A new, harmless mesocosm design for field rearing ranid embryos and determining clutch sizes

摘要: Results.—The device was tested on 193 Rana dalmatina egg masses during two breeding seasons, in 2007 (N= 80) and in 2010 (N= 113). Only those clutches from 2010 were protected from above. The hatching success, ie, the ratio of embryos that developed to hatching to total clutch size, varied significantly between seasons (2007 mean±SE: 0.86±0.02; 2010: 0.91±0.01; Wilcoxon rank-sum test: W= 5334, p= 0.03). Even though such inconclusive differences could be due to factors other than egg protection (eg, environmental conditions that varied between years, inclusion of different individuals in the samples), we observed egg predation by diving beetles (Dytiscus spp.) only in the case of unprotected clutches.Discussion.—Using this device has several limitations. It can only be used for amphibians that spawn separate and compact eggs masses near the water surface, as well as having herbivorous larvae. The method should not be applied in case of clutches placed deeper in the water column. The change of clutch position in a water column can lead to changes in environmental conditions (eg, water temperature, intensity of UV-B radiation) between the original, deeper location and a new one on the water’s surface. This could have possible negative effect on embryonic survival (Pounds 2001). The stocking’s inner surface quickly becomes covered by algae and organic particles. Thus, the device provides appropriate food conditions mainly for generalized grazers scraping the substrate (Duellman and Trueb 1994). The method is particularly suitable for ranid frogs. Because cannibalism has been observed in ranid tadpoles (reviewed in …