Ligophorus kaohsianghsieni (Gusev, 1962) Gusev, 1985 was collected from the so-iuy mullet Planiliza haematocheilus (Temminck & Schlegel, 1845) from the Black Sea and the Sea of Japan. DNA sequences data for L. kaohsianghsieni, as well as its morphological characters from the Sea of Japan were obtained for the first time. Significant morphometric and genetic diversity between specimens of L. kaohsianghsieni from the Black-Azov Sea region and the Sea of Japan were not found. For the first time, the molecular phylogeny of L. kaohsianghsieni based on three fragments of the nuclear DNA ribosomal cluster (18S, internal transcribed spacer 1 and 28S) was reconstructed. Molecular analysis of Ligophorus species from the Atlantic and Pacific Oceans revealed a significant phylogenetic distance between L. kaohsianghsieni and two others species (Ligophorus pilengas and Ligophorus llewellyni) from the same host (P. haematocheilus) and region. This result indicates the lack of correspondence between the phylogenetic and geographical closeness of the hosts and the relation of their parasites from the genus Ligophorus.

The life-history of Mazocraes alosae Hermann, 1782 on one of its hosts, the Pontic shad Alosa immaculata Bennett, is described for the first time. This anadromous fish, which occurs off the coast of the Crimea and migrates from the Black Sea to the Sea of Azov and into the River Don for spawning, was studied throughout its migration and during all seasons. It is demonstrated that the period of reproduction of this monogenean is significantly longer than that reported for the population in the Caspian Sea, lasting from April to November with a peak in April-May, and continues both in the sea and the river. Experiments showed that water salinity does not limit the development of the eggs or the hatching of the oncomiracidia. Our data reveal that the abundance of M. alosae is not determined by the size or sex of mature fish and that shad of less than two years old can also be infected with this monogenean, although less frequently than older fish. The direction of the migration of A. immaculata, either from the Black Sea to the Sea of Azov and into the rivers or in the opposite direction, does not influence the number of monogeneans present on the host. The main factor affecting the dynamics of the abundance of this monogenean is season, and, as has been indicated previously in the Caspian Sea basin, there is a synchronisation between the parasite's life-history and both the host's spawning behaviour and the duration of its migration.

Most gyrodactylids have a haptor armed with a pair of hamuli, two connecting bars and 16 marginal hooks. In some gyrodactylids, however, the haptor is disc-shaped and reinforced by additional sclerites. The genus Polyclithrum has arguably the most elaborate haptor in this group. This study aimed to gain better understanding of the anatomy of Polyclithrum by examining neuromusculature and haptoral armament of Polyclithrum ponticum, a species parasitizing Mugil cephalus in the Black Sea, with emphasis on haptoral sclerites and musculature in connection with host-attachment mechanisms. Musculature was stained by phalloidin, the nervous system by anti-serotonin and anti-FMRFamide antibodies, and haptoral sclerites were visualized in reflected light. The study provided new information on sclerites: in addition to previously described supplementary sclerites (A1–6), ear-shaped sclerites (ESSs) and two paired groups of ribs, reflected light revealed a rod-shaped process on the ESSs and a pair of small posterior sclerites. The sclerites were shown to be operated by 16 muscles, the most prominent of which were two transverse muscles connecting the hamular roots, three muscles attached to sclerite A2, the muscle fibres of anterior ribs and a set of extrinsic muscles. The nervous system consists of a pair of cerebral ganglia connected by a commissure and three pairs of nerve cords that unite in the haptor to form a loop between the opposite cords. The arrangement of sclerites and muscles suggests that Polyclithrum initiates the attachment by clamping a host's surface with longitudinally folded haptor and then secures its position with marginal hooks.

In this paper, we analyzed the diversity and structure of helminth communities of 12 common fish species from the coastal zone of Crimea. A total of 53 helminth species were found. The total number of parasite species per host fish ranged from 3 to 18. Species richness at the infracommunity and component community levels were from 1.4–4.2 to 1.7–7, respectively. The Brillouin index for the infracommunites was 0.1–1, while the Shannon index for the component communities was 0.3–1.2. Component communities demonstrated a bi- or tri-modal distribution of the parasite prevalence and positive correlations between the prevalence and log-transformed abundance indices, thus following the “core–satellite” conception. Overall, the prevalence and abundance index of the dominant parasite in the component communities ranged from 18 to 80% and from 0.6 to 61.5 ind. per fish, respectively. The structure of the helminth component communities demonstrated good accordance with the nestedness mode where the rarest species occurred in the most diverse infracommunities, while the poorest infracommunities were composed of a few dominating species. More than two-thirds of the studied helminth species had an aggregated distribution indicating well-structured and developed communities. Our data provide a basis for further research and may be used for fish resource monitoring and management.

In this study, nematodes were first reported in the amphipods, Ischyrocerus commensalis, collected from the introduced and commercially important red king crabs, Paralithodes camtschaticus, in the coastal Barents Sea in July 2022. Commensal amphipods were registered on all red king crabs captured (n = 70, prevalence 100%). Further laboratory analysis revealed that 11 out of 467 amphipod individuals (prevalence 2.4%) harbored single third-stage larvae of Hysterothylacium sp. l. (Nematoda: Anisakidae). The nematode larvae ranged from 0.63 to 6.10 mm in body length. Due to the low prevalence of nematodes and lower vulnerability of the host amphipods to fish predators, negative effects on the Barents Sea ecosystem through the range expansion of crab-associated amphipods and their parasites are unlikely.