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Living in a Japanese onsen: Field observations and physiological measurements of hot spring amphibian tadpoles, Buergeria japonica


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The Japanese stream tree frog, Buergeria japonica, is widely distributed across the southern islands of Japan and Taiwan. While the species is known to inhabit hot springs, this has only been reported in Taiwan. To further understand the utilization of hot springs by B. japonica, we conducted field observations of tadpoles from a hot spring on Kuchinoshima Island, a tiny volcanic island of southwestern Japan. We found that tadpoles on Kuchinoshima Island inhabited hot spring pools with extremely high temperatures that exceeded temperatures in which any other amphibians have been found. In addition, we conducted thermal tolerance measurements and found that the thermal tolerance of B. japonica tadpoles was high. These findings suggest that high thermal tolerance of B. japonica is maintained even at the northern tip of its distribution, and this has allowed them to widen their available niche and inhabit a hot spring on the tiny island of Kuchinoshima.
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Living in a Japanese onsen: field observations and physiological measurements of hot spring
amphibian tadpoles, Buergeria japonica
Shohei Komaki1, Quintin Lau2, Takeshi Igawa3
1Global Career Design Center, Hiroshima University, 1-7-1, Higashi-Hiroshima, Hiroshima 739-
8514, Japan
2Department of Evolutionary Studies of Biosystems, Sokendai (The Graduate University for
Advanced Studies), 1560-35, Hayama, Kanagawa 240-0193, Japan
3Division of Developmental Science, Graduate School of International Development and
Cooperation, Hiroshima University, 1-5-1, Higashi-Hiroshima, Hiroshima 739-8529, Japan
Manuscript type: short note
Word counts: 2224 (whole manuscript), 151 (abstract)
Corresponding author:
Shohei Komaki
Global Career Design Center, Hiroshima University, 1-7-1, Higashi-Hiroshima, Hiroshima 739-
8514, Japan
Iwate Medical University
The Japanese stream tree frog, Buergeria japonica, is widely distributed across the southern
islands of Japan and Taiwan. While the species is known to inhabit hot springs, this has only
been reported in Taiwan. To further understand the utilization of hot springs by B. japonica, we
conducted field observations of tadpoles from a hot spring on Kuchinoshima Island, a tiny
volcanic island of southwestern Japan. We found that tadpoles on Kuchinoshima Island inhabited
hot spring pools with extremely high temperatures that exceeded temperatures in which any
other amphibians have been found. In addition, we conducted thermal tolerance measurements
and found that the thermal tolerance of B. japonica tadpoles was high. These findings suggest
that high thermal tolerance of B. japonica is maintained even at the northern tip of its distribution,
and this has allowed them to widen their available niche and inhabit a hot spring on the tiny
island of Kuchinoshima.
Geothermal hot spring; Rhacophorus; thermal tolerance; volcanic island
Main text
The distribution of ectotherms is critically affected by their surrounding thermal environment.
This is important for amphibians that occupy aqueous and terrestrial environments throughout
their lifecycles (Wells, 2007). Specifically, water temperature strongly affects amphibian
survival rate during early developmental stages, whereby most tadpoles generally avoid very
cold or hot waters (Wells, 2007). However, a few studies have reported the occurrences of
amphibian species in hot springs (Brues, 1932; Mason, 1939; Chen et al., 2001; Wu and Kam,
In central and northern Taiwan, tadpoles of the Japanese stream tree frog, Buergeria japonica,
reportedly inhabit hot springs (called “onsen” in Japanese) [Jentse, Rushan, and Wulai hot
springs (Chen et al., 2001; Wu and Kam, 2005)]. These populations showed extremely high
thermal tolerance (> 40 °C) and preferred water temperatures around 37 °C (Chen et al., 2001;
Wu and Kam, 2005), which was assumed to enable them to inhabit the hot springs (Chen et al.,
2001). This species is widely distributed almost throughout the Ryukyu Archipelago of Japan
and Taiwan, and is characterized as the only naturally occurring amphibian species in the islands
of the Tokara Archipelago (Maenosono and Toda, 2007). Recently, Komaki et al. (2016)
investigated thermal and salinity tolerances of multiple populations of this species
(Amamioshima and Tokashikijima Islands of Japan and Yilan and Hualien in Taiwan) and
confirmed that the thermal tolerances have been maintained across island populations. Due to
small island sizes and volcanic origin, naturally forming freshwater resources in Tokara Islands
may be limited predominantly to hot springs. Therefore, the high thermal tolerance of B.
japonica may have allowed the species to utilize the hot springs as the site for reproduction and
survive on these islands prior to modern-human colonization which brought upon freshwater
sources with lower temperatures (Komaki et al., 2016). However, the thermal tolerance and
inhabitance in hot springs of Tokara populations were not confirmed. To better understand the
utilization of hot springs by B. japonica, we performed field observations and measurements of
thermal tolerance of B. japonica tadpoles from a hot spring located on an island of the Tokara
On 9th September 2015, we performed field observations in Seramma Onsen (Seramma Hot
Spring), a geothermal hot spring on Kuchinoshima Island (29° 57.32N 129° 55.63E, Fig. 1),
and found Buergeria japonica tadpoles inhabiting the hot spring. The study area included
shallow and narrow streams and small pools of hot spring water (Fig. 1). The chains of small
shallow pools (< 1.5 m in diameter and < 10 cm in depth) within the streams were formed by
fallen leaves, branches, or stones that dammed up the water. In the water pools, few tadpoles
were readily visible, thus, we searched for individuals hidden under fallen leaves or stones in the
pools using landing nets, and inferred the existence and abundance of tadpoles. We measured
water temperatures of each pool and site tadpoles were found, using two electrical thermometers
(Japan Pet Design Co., Ltd. Tokyo, Japan) for accurate measurements. Water temperatures were
remarkably variable between pools, ranging from 34.6 to 51.8 °C. Water temperature was higher
in pools closer to the hot spring sources. The highest temperature of a pool inhabited by B.
japonica was 46.1 °C, but only a single tadpole was found. The maximum water temperature of
pools in which multiple (at least four) tadpoles were found was 41.5 °C. There were also cooler
pools that ranged from 34.6 to 37.2 °C, each inhabited by more than 20 tadpoles. No dead
tadpoles, juveniles or adults were found.
The occurrence of B. japonica tadpoles in the geothermal hot spring on Kuchinoshima Island
indicates that habitation in hot springs is not confined to the populations in Taiwan. Since
sustained high thermal tolerance of B. japonica tadpoles among northern and central Ryukyu
Archipelago and Taiwan were revealed by other studies (Chen et al., 2001; Wu and Kam, 2005;
Komaki et al., 2016), the potential use of hot springs may be maintained across their distribution,
specifically to the northern tip of its distribution, Kuchinoshima Island. As tadpoles in Seramma
tended to occur in cooler pools at temperatures below 37 °C, their thermal preference is similar
to those of the Taiwanese populations (Wu and Kam, 2005).
We observed no tadpoles swimming across to neighbouring pools, but most tadpoles were
under fallen leaves in each pool. Therefore, it is possible that tadpoles had stayed for extensive
periods at the location observed, whereby individuals did not migrate frequently between pools.
However, a single individual that was found in 46.1 °C may have accidentally or temporarily
drifted from neighbouring cooler pools. Nevertheless, the identification of multiple tadpoles at
41.5 °C indicates they can regularly inhabit such high temperatures. While more detailed surveys
are needed to track individual movement and understand the temperature range and duration of
inhabitation in hot spring pools, these are the highest recorded water temperatures in which
tadpoles were found compared with previous studies [Taiwanese populations of Buergeria
japonica at 39.7 °C (Wu and Kam, 2005); Rana sp. (likely R. pretiosa) at 106 °C (41.1 °C)
(Brues, 1927); Spea bombifrons at 38.4 °C (observed temporarily at 45 °C but soon returned to
cooler water) (Brues, 1932)].
The constant water temperature year around is advantageous to amphibians (Scott and
Jennings, 1985). In addition, high water temperature is expected to increase the growth rate of
tadpoles and decrease intra- and interspecific competition (Licht, 1971; Chen et al., 2001). It is
also suggested that temperature is linked to immune responses of amphibians, whereby warmer
water temperatures can enhance the effectiveness of immune defences (Forrest and Schlaepfer,
2011). Therefore, tadpoles may actively select hot water to maximize these benefits.
Alternatively, some individuals may simply be forced to inhabit unfavourable conditions due to
the limitation of water resources and temporary barriers for movement. These uncertainties
should be addressed by comparative studies of B. japonica tadpoles that focus on their
behavioural patterns, biotic factors such as food availability and predation pressures among
habitats with different thermal conditions, and experiments on survival and growth rates at
variable temperatures.
To measure the thermal tolerance of B. japonica tadpoles, we collected 40 tadpoles at Gosner
stages 26–28 (Gosner, 1960) and water from pools around 37 °C in the Seramma Hot Spring.
The hot spring water that contained tadpoles was kept inside a hotel room near the hot spring for
3 h to reduce the water temperature to that of artificial freshwater pools formed on the island
(approximately 26 °C). For the control, we prepared a main tank that contained 4000 ml of water
around 26 °C, and the water temperature was stably maintained by air conditioning so that we
could measure the survival rate of tadpoles without the thermal effect of hot spring. We then
placed a sub-tank filled with 400 ml of the hot spring water into the main tank and kept 10
tadpoles in the sub-tank. For the heat treatment, we prepared another main tank similar to the
control and placed three sub-tanks with 400 ml of hot spring water, each containing 10 tadpoles
(Table 1: experimental groups 1-3). The temperature of the main tank was then increased about
0.16 °C per min using a heater (EVERES Co., Ltd. Tokyo, Japan) with a thermo-controller
(IWAKI Co., Ltd. Tokyo, Japan). Water in the main tanks were automatically circulated, and
sub-tanks were rearranged every 10 min to eliminate the bias of water temperature resulting from
position within the main tank. We also monitored water temperature at the surface and bottom of
the main tanks to confirm that there was no temperature gradient. We stopped measuring thermal
tolerance of each individual immediately when it lost balance, ability to swim, or died, and
recorded the temperature at that time as the thermal limit. Individuals were pithed immediately
after losing such righting behaviours. Tadpoles were not fed before or during the experiment.
For the thermal tolerance measurement, all tadpoles died or lost the righting behavior around
46 °C (Table 1). The lowest and highest thermal limits were 46.0 and 46.2 °C, respectively.
Meanwhile, no mortality was observed in the control group. While Chen et al. (2001) and Wu
and Kam (2004) also measured thermal tolerance of B. japonica tadpoles, direct comparisons to
our study cannot be made due to different criteria and different heating rates applied which can
affect the thermal tolerance (Rezende et al., 2011). Nevertheless, it seems likely that B. japonica
tadpoles of Seramma population have substantially high thermal tolerance similar to that found
in Taiwanese populations (43–44 °C) (Chen et al., 2001; Wu and Kam, 2005).
In conclusion, we found that B. japonica tadpoles on Kuchinoshima Island inhabit the highest
water temperatures ever recorded for any amphibian tadpole and that the B. japonica thermal
tolerance is retained across their entire distribution (see also Komaki et al., 2016). The high
thermal tolerance is essential for tadpoles to inhabit warmer hot spring pools, contributing to the
widening of the species’ ecological niche on the tiny volcanic island. On the other hand, tadpoles
were distributed in hot spring pools close to the fatal temperature that were adjacent to hotter
pools exceeding the fatal temperature. Therefore, to understand the mechanisms of how B.
japonica tadpoles inhabit the geothermal hot spring and avoid fatal temperatures, further studies
on ecological and behavioural features are important.
Permission to perform field sampling and experiment on Kuchinoshima Island was granted by
the mayor of Toshima village. This study was supported by a Grant-in-Aid for JSPS Fellows
(number 25-5065) from Japan Society for the Promotion of Science and a grant from Hiroshima
University Education and Research Support Foundation to Komaki S.
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Table 1. Sample sizes (n) and the thermal limits (mean, range and SD) of five experimental
Mean (range) (°C)
46.1 (46–46.2)
46.06 (46–46.2)
46.04 (46–46.2)
Fig. 1. Stream of hot spring in Seramma Onsen. Water pools were continuously formed.
... In addition, the water environments utilized by these species are also diverse such as streams, ponds, rice fields, shallow puddles, artificial diches, and so on, with different thermal conditions. Particularly, tadpoles of B. japonica were found in geothermal hot springs and were frequently found in water where temperatures reach 40 °C in a natural hot-spring stream in Taiwan and Kuchinoshima Island in Japan (Chen et al. 2001;Wu and Kam 2005;Komaki, Lau, et al. 2016). Buergeria japonica tadpoles generally possess extreme heat tolerance and their upper thermal limit reaches as high as 42 °C (Wu and Kam 2005;Komaki, Lau, et al. 2016;Komaki et al. 2020). ...
... Particularly, tadpoles of B. japonica were found in geothermal hot springs and were frequently found in water where temperatures reach 40 °C in a natural hot-spring stream in Taiwan and Kuchinoshima Island in Japan (Chen et al. 2001;Wu and Kam 2005;Komaki, Lau, et al. 2016). Buergeria japonica tadpoles generally possess extreme heat tolerance and their upper thermal limit reaches as high as 42 °C (Wu and Kam 2005;Komaki, Lau, et al. 2016;Komaki et al. 2020). ...
... 1C). The average CT max was 42.6 ± 0.2 °C (n = 8) under our experimental conditions and was in the range of previously reported CT max values for B. japonica tadpoles (40.6-46.1 °C), although the heating rate varied from 0.16 °C/min to 1 °C/ 12 h in previous studies (Chen et al. 2001;Wu and Kam 2005;Komaki, Lau, et al. 2016). ...
Full-text available
Environmental temperature is a critical factor for all forms of life, and thermal tolerance defines the habitats utilized by a species. Moreover, the evolutionary tuning of thermal perception can also play a key role in habitat selection. Yet, the relative importance of thermal tolerance and perception in environmental adaptation remains poorly understood. Thermal conditions experienced by anuran tadpoles differ among species due to the variation in breeding seasons and water environments selected by parental frogs. In the present study, heat tolerance and avoidance temperatures were compared in tadpoles from five anuran species that spatially and temporally inhabit different thermal niches. These two parameters were positively correlated with each other and were consistent with the thermal conditions of habitats. The species difference in avoidance temperature was 2.6 times larger than that in heat tolerance, suggesting the importance of heat avoidance responses in habitat selection. In addition, the avoidance temperature increased after warm acclimation, especially in the species frequently exposed to heat in their habitats. Characterization of the heat-sensing TRPA1 ion channel revealed an amphibian-specific alternatively spliced variant containing a single valine insertion relative to the canonical alternative spliced variant of TRPA1, and this novel variant altered the response to thermal stimuli. The two alternatively spliced variants of TRPA1 exhibited different thermal responses in a species-specific manner, which are likely to be associated with a difference in avoidance temperatures among species. Together, our findings suggest that the functional change in TRPA1 plays a crucial role in thermal adaptation processes.
... Furthermore, Komaki, Lau, and Igawa (2016) performed field observations and physiological measurements of B. japonica tadpoles in the Seranma hot spring in Kuchinoshima Island and reported tadpoles inhabiting hot spring pools of up to 46.1°C in the wild and experimentally determined that tadpoles have a thermal limit of 46.2°C. This is the highest ever recorded water temperature inhabited by any frog population worldwide. ...
... After collection, we spread the litter on a plastic laboratory tray and sorted out and counted the tadpoles. In a previous study, the highest water temperature where a tadpole was observed was 46°C (Komaki, Lau, et al., 2016). Thus, we only used study sites with water pool temperatures of 46°C or lower. ...
... Locations of the hot spring sources were unchanged for at least 5 years since the previous study in this area (Komaki, Lau, et al., 2016); therefore, the major spatial gradient of water temperature in the study area seems to be stable. Conversely, water temperatures in pools where hot and cool water meets were relatively unstable: ...
Full-text available
“Hot spring frog” is an informal name used for the Japanese stream tree frog (Buergeria japonica), which is widely distributed in Taiwan and the Ryukyu Archipelago in Japan. Some populations of the species are known to inhabit hot springs. However, water temperature can be extremely high around the sources of hot springs. Thus, it is questionable whether B. japonica selectively inhabits such dangerous environments. To address this question, we conducted a series of observations of water temperature preferences of a hot spring population of B. japonica in Kuchinoshima Island in Japan: (a) a field observation of tadpole density in water pools of different temperatures, (b) a field observation of water temperatures where adult males appear for breeding, and (c) an indoor observation of water temperatures selected by adult females for oviposition. As a result, tadpoles showed a higher density in cooler water. Adult males avoided water pools hotter than 37°C, and adult females selected cooler pools for oviposition. Camera records also showed that adult individuals tend to appear around cooler pools. Thus, we did not find any support for the hypothesis that hot spring frogs prefer hot water. Conversely, they apparently tended to prefer cooler water if it was available. Water temperatures around the sources of the hot spring exceed thermal tolerances of the species and could be a strong selective pressure on the population. Thus, the ability to sense and avoid lethal temperatures may be a key ecological and physiological characteristic for the species that inhabit hot springs. Hot spring frog (Buergeria japonica) inhabits hot springs and exhibits high thermal tolerance. Although, behavioral observations showed that they do not prefer hot waters. Our study suggests that ability to sense and avoid lethal temperatures are key ecological and physiological characteristics for the species that inhabit hot springs.
... Currently, six species have been identified, and several populations of three species, B. otai, B. choui, and B. japonica, are further found in geothermal hot springs (Morita, 1988;Chen et al., 2001;Wu and Kam, 2005;Komaki et al., 2016b). ...
... However, due to the nature of hot springs, the water temperature can fluctuate, and tadpoles must face inevitable temperature spikes in this fully aquatic life stage. Indeed, Buergeria frogs living in hot springs can tolerate hot water: a previous study measured 30 Buergeria tadpoles collected from a hot spring habitat and revealed tolerance of temperatures up to 46 • C, whereas above this temperature, tadpoles lost balance and swimming ability or died (Komaki et al., 2016b). Such high thermal tolerance has been observed not only in hot spring populations but also in freshwater Buergeria populations across their distribution (Komaki et al., 2016a). ...
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Buergeria frog tadpoles exhibit high thermal tolerance and are occasionally found in water pools that temporarily exceed 40°C. With the aim of understanding how they can cope with the severe heat stress, we performed RNA-seq of three heat-treated (38°C) and three control (25°C) tadpoles and compared their transcriptomic profiles. We identified 382 differentially expressed transcripts. A protein-protein interaction (PPI) network analysis of these transcripts further identified hub proteins involved in protein degradation, stress granule assembly, and global suppression of DNA transcription and mRNA translation. Along with the avoidance behavior against high water temperature, these endurance mechanisms potentially support tadpoles to survive in high temperatures for short periods of time. Similar mechanisms may exist in many other amphibian species whose habitats are prone to high temperatures.
... As Inger (1950) described, survival on small islands is generally unsuccessful because of, for example, inbreeding depression or environmental fluctuations (Eldridge et al., 1999). However, B. japonica inhabits various environments, from coastal to mountainous areas and geothermal hot springs (Chen et al., 2001;Wu & Kam, 2005;Komaki et al., 2016b), and therefore they could have utilized a wide range of ecological niches, even on the tiny volcanic islands of the Tokara Archipelago, allowing them to survive and sustain their genetic diversity on each island without substantial impacts of inbreeding depression or catastrophes. ...
Aim Amphibians are considered poor transoceanic dispersers because of their permeable skin. However, overseas dispersal of amphibian species has been revealed by recent phylogeographical studies and the role of overseas colonization of amphibians on their evolution and diversification has also been highlighted. However, no studies have investigated in detail the demographic processes related to these overseas colonization events. To clarify how amphibians achieve overseas colonization, we estimated the demographic history of the Japanese stream tree frog, Buergeria japonica, which is distributed on Amami Island and four northern neighbouring islands of the Tokara Archipelago, Japan. Location South-western islands of Japan and Taiwan. Methods We analysed the mitochondrial cytb gene and 20 microsatellite loci, and constructed phylogenetic trees based on these data. We also performed demographic analyses by applying approximate Bayesian computation (ABC) method and an isolation-with-migration model. Results Phylogenetic and demographic analyses based on cytb and 20 microsatellite genotype data revealed that divergence among island populations took place recently, mostly within the last few thousand years. Populations from the northern islands had reduced genetic diversity compared with southern islands, and ABC analyses supported the hypothesis that the species colonized islands from south to north. Main conclusions Given that the islands are separated from each other by deep sea, the recent divergences observed indicate overseas colonization events among the five islands. ABC analyses support the hypothesis that B. japonica underwent a stepping-stone overseas colonization from southern to northern neighbouring islands during the past few thousand years accompanied by multiple founder effects. These results support the hypothesis that overseas colonization could have had a substantial impact on amphibian evolution and diversification.
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The mitochondrial genome (mt-genome) is one of the promising molecular markers for phylogenetics and population genetics. Recently, various mt-genomes have been determined rapidly by using massively parallel sequencers. However, the control region (CR, also called D-loop) in mt-genomes remain difficult to precisely determine due to the presence of repeat regions. Here, using Nanopore sequencing, we succeeded in rapid and collective determination of complete mt-genome of the hot-spring frog, Buergeria japonica, and found that its mt-genome size was 22,274 bp including CR (6,929 bp) with two types of tandem repeat motifs forming repeat regions. Comparison of assemble strategies revealed that the long- and short-read data combined together enabled efficient determination of the CR, but the short-read data alone did not. The B. japonica CR was longer than that of a congenic species inhabiting cooler climate areas, Buergeria buergeri, because of the long repeat regions in the former. During the thermal adaptation of B. japonica, the longer repeat regions in its CR may have accumulated within a short period after divergence from B. buergeri.
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Rising temperatures represent a significant threat to the survival of ectothermic animals. As such, upper thermal limits represent an important trait to assess the vulnerability of ectotherms to changing temperatures. For instance, one may use upper thermal limits to estimate current and future thermal safety margins (i.e., the proximity of upper thermal limits to experienced temperatures), use this trait together with other physiological traits in species distribution models, or investigate the plasticity and evolvability of these limits for buffering the impacts of changing temperatures. While datasets on thermal tolerance limits have been previously compiled, they sometimes report single estimates for a given species, do not present measures of data dispersion, and are biased towards certain parts of the globe. To overcome these limitations, we systematically searched the literature in seven languages to produce the most comprehensive dataset to date on amphibian upper thermal limits, spanning 3,095 estimates across 616 species. This resource will represent a useful tool to evaluate the vulnerability of amphibians, and ectotherms more generally, to changing temperatures.
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We compared the reproductive phenology and tadpole thermal physiology of two populations of Buergeria japonica in Taiwan. The population in Jentse breeds in a geothermal hot spring whose water is a warm, over 35°C year-round, whereas the population in Chukou breeds in a stream whose water tempera- ture is closely correlated with fluctuations in air temperature and averages 23°C. The B. japonica population in Jentse breeds year-round, but the Chukou population breeds only from March to October. Tadpoles from Jentse that reared at 32°C attained larger sizes than tadpoles from Chukou that reared at 32°C, whereas tadpoles from both sites attained similar body size when reared at 22°C. Chukou tadpoles reared at 40°C died within 1-2 d, whereas Jentse tadpoles survived for 9 d at 40°C. All tadpoles tolerated heat well, but Jentse tadpoles had higher critical thermal maxima (CTMax) than Chukou tadpoles. Tadpoles from Jentse showed no metabolic compensation but those from Chukou showed an "inverse" metabolic compensation. Results suggest that B. japonica that breed in the geothermal hot spring has a prolonged breeding season, high growth rates and the reduction or absence of intra- and inter-specific competition. Evolution of a high level of heat tolerance and, possibly, behavioral temperature selection, enable tadpoles in the hotsprings to offset the thermal stress imposed by warm water.
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We measured the critical thermal maxima (CTMax) of tadpoles of Buergeria japonica inhabiting streams and geothermal hot springs. The CTMax of all B. japonica tadpoles was over 41 C, indicating that the tadpoles are heat-tolerant. At Rushan, we monitored the temporal and spatial distribution of tadpoles over a 24-h period in a 220 cm X 70 cm plot that is fed by a geothermal hot spring. The thermal profiles of the plot varied seasonally, but the highest water temperature selected by tadpoles remained similar (ca. 37 C). In the winter, the thermal range of the plot was lower (29.5-40.6 C), and tadpoles could be found in the majority of the plot. In contrast, during the spring and summer the temperature ranges of the plot were higher (33.1-43.8 C and 33.0-42.4 C, respectively), and tadpoles could be found only in the cooler sections of the plot, especially around 1000 h with direct solar radiation. These results suggest that extreme heat tolerance and behavioral thermoregulation (i.e., heat avoidance behavior) are the main means by which B. japonica tadpoles cope with the high temperatures in geothermal hot springs.
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1. Current studies indicate that estimates of thermal tolerance limits in ectotherms depend on the experimental protocol used, with slower and presumably more ecologically relevant rates of warming negatively affecting the upper thermal limits (CTmax). Recent empirical evidence also gives credence to earlier speculations suggesting that estimates of heritability could drop with slower heating rates. 2. Using published data from the fruit fly Drosophila melanogaster, we show that empirical patterns can be explained if flies’ physical condition decreases with experimental time in thermal tolerance assays. This problem could even overshadow potential benefits of thermal acclimation, also suggesting that a drop in CTmax with slower heating rates does not necessarily rule out beneficial acclimatory responses. 3. Numerical results from a simple illustrative model show that no clear conclusions can be obtained on how the phenotypic variance in CTmax will be affected with different rates of thermal change. Conversely, the genetic variance and estimated heritabilities are expected to drop with slower heating rates, hence ramping rates in experiments aiming to study the evolutionary potential of thermal tolerance to respond to global warming should be as fast as possible (within the range in which measurement accuracy and physical condition are not affected). 4. Measurements under ecologically realistic warming rates should also consider the impact of other physiological and behavioural strategies that might partly compensate the negative effects of slow heating rates. However, there are situations in which slow heating rates closely mimic natural conditions, as those encountered by some aquatic ectotherms. These heating rates may be an issue of major concern in these species, given its negative impact on CTmax and its adaptive potential.
Physiological tolerance to variable environmental conditions is essential for species to disperse over habitat boundaries and sustain populations in new habitats. In particular, salinity and temperature are one of the major factors determining species’ distributions. The tree frog Buergeria japonica is the most widely distributed amphibian species found in the Ryukyu Archipelago in Japan and Taiwan, and uses a wide range of breeding sites. Such characteristics suggest a high salinity and thermal tolerance in B. japonica tadpoles. We measured the salinity and thermal tolerance of tadpoles from three islands to determine if physiological tolerance could have contributed to the wide dispersal and survival across different environments. The critical salinity of B. japonica was 10–11‰, a value markedly below seawater. We also observed a critical maximum temperature of approximately 40°C, a value which is higher than what is commonly observed for other anuran species. This high thermal tolerance may have favoured island dispersal and survival, particularly in volcanic islands.
Embryonic thermal adaptations of the frogs, Rana aurora aurora and Rana pretiosa pretiosa from the Pacific Northwest, are described. Limits of temperature tolerance of young R. aurora embryos are about 4-21 degrees C, both the upper and lower lethals being the lowest for any North American ranid frog. For R. pretiosa, the lethal thermal limits of young embryos are approximately 6-28 degrees C. The tolerance limits broaden as embryos become older, and embryos of both species can survive short-term exposure to normally lethal chronic cold temperatures. The developmental rates for embryos of both species at a wide range of constant temperatures are given. Egg masses of both species are compact and globular. The ova of R. aurora average 3.03 mm, those of R. pretiosa 2.31 mm. R. aurora embryos hatch at stage 21, and R. pretiosa embryos hatch at stage 19, a difference that may reflect the O"2 needs of the hatching embryos. The O2 consumption by R. aurora embryos between developmental stages 12-15 at 18.5 degrees C averaged 0.59 cmm O2/egg per hour. R. pretiosa embryos at the same stage and temperature averaged 0.57 cmm O2/egg per hour. Field observations of breeding frogs indicate a correlation between breeding habits--such as initiation of breeding season, time of daily sexual activity, male calling behavior, and spawning site--and embryonic thermal requirements. High mortality of R. pretiosa embryos in the field often results from freezing temperatures at night and desiccation of egg masses. These factors do not greatly affect R. aurora embryos. These thermal adaptations of the two western species of Rana are compared with those of species from eastern North America, as an aid in broadening our understanding of the evolutionary strategies within the genus in North America.