In aquatic systems, chemical cues are one of the major sources of information through which animals can assess local predation risk. Non-native red-eared sliders (Trachemys scripta elegans) have the potential to disrupt aquatic ecosystems in Central Europe because of their superior competitive abilities and omnivorous diets. In this study, we examined whether continuous predator-borne cues are tied to changes in the developmental rates, growth rates and sizes at metamorphosis of common frog tadpoles (Rana temporaria). Our results show rather rarely documented types of amphibian prey responses to caged predators. The presence of turtles shortened the time at metamorphosis of tadpoles from 110 ± 11.7 days to 93 ± 13.0 days (mean ± S.D.). The first metamorphosed individuals were recorded on the 65th day and on the 80th day from hatching in the predator treatment and in the control group, respectively. The froglets were significantly smaller (12.8 ± 0.99 mm) in the presence of the predator than in the control treatment (15.2 ± 1.27 mm). The growth rate trajectories were similar between the predator treatment and the control. Thus, predator-induced tadpole defences were evident in higher developmental rates and smaller sizes at metamorphosis without significant changes in growth.
Abstract:The freshwater bryozoan species Pectinatella magnifica was found in 6 sandpits and in 19 mostly extensively managed ponds in the Protected Landscape Area and Biosphere Reserve Třeboňsko (Czech Republic) from its first record (in 2003) to 2012. Mean fresh biomass and abundance of P. magnifica colonies were 0.6 ± 1.5 kg m −2 and 0.7 ± 1.1 colony m −2 (± SD), respectively, in the shoreline zone during the growing season 2006-2011. The maximum biomass was mostly recorded during the first half of August in all basins. Colonization of further localities was recorded rather than increasing of P. magnifica biomass or abundance in 2012. There were no correlations between water temperature or water transparency and biomass/abundance of P. magnifica during the growing season. P. magnifica colonies preferred to grow on the branches or roots (especially of Salix sp.) to aquatic macrophytes and stones. Most of the water bodies, where this bryozoan species occurred, had lower concentration of total phosphorus in the water when compared with the typical fishponds in the Czech Republic. Inner space of colonies of P. magnifica provided suitable higher trophic level substrate when compared with the water of the sandpits/fishponds especially for green coccal algae. A massive algal colonization was indentified in decomposing colonies at the end of the growing season.
Feeding selectivity of Nile tilapia (Oreochromis niloticus L.) juveniles (9.3-20.9 g) to four aquatic macrophyte species and tilapia growth were studied in 4 consecutive experiments. Plant diet was provided to 8 tanks containing 20 fishes for 5 days. The fish were fed a carp diet between 4 experiments for 14 days. The consumption of 4 aquatic macrophyte species differed significantly [F(3,252) = 39.6; P < 10 -6 ]. Elodea canadensis was the most preferred plant (Chesson selectivity index = 0.50 ± 0.05, n = 4). Potamogeton pectinatus and Spirodela polyrhiza were consumed with about equal preference. Myriophyllum spicatum was the least preferred species. Elodea canadensis contained relatively more phosphorus, potassium and ash than the other three species. The daily plant dry weight intake ranged between 0.79 and 2.26% of body weight. The fish grew during the first two experiments (SGR = 2.54 and 3.18%/d, respectively), but lost weight during the 3rd and 4th experiments (SGR = -1.75 and -1.71%/d, respectively).
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