Causative agents of black spot disease in Middendorff's eelpout, Hadropareia middendorffii (Teleostei: Zoarcidae), from the Sea of Okhotsk.

Data on the infection of Middendorff's eelpout, Hadropareia middendorffii, by metacercariae of the trematodes Cryptocotyle lingua Creplin, 1825 and Liliatrema skrjabini Gubanov, 1953, which are causative agents of black spot disease, is provided here for the first time for Taui Bay, Sea of Okhotsk. The prevalence of infection of fish by larvae of C. lingua reached 79%, while the intensity of infection varied from one to 278 individuals (with an average of 53). The prevalence of infection by L. skrjabini was lower (40%), with an intensity from one to 22 larvae (on average, eight). Metacercariae of both trematode species were surrounded by two envelopes: an outer, connective tissue capsule formed by the host's cells, and an inner cyst formed by the parasite. The examined metacercariae were found in all regions of the fish's body (head, trunk, and fins), with approximately equal numbers of individuals of both species found in tissues of the head and trunk regions. Metacercariae of C. lingua were localised in the fishes' muscles, not only immediately under the skin but also in deeper layers. The capsules formed around metacercariae of both trematode species were dominated by fibroblasts and collagen fibres and did not show pronounced signs of inflammation. Studying the distribution of black spot disease and its effects on fish is crucial for understanding disease patterns in relation to fish population dynamics. It may also inform the development of effective anthelmintic treatments for use in aquaculture farms.