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.

Existing data on Myxozoa parasites infecting mullets were reviewed. The validity of nine species names was updated. Sixteen species were registered during analysis of original material collected in the Mediterranean, Black, Azov, and Japan Seas in 2004–2005. A new bivalvulid myxozoan parasite, Myxobolus adeli n. sp., was described from the inner organs of the golden grey mullet Liza aurata (Risso, 1810) collected in the Mediterranean (Ebro Delta, Spain), Black Sea (Kerch Strait, Ukraine), and Azov Sea (Genichesk, Ukraine) coastal waters. It is characterized by the presence of elongated, spindle-like cysts 0.5–1.3 mm in size, filled with wide transverse-oval spores about 6.2 × 7.2 × 4.6 μm in size, with two equal polar capsules measuring about 3.0 × 1.8 μm and short polar filament, turned into four coils. The obtained data show that this species differs from all previously described Myxobolus spp. with equal polar capsules. Comparative study of Myxobolus spp. recorded in worldwide mullets indicates a close relationship with M. adeli n. sp. and Myxobolus improvisus Isjumova, 1964 registered in mullets. Probably, the last species includes representatives of some different species, infecting freshwater and marine hosts.

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A new multivalvulid myxozoan parasite, Kudoa unicapsula n. sp., is described from the intestinal mesentery, intestine and pyloric caeca of the thin-lipped grey mullet Liza ramada (Risso 1826) and the golden grey mullet L. aurata (Risso, 1810) from the Mediterranean coastal waters of Spain. It is characterized by the presence of elongated, rice corn-like white cysts of 0.47-0.56 x 0.18-0.38 mm, filled with tetracapsulate, slightly asymmetric spores, rectangular in apical view and tear-shaped in lateral view with four polar capsules of considerably different size and slightly unequal spore valves with rounded edges, overlapping each other on the apex of the spore. One large polar capsule includes a polar filament coiled in two to three turns, and the other three polar capsules, which are very small, posses only a rudimental filament. Both light and electron microscopy data showed that this species differs from all previously described Kudoa spp. with unequal polar capsules. The molecular analysis based on 18S and 28S ribosomal deoxyribonucleic acid DNA sequence data of K. unicapsula n. sp. indicates a close relationship and thus phylogenetic clustering together with K. trifolia, a myxozoan from the same host and the same geographical location.

Myxosporeans of the genus Ortholinea have a worldwide distribution and infect organs and tissues of exclusively marine fishes. Here we describe the morphological and molecular characteristics of Ortholinea mullusi sp. nov. parasitizing the urinary bladder and kidney tubules of red mullet Mullus barbatus collected from the coastal zone of Sinop in the Black Sea, Turkey. Polysporic plasmodia with immature spores were either elongate, 37.0 ± 4.5 SD (30-50) µm long and 45.0 ± 3.8 (40-55) µm wide, or were round, up to 100.0 µm in diameter. Mature, free spores were spherical in the frontal view and measured 9.3 ± 0.2 (9.0-9.7) µm in length, 8.7 ± 0.3 (8.2-9.3) µm in width and 7.7 ± 0.1 (7.5-7.9) µm in thickness. We observed 2 polar capsules of equal size, which measured 3.1 ± 0.1 (3.0-3.2) µm long by 2.5 ± 0.1 (2.4-2.6) µm wide, and the tips of the polar capsules were open towards the sutural line. The prevalence of infection by O. mullusi sp. nov. was 24.5%. Phylogenetic analysis based on nuclear small subunit ribosomal DNA (SSU rDNA) clearly suggested O. mullusi to be a new species, clustered within a lineage comprising O. labracis and O auratae. Pairwise nucleotide similarities and DNA distance values between O. mullusi sp. nov. and sister Ortholinea species also supported this suggestion.

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.

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The Nature Protection Aspect of the Black Sea Fish Myxosporean (Myxozoa, Myxosporea) Studies. Yurakhno V. M. -The species composition and indices of the myxosporeans infecting fish from the different nature conservation Black Sea regions were considered on the base of the original investigations and literature data. Myxosporean inhabiting the Red Book fish hosts were investigated. The myxosporean species potentially pathogenic for the fish health were identified and epizootological situation was estimated.