First data on Bivalvulida myxosporeans of Nha Trang Bay mullets (Vietnam)

Mullets are objects of fishery in Vietnam; some of them are raised in rice paddies. Intensive development of fish mariculture in this country, as well as a large proportion of hydrobionts in the population diet, requires knowledge not only of the species composition of parasites, but also of their life cycles, pathogenicity, medical and epizootiological significance. The fauna of marine myxosporeans in this area has been studied very poorly. Totally 36 species of Myxosporea in freshwater fishes of Vietnam are known and 10 are not identified; 7 species of myxosporeans are found in marine fishes and 9 are not identified. In March – April of 2018 in the Nha Trang Bay (Vietnam, South China Sea, which the Vietnamese call the East Sea) 71 specimens of 7 species of Mugilidae fish were examined for identifying myxosporeans by the method of incomplete parasitological autopsy (4 specimens Chelon planiceps, 25 specimens Valamugil speigleri, 10 specimens V. formosae, 1 specimen Planiliza subviridis, 19 specimens Paramugil parmatus, 9 specimens Mugil cephalus, 3 specimens Moolgarda seheli). All organs were examined under Olympus SZ61 binocular microscope at magnification from ×15 to ×25. Smears from the tissues of various fish organs were examined under Olympus CKX53 microscope with phase contrast at magnification ×400 and under Olympus CX41 microscope with the Olympus SC50 digital camera and phase contrast at magnification ×800. Myxosporeans were fixed in glycerol-gelatin according to the standard technique. Parasite measurements were based on 20 Myxobolus spinacurvatura spores, 25 Sphaerospora dicentrarchi spores, and 10 Henneguya sp. spores. All measurements were carried out by the standard method. To assess the number of parasites, data on prevalence and intensity of invasion were used. The first data on Bivalvulida myxosporeans of Mugilidae fish in Nha Trang Bay were obtained: Myxobolus spinacurvatura was found in the gallbladder and intestine of 78 % Mugil cephalus with the intensity of invasion of unit spores in the gallbladder smear and 5–11 cysts in the intestine; Sphaerospora dicentrarchi was found in the gallbladder of 5 % Paramugil parmatus, 12 % Valamugil speigleri, 20 % V. formosae and 56 % Mugil cephalus with the intensity of invasion of unit spores in the smear; Henneguya sp. was found in the gallbladder of 8 % V. speigleri and 10 % V. formosae with the intensity of invasion of unit, tens of spores in the smear. Description of the revealed species was made. Their distribution in other regions of the World Ocean was considered. S. dicentrarchi turned out to be a new species for the fauna of the East Sea and Vietnam. This species was found not only in Mugil cephalus, the typical host in other regions, but also in new hosts Paramugil parmatus, Valamugil speigleri, and V. formosae. Myxobolus spinacurvatura was first found in the Nha Trang Bay. Since Myxobolus spinacurvatura and Sphaerospora dicentrarchi are known in Mugil cephalus not only in the Pacific Ocean basin but also in the Atlantic Ocean one, taking into account the connection of these water bodies in ancient times, it is assumed that each of these parasites had a common large distribution area in the past, which was then broken due to powerful geological transformations of our planet.