Critical thermal minimum of the embryonic development in the common frog (Rana temporaria)
Abstract: The embryonic development of anurans is known to be temperature-dependent and well-studied. Oviposition of Rana temporaria (common frog) in the northern temperate climate zone occurs as soon as the ice melts on water bodies. This early oviposition can take place in waters below 5 °C. The aim of this study was to find the critical thermal minimum of the embryonic life-stage in R. temporaria to use in degree-day calculations and investigate the survivability in low temperatures. The end of the embryonic period for R. temporaria was set to development stage 23 (hatching and tooth development, Gosner 1960). This study used 15 egg clutches, and subjected the embryos to seven constant temperature treatments between 1 – 7 °C. The sampling took place in south-central Sweden (59°N). The aim was achieved by investigating survival and development of the embryos. Studying survival was important to investigate if the critical thermal minimum of populations with no gene-flow between them differed. The maximum development stage was used to determine the developmental temperature threshold (base temperature) for the use of degree-days calculations. Lethal dose 50 (LD50) was used as a measurement of survival, and maximum development stage (DSmax) was used for development over temperature treatments. There was a significant local divergence for critical thermal minimum between populations based on survival. Two populations were not significantly different for survival; 4.96 °C, and 4.94 °C, the third population had a significant lower LD50 value of 4.37 °C. Developmental thresholds were temperature-dependent and there was no singular threshold for the embryonic development of R. temporaria. This might complicate the degree-day usage on anurans; however, this complicates the usage of degree-days on all organisms. When using the degree-day formula on embryonic development of R. temporaria I strongly suggest that 3.89 °C or 2.88 °C is to be used for the calculations, depending on the stopping point of development i.e. stage 23 or stage 20 respectively. Two of three populations had a currently unknown to Europe algae (Oophila) present in the embryo envelope. Embryos with the algae present had significantly higher survival; 64 % vs. 62 %. The populations without this alga had lower development maxima and the presence of alga might have skewed the results. Future studies should focus on the critical thermal minimum over latitudinal clines, distribution of the algae in Europe, and the possible symbiotic relationship between European anurans and Oophila.
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