Host-parasite relationships are often characterized by the rapid evolution of parasite adaptations to exploit their host, and counteradaptations in the host to avoid the costs imposed by parasitism. Hence, the current coevolutionary state between a parasite and its hosts is predicted to vary according to the history of sympatry and local abundance of interacting species. We compared a unique reciprocal coevolutionary relationship of a fish, the European bitterling (Rhodeus amarus) and freshwater mussels (Unionidae) between areas of recent (Central Europe) and ancient (Turkey) sympatry. Bitterling parasitize freshwater mussels by laying their eggs in the gills of mussel and, in turn, mussel larvae (glochidia) parasitize the fish. We found that all bitterling from both regions avoided one mussel species. Preferences among other mussel species tended to be related to local mussel abundance rather than duration of sympatry. Individual fish were not consistent in their oviposition choices, precluding the evolution of host-specific lineages. Mussels were demonstrated to have evolved strong defenses to bitterling parasitism in the area of ancient sympatry, but have no such defenses in the large areas of Europe where bitterling are currently invasive. Bitterling avoided glochidia infection irrespective of the duration of sympatry. K E Y W O R D S :Coevolutionary arm races, evolutionary lag, gentes, host race, specialization, symbiosis.

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The effects of a commercial trout feed (45% crude protein, 12% crude fat) and a carp diet (35% crude protein, 10% crude fat) on growth performance of Russian sturgeon (Acipenser gueldenstaedtii) was studied and expressed as weight gain (WG), specific growth rate (SGR), feed conversion ratio (FCR) and body composition (protein, lipid and dry matter).). 23-months old sturgeons were hand-fed (pelleted diets) twice daily throughout the 330-day experimental period. Fish were grown at ambient temperatures (seasonal range: 6.2-26.3 ºC). Initial mean weight in the two groups was 399 ± 12 g and 404 ± 13.1 g, respectively. Mean weight gain in the two groups was calculated to be 95.3% in fish fed the trout pellets (Group 1) and 77.3% in fish fed the carp pellets (Group 2). Mean specific growth rate (SGR) in sturgeons fed the trout and carp diets was found to be 0.19 and 0.16, respectively. Mean feed conversion ratio (FCR) for both test groups was 4.9 and 5.2, respectively. Corresponding crude protein values, were 19.04% and 18.21% in muscle tissue, respectively. The results indicated that growth performance of Russian sturgeons using trout diets seem to be somewhat better than that of the fish fed the carp feed. However, growth performance (WG, SGR and FCR) and body composition were not statistically different in two groups.

In aquatic ecosystems, fish play a key role in parasite accumulation and transmission to predacious animals. In the present study, realized on seven populations of a small cyprinid fish species, the European bitterling Rhodeus amarus, we investigated (1) the role of the European bitterling as a potential intermediate or paratenic host, (2) the ability of the fish to accumulate parasites with similar final host group, and (3) its significance as a potential source of parasite infection in the ecosystem in respect to habitat characteristics. A total of 36 parasite species were recorded; 31 species (90% of all parasite specimens) were classified as endoparasites. Most of the endoparasites were found in the larval life stage, using bitterling as an intermediate or paratenic host. In particular, parasite community structure showed significantly higher proportions of allogenic parasites in comparison with autogenic. The supposed co-occurrence of parasite species with identical final host groups showed only a weak association. The adjacent reservoir areas were a significant determinant of both the total and infracommunity parasite species richness and for the mean parasite abundance. No relationship between the distance of sampling site from the adjacent reservoir and parasite community characteristics was found. As a smallsized fish with a wide distribution range and high local abundances, the European bitterling can represent a natural prey for a wide range of piscivorous predators. Due to its susceptibility to the number of larval endoparasites, this fish species may therefore fulfill the role as important transmitter of parasites to their final hosts.

The growth performance of juvenile sturgeon (Acipenser gueldenstaedtii Brandt & Ratzenburg, 1833) was studied in experimental net cages (2 m · 2 m · 2 m) in freshwater Sapanca Lake. Fertilized eggs were imported from Krasnodar, Russia in January 2001. Two experimental stocking densities were employed (12 and 8 ind. m )3 ). Sturgeon were hand-fed a pelleted commercial trout feed twice daily throughout the 203-day experiment period. Fish were grown at ambient temperature (12.7-28.5°C) and natural photoperiod. Initial and final mean body weights were 279.5 g (±31.27) and 1112.8 g (±234.77) in the first group, and 271.1 g (±28.21) and 1140.5 g (±213.31) in the second group, respectively. Results were a fourfold biomass density increase in both trials from an initial 3.48 to 12.4 kg m )3 (high density) and 2.2 to 8.8 kg m )3 (low density). Mean specific growth rate (SGR) was determined as 2.69% day )1 in both groups. Mean condition factor (CF) at the end of the experiment was 0.36 ± 0.05 in the high-density group and 0.41 ± 0.05 in the low-density group. Mean feed conversion ratio (FCR) was calculated to be 5.7 and 5.8, respectively. Survival was 92.7% in the first group (high density) and 96.1% in the second group (low density). The results demonstrated that sturgeon juveniles can be raised in cages and that growth performance (CF, SGR and FCR) and mortality of sturgeon were not significantly different at the two stocking densities (P > 0.05) tested.