The East Asian Invader Thermocyclops taihokuensis (Harada 1931) and the Native Thermocyclops oithonoides (Sars 1863) (Crustacea, Cyclopoida): A Comparative Analysis of the Morphology of Two Related Species from Water Bodies of European Russia.

Details of the body structure of sexually mature copepods (females and males) of two morphologically similar species of the genus Thermocyclops are presented. One was the invader, T. taihokuensis from the Shat Reservoir, Tula oblast, while the other was the native T. oithonoides from Pleshcheyevo Lake, Yaroslavl oblast. Both were studied to clarify the known morphological features and to identify new ones in order to distinguish reliably between these species. The structure was analyzed using programs for visualization and measurement of objects observed under the AxioVision Rel. 4.8 and ScopePhoto 3.0 microscopes. Drawings of the fine structures were made from photographs. The distinctions between these two species are shown to be mainly based on qualitative characteristics (the shape of the element, its location, etc.). No taxonomically significant differences have been found in the measured parameters of the elements of the fourth, fifth, and sixth (in males) legs, the caudal rami, and their setae, all of which are commonly used to identify species of this genus. The shape of the seminal receptacle, the apical setae of the caudal rami, the apical spines of the endopodite of the fourth legs (P4), and the position of the lateral seta on the caudal rami have been found to be the most important for identifying the females of these species. Interspecies differences were revealed in the armature of the prominences of the intercoxal coupler, the ornamentation of the P4 coxopodite and its intercoxal coupler, and the armature of the 14th segment of the antennules of females. These characteristics are proposed as additional features for species identification. Thermocyclops taihokuensis males are shown to be significantly larger than those of T. oithonoides, and they also differ in the position of the lateral setae of the caudal rami, the P4 armature, and the length of the central apical setae. A key is proposed for identifying six species of the genus Thermocyclops common in European Russia. [ABSTRACT FROM AUTHOR]