juveniles in shaded ponds. Southern leopard frogs exhibited reduced body size at metamorphosis only when shaded. These studies suggest that pond canopy closure, not larval competition, may be contributing to the decline of the dusky gopher frog. The different responses to canopy closure suggest a potential mechanism for the loss of dusky gopher frogs and the persistence of southern leopard frogs. Removal of trees from historically open-canopy ponds may help facilitate the recovery of dusky gopher frogs and benefit similar species.     

同水塘分布两种无尾两栖类蝌蚪的竞争策略

实验室条件下,通过活动性水平,变态时的体重、增长率和完成变态所需时间考察同水塘分布的中华蟾蜍(Bufo gargarizans)和高原林蛙蝌蚪(Rana kukunoris)的竞争策略。实验按照2×3因子设计,即:食物资源2个水平(高、低),组合方式3个水平(10只中华蟾蜍蝌蚪,记为B组;5只中华蟾蜍蝌蚪和5只高原林蛙蝌蚪,记为BR组;10只高原林蛙蝌蚪,记为R组)。中华蟾蜍蝌蚪的活动性在食物水平低时显著低于食物水平高时,而高原林蛙蝌蚪的活动性在不同食物水平下无显著差异;食物水平低时,混合组的高原林蛙蝌蚪变态时体重和体重增长率都显著高于R组,而混合组中华蟾蜍蝌蚪与B组相比无显著差异;在不同处理组中,食物水平低时混合组中华蟾蜍蝌蚪幼体期最短。这些结果表明:中华蟾蜍蝌蚪在不同食物资源条件下所选择的生存策略可能不同,即食物资源充足时,增加活动性获取更多食物;食物资源有限时,降低活动性且提前完成变态;与中华蟾蜍蝌蚪相比,在食物资源有限时高原林蛙蝌蚪获取食物能力更强。     

Non-equivalence of growth arrest induced by predation risk or food limitation: context-dependent compensatory growth in anuran tadpoles

1. To gain insight into the evolution of compensatory growth, we studied the growth patterns of anuran (Rana temporaria) larvae following either a period of exogenous growth depression (food restriction) or a period of endogenous depression (exposure to predators). We also investigated the potential deferred costs that larval compensatory growth could impose on post-metamorphic individuals. 2. Food-deprived larvae exhibited full compensatory growth in response to reduced growth rates caused by food limitation, and the growth trajectories of low- and high-rations tadpoles converged before the onset of metamorphosis. 3. According to our predictions, individuals exposed to larval predators did not show growth compensation following predator removal despite undergoing a significant reduction in growth rate associated with low activity levels. 4. Jumping ability of individuals exposed to predators during only 20 days from the commencement of the larval phase was equivalent to that of non-exposed animals, and greater than the jumping capacity of those maintained with predators until the time of metamorphosis. This pattern was consistent with the pattern observed for variation in relative leg length. 5. These results support the suggestion that submaximum and compensatory growth could have evolved to minimize the overall growth/mortality costs in environments with high spatiotemporal variation in predation intensity.