Key-words:non-indigenous fish, host-parasite coevolution, quantitative parasitology, DNA barcoding, molecular markersIn this study we analyzed comparatively the host-parasite associations between two fish host species invasive in Europe (Lepomis gibbosus and Pseudorasbora parva) and one known generalist parasite species, the copepod Lernaea cyprinacea. We used a fragment of the hypervariable region D1-D2 of the 28S rRNA to confirm that the copepod specimens collected on both host species in our study are indeed conspecific. The prevalence of infection was significantly different between the two host species in all three aquatic ecosystems. Two populations of L. gibbosus exhibited a positive correlation coefficient between the standard body length and infection intensity, while a negative correlation coefficient was observed in one population of P. parva. This is one of the few studies providing parasitological parameters of infections of Lernaea cypriancea in Lepomis gibbosus and Pseudorasbora parva.

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SUMMARY (Rhodeus amarus).These are the first estimated parameters of length-weight relationships for the fish species inhabiting the Timiş River basin. In addition, from all studied species data regarding LWRs for cyprinids Romanogobio albipinnatus Lukasch, 1933 and R. kesslerii Dybowsky, 1862 were unknown before to FishBase.

In the protected area Natural Park Comana located in Giurgiu County, a survey was carried out between April and June 2021, in order to analyze the water quality related to the aquatic diversity. In the samples collected from Comana Pond, several representatives belonging to four kingdoms were identified: Bacteria (blue-green algae, bacteria), Protista (single-celled and colonial green algae, diatoms, euglenoid flagellates, ciliates), Plantae (filamentous green algae, aquatic freshwater plants) and Animalia (European perch, rudd, pike, Prussian carp). Plankton organisms Achromatium sp., Merismopedia sp., Spirilla sp., Spirulina sp. (bacteria); Gomphonema sp., Melosira sp., Paramecium caudatum (chromists); Desmococcus sp, Ulothrix sp., Volvox sp. (chlorophyte green algae) and also the parasitic nematode Philometroides sanguineus were mentioned for the first time in the Comana Pond. A biometric analysis was performed on a total number of 91 Carassius gibelio individuals and negative pattern of growth and also a fish well-being were postulated. The parasite P. sanguineus was detected in the caudal fin of two Prussian carp individuals. Water samples were collected from 7 sampling points (SP) and were subjected to physico-chemical analysis. It were determined the following parameters: pH, electrical conductivity (EC), total hardness (TH), chemical oxygen demand (COD), phosphate phosphorus (P-PO<sub<4</sub<<sup<3-</sup<), nitrate nitrogen (N-NO<sub<3</sub<<sup<-</sup<), nitrite nitrogen (N-NO<sub<2</sub<<sup<-</sup<). Generally, it was concluded that Comana Pond is a dynamic ecosystem that meets favorable conditions for the growth and development of aquatic organisms.