Four gobiid species, Babka gymnotrachelus, Neogobius melanostomus, Neogobius fluviatilis, and Proterorhinus semilunaris, were parasitologically studied in different localities of the Dnieper and Vistula river basins. The highest number of parasitic species was found in N. fluviatilis (35 taxa). The parasite fauna of N. melanostomus, B. gymnotrachelus, and P. semilunaris consists of 23, 22, and 15 taxa, respectively. The species accumulation curves show stable accumulation of parasite species by all four fish hosts along the studied part of the corridor, from the Dnieper Estuary to the Vistula River delta. The plot reveals also that the studied gobies lose the parasites common in the host native range and accept new parasites from the colonized area. In the case of N. melanostomus, it complies with the enemy release hypothesis, as the parasite load was low in the invaded area if compared to the native range. The three other alien gobies are vector for Gyrodactylus proterorhini in the Baltic basin. Moreover, populations of this alien monogenean tend to be more abundant in their new range in comparison with the Black Sea basin. In general, the number of parasite species in the colonized area was of the same rank as in the native one for N. fluviatilis, and even higher for B. gymnotrachelus. This results from accumulating new parasite species along the gobiid invasion route. In particular, the N. fluviatilis, B. gymnotrachelus, and P. semilunaris lost some of their native parasites and gained the local ones after entering the post-dam part of the Vistula River; it can be interpreted as a partial escape from parasites.

The genus Grillotia Guiart, 1927 is cosmopolitan in its distribution and the type-species, G. erinaceus (van Beneden, 1858), has been relatively well studied. However, this study provides infection indices of Grillotia erinaceus from southern and northern Black Sea whiting Merlangius merlangus for the first time. The specimens of Grillotia erinaceus were obtained from subserosa of the anterior oesophagus, stomach, pyloric caeca, liver, ovaries and mesenterium of whiting caught by commercial fishing vessels off Sinop, Turkey and off Balaklava, Ukraine. Fish were examined during the period from May 2011 to April 2012. Prevalence and mean intensity values in 268 fish collected off Sinop in the Black Sea were 18.66% and 1.82 ± 0.16 parasites per infected fish, respectively. In Ukrainian 166 whiting samples collected off Balaklava in the Black Sea, however, G. erinaceus plerocercus infection prevalence was 10.24% and mean intensity 1.71 ± 0.75 parasites per infected fish. Infection parameters were also determined at both sampling sites in relation with host length, sex and season.

Trichodina gobii was identified during a parasitological survey conducted on the parasite fauna of whiting, Merlangius merlangus at both Sinop (Turkey) and Sevastopol (Russia) coasts of the Black Sea in the period between May 2011 and March 2014. Its morphological characteristics and taxonomic affinities with other previous reports on this species were revealed. The ecological occurrences in relation with seasonality, host length and sex of whiting were also determined. Parasitological indices were calculated from 690 and 423 whiting specimens collected in Sinop and Sevastopol, respectively. Overall infestation prevalence (%) and mean intensity and mean abundance values were 45.6%, 70.1±10.4 and 32.0±4.9, respectively, in Sinop, while they were 6.9%, 29.2±10.1 and 2.0±0.8 in Sevastopol. The taxonomic affinities to other trichodinid species and ecological data are discussed in detail.

SummaryIn the present study, we investigated the comparative infection levels, ecology and host-parasite interrelationship of a nematode Hysterothylacium aduncum (Rudolphi, 1802) in whiting, Merlangius merlangus (L., 1758) in southern (Sinop) and northern (Balaklava Bay) coasts of the Black Sea for the fi rst time. Fish were collected throughout a period from May 2011 to March 2014 from local fi shermen. A total of 690 fi sh specimens near Sinop and 423 fi sh near Balaklava Bay were examined for parasites. Hysterothylacium aduncum was the only nematode species identifi ed in the digestive tract of the Black Sea whiting Merlangius merlangus. Prevalence of infection, mean intensity and mean abundance values were determined according to season, length classes and sex of fi sh at both localities. These infection indices were always higher in Sinop samples than those of Balaklava Bay samples.