The Eurasian otter Lutra lutra is a generalist carnivore that is widely distributed in many aquatic ecosystems. Based on its inherent attributes of opportunistic foraging behaviour and broad dietary range, it is naturally considered a potential sampler of the diversity of aquatic vertebrates. To test the ability and efficiency of otters as a diversity sampler, we used DNA metabarcoding to investigate the composition in vertebrates of the diet of otters that inhabit a forest stream area in northeast China. Twenty vertebrate prey taxa were detected in 98 otter spraints. Otter diet mainly comprised aquatic fishes (59.4%) and amphibians (39.0%). We also used traditional approaches to investigate fish communities at 60 sampling sites in the same area to determine the relationship between fish population composition in the environment and otter diet. The comparison revealed that 28 species of fish were distributed in this area, of which five are simultaneously detected in otter spraints. This indicates that molecular analysis of the diet of otters is not an ideal approach for investigating fish diversity, at least when using the 12SV5 primer pair. Based on a review of the available molecular research on otter diet, we conclude that the low species resolution may be due to the presence of many closely-related prey species in native habitats and lack of suitable barcodes. Considering the remarkable power of diet metabarcoding analysis in capturing elusive and rare species, it represents an approach that can compensate for the defects associated with fishing methods and we suggest that it can be used as an auxiliary means of measuring traditional fish diversity.
In order to determine the influence of of salinity on the survival, growth, development, and neonate production of the cladoceran Scapholeberis mucronata (O. F. Müler), long-term experiments were carried out at four salinity gradients (1, 2, 3, and 4) and a control (freshwater) using Chlorella pyrenoidesa as feed. The acute effect of salinity on this species indicated that salinity gradients 4 and 4.5 were its upper tolerance limits for reproduction and survival. The survival and growth rates of individuals reared in salinity gradients 1 and 2 were higher than those reared in other salinity gradients. The mean size of the female adults decreased from 820 to 743 μm when the salinity increased from salinity gradients 1 to 4. For individuals reared in salinity gradients 1 and 2, the rm of population was 1.021 and 0.903, respectively; the rate of egg production was 1.281 and 1.390, respectively; the cumulative egg production was 83.2 and 106.0 and mean life span was 16.05 and 17.30, respectively. These values of life-history parameters were higher than those of individuals reared in salinity gradient 3. No eggs were produced by females reared in gradient 4 during the entire experiment. Furthermore, individuals reared at gradients 1 and 2 had faster embryonic development. The above results imply that S. mucronata prefer an environment with lower salinity (1-2). Resting egg formation and sexual reproduction did not occur in any of the tested salinity gradients.
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