Effect of Temperature on the Hatching Time of Eggs of Three Rhithrogena Spp. (Ephemeroptera) from Austrian Streams and an English Stream and River

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Abstract
Eggs of R. semicolorata from the Wilfin Beck and River Lune (England), R. cf. hybrida from the Seebach (Austria) and R. loyolaea from the Herrnalmbach (Austria) were kept at constant temperatures in the laboratory. The percentage of eggs that hatched at each temperature varied from 0-26% for R. semicolorata, 1-33% for R. cf. hybrida and 0-48% for R. loyolaea. These variations were related to temperature in 2 spp.; i.e., no hatching below 5.degree. C and maximum values at 15.8.degree. C for R. semicolorata, few eggs hatching above 10.degree. C and maximum values in the range 1.9-5.2.degree. C for R. loyolaea. Hatching time (days after fertilization for 10, 50 and 90% of the eggs to hatch) decreased with increasing temperature and the relationship between the 2 variables was well described by a power-law. The relationship for R. loyolaea was also well described by a hyperbola, but both models were not applicable above 8.4.degree. C because hatching time in this species then increased with increasing temperature. There were interspecific differences in hatching times for the 3 spp., but the times for 50% and 90% of eggs hatched were not significantly different for the 2 populations of R. semicolorata. The length of the period over which eggs were hatching was remarkably short; .ltoreq. 9 days for R. semicolorata and R. cf. hybrida, and about 35 days for R. loyolaea. Quantitative information on the hatching times of 12 spp. [including Ephemerella ignita, Tricorythodes minutus, Baetis rhodani, B. vernus, Ecdyonurus picteti, E. venosus, E. dispar, E. torrentis and E. insignis] and 20 populations of Ephemeroptera was briefly summarized and discussed. The various mathematical models used to describe the relationship between temperature and hatching time in eggs of poikilotherms were reviewed and the general equation for a hyperbola and power-law may frequently be an adequate empirical model.