On finding of an abundant population of free-living stages of ergasilids (Copepoda, Cyclopoida) in the Lyutoga river of Southern Sakhalin

Abstract

In the littoral zone of the oxbow of the Lyutoga River (Southern Sakhalin), which remains connected to the main channel, a high population density of parasitic copepods was found (103 ind./m2). Ergasilus briani Markewitsch, 1933, widely distributed in Eurasia, but not previously recorded in South Sakhalin and noted only for one water body in northwestern Sakhalin (Lake Sladkoe), demonstrated the highest population density (98 ind./m2). The abundance of the second species, Thersitina gasterostei (Pagenstecher, 1861) was significantly lower (5 ind./m2). T. gasterostei has previously been noted in South Sakhalin, but the present finding is the first for the Lyutoga River. The high abundance of free-living stages of ergasilids is presumably related to the reproductive peak and the hydrological nature of the collection site (littoral area with a slow current and strong overgrowing with macrophytes).