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We describe life history traits of the ostracod Darwinula stevensoni through laboratory experiments. This small (about 0.7 mm), ancient, obligate parthenogenetic species appeared to be particulary difficult to handle as its long life cycle (up to 3-4 years) has made lab experiments over several generations very difficult. D. stevensoni is an eurythermal and euryhaline species with low variability in size and shape (both of the carapace and the soft parts). Its genetic variability has also been found to be very low. Survival, clutch size, deposition timing and hatching were evaluated in acclimated and non-acclimated females from seven populations: six from Northern Italy and one from Spain. The samples were collected from three different habitats: four lakes, two streams and one spring. A genetic survey using starch gel electrophoresis had previously revealed that only Glucose phosphate isomerase (Gpi) locus was polymorphic. A clone, homozygous at Gpi locus, is the most common in lacustrine and spring habitats in the whole biogeographic range. Surprisingly, two heterozygous clones dominate in Northern Italian lotic environments. Enzymatic activity of heterozygous and homozygous genotypes at Gpi locus was assayed in order to evaluate the relationship between Gpi activity and fitness. Survival and developmental time were not affected by acclimation, while reproductive potential decreased in acclimated females. In females from running waters, reproduction started later and lasted for a shorter time, clutch size was smaller, and hatching percentage lower than females from lakes. These differences are not directly correlated with differences in enzymatic activities at Gpi locus. This does not rule out an overall genetic control of these characteristics since, in a parthenogenetic species, the whole genome is transmitted without recombination. We stress the intriguing case of D. stevensoni in relation to the concept of the "general purpose genotype"

Relatively few laboratory tests of the effect of salinity on ostracod species exist. Here we report the results of acute and chronic bioassay tests to study the effects of six different salinities (deionised water, 22, 120, 922, 9600 μS-cm -1 , and marine water) on survival and reproductive parameters (number of eggs and hatching). The test was carried out on individuals of Darwinula stevensoni from different localities characterised by different salinity (from 111 μS-cm -1 at Monate, Italy, to 3440 μS-cm -1 at Hollandersgatkreek, Belgium). Individuals belonging to three life cycle stages were used (pre-adult, adult, ovigerous). Surprisingly low mortality was observed in all treatments and for all life stages, independently of population of provenance. Only in marine water was mortality noticeable although reproductive parameters were hardly affected. Given the high genetic homogeneity of the species, our results seem to support the hypothesis of high phenotypic plasticity for salinity tolerance in D. stevensoni.

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