Title : Conditionally pathogenic microparasites (Microsporidia and Myxosporea) of mullet fish potential objects of mariculture in the black and azov seas
Abstract:
Mullet fish have high nutritional value and excellent taste, which is why they are farmed in various countries around the World (on all continents except Antarcti?a). In the Black Sea and Sea of Azov, these fish are not farmed, but rather exploited for commercial purposes. However, they may eventually be bred in culture farms, so it is important to understand the parasites that can harm them and spoil the commercial value of fish products. In the Black Sea and the Sea of Azov, we highlight one species of microsporidian from the genus Loma Morrison & Sprague, 1981, and three species of myxosporeans from the genus Myxobolus Bütschli, 1882, which are ?onditionally pathogenic microparasites for mullet hosts:
– Loma mugili Ovcharenko, Sarabeev, Wita & Czapli?ska, 2000 was found in the Molochny Estuary of the Sea of ??Azov in the endothelial cells of the gill filaments in 4.5% of the So-iuy mullet Planiliza haematocheilus (Temminck & Schlegel, 1845) with an intensity of 1–36 (9.7) cysts and an abundance of 0.39 cysts (Ovcharenko et al., 2000). According to V. N. Maltsev (1999), this parasite caused mass mortality of mullet fingerlings in this region in the autumn of 1996. Incidentally, the So-iuy mullet was successfully introduced to the Azov and Black Seas by Soviet scientists in the late 1970s. They brought it from the Sea of Japan, which belongs to the Pacific Ocean basin (Yurakhno, 2020).
– Myxobolus exiguus Thélohan, 1895 caused a mass mortality of the native species of Flathead grey mullet Mugil cephalus (Linnaeus, 1758) and Golden grey mullet Chelon auratus (Risso, 1810) in the spring of 1949 in the Kerch Strait and Maly Knut Bay, due to gill swelling and bleeding (Petrushevsky, Schulman, 1958). This phenomenon coincided with a natural fish kill due to deteriorating hydrological conditions and never recurred on such a scale. A little later, T. P. Pogoreltseva (1966) published an article on mullet diseases associated with mass infestation by M. exiguus. However, according to our data (Yurakhno, 2018), M. exiguus has become a rare species in the Black Sea and Azov Seas in recent decades. Since 1987, we have never encountered it in Sevastopol, and in Kerch and Genichesk it had extremely low rates of prevalence in mullet (1 and 3%, respectively) with an intensity of single spores in a smear.
– Myxobolus parvus Schulman, 1962 was found by us in 1996–1997 on the gills of Planiliza haematocheilus with a maximum infestation prevalence of 54% and high intensity. In some cases, we observed a white, fluid film consisting of the parasite's spores and plasmodia coating the gills of this fish species. This latter circumstance inevitably affected the respiratory function of the affected organ (Yurakhno, 2018, 2020).
– Myxobolus episquamalis Egusa, Maeno & Sorimachi, 1990 was found on the scales of Mugil cephalus in the Black Sea near Zhelezny Rog Cape in May (at a water temperature of 18.4 °C), June (19.5 °C), and October (14.0 °C) 2015. The proportion of sick individuals varied from 3 to 15% in spring, reached 40% in summer, and decreased to 2.5% by autumn. No mortality cases were noted. However, extensive scale damage reduced the commercial value of the fish (Kazarnikova et al., 2018, 2020). This species was also found on the scales of M. cephalus in Turkey in 2017-2018 (Simsek, 2019), and, in 2025, we encountered it in photographs taken by Crimean spearfishers, who were concerned about the presence of many fish with severe scale damage in schools of mullets. No parasitologists had identified this species in its fauna until the first discovery of myxosporeans in the Black Sea in the late 1930s. It was most likely introduced to the Black Sea by the M. cephalus, which migrated from the Mediterranean Sea, where it had previously been known to infect this host.
The fact that these species of myxosporeans can be transferred to adjacent water bodies and parasitize other species of host fish emphasizes the need for increased monitoring of the parasitological situation associated with microparasites of mullets in the Black and Azov Seas.
This work is supported within the framework of the IBSS state research assignment “Biodiversity as the basis for the sustainable functioning of marine ecosystems, criteria and scientific principles for its conservation” (No. 124022400148-4) (Russian Federation).
