A FURTHER CRITERION FOR THE IDENTIFICATION OF SPECIES IN THE GENUS CYCLOPS S. STR. (COPEPODA, CYCLOPOIDA) BY ULRICH EINSLE Landesanstalt für Umweltschutz Baden-Wurttemberg, Institut für Seenforschung und Fischereiwesen, Aufienstelle Konstanz, F.R. Germany GENERAL REMARKS ' The taxonomy of the "strenuus-group" of the genus Cyclops has been examin- ed in a number of morphometrical studies (Rzoska, 1930; Kozminski, 1927, 1933) and culminated in Lindberg's (1957) monograph which recognized 52 species and subspecies. Morphometrical studies on the genus were continued by Kiefer (1939) and Einsle (1964, 1975). In contrast to Lindberg, who often examined few individuals, the latter authors considered variability by measur- ing thousands of individuals. These studies (Einsle, 1975) revealed problems in the application of Lindberg's classification system. Chromatin-diminution (Beerman, 1959) studies were employed to clarify these taxonomic problems. They had the advantage of being independent of external morphological structures, and it proved possible to distinguish species or at least groups of two or three species by their patterns of chromatin- . diminution. However, the application of this technique is limited by the need to have living females with eggs in a certain stage of cleavage division and by the expertise required to prepare specimens. Therefore, the search for a mor- phological criterion that easily allows the determination of Cyclops species has . been continued. One promising possibility seems to be found in the pattern of spines on the posterior face of the coxa of the 4th pair of swimming legs (fig. 1). The patterns are relatively uniform within a species, but remarkable variation exists among species. In this paper, six groups of spines are described and categorized by the letters A to F. Spine patterns can be described by a few letters, and each species has one or two typical spine patterns. The investigation of the coxa by scanning electron microscopy would show more details, but the phenotypes described in this paper can be recognized easily by light microscopy. Some of the species in the "strenuus-group" are easily recognized by their body shape and by the characteristics of their furcal setae (C. scutifer, C. vicinus, C. insignis). In other species (C. abyssorum, C. strenuus, C. furcifer) a high degree 300 of morphological variation exists. In these latter cases, examination of the spine pattern of the fourth pair of legs greatly aids identification. In western Europe the following species are important for these studies: Cyclopsabyssorum (Sars, 1863) Cyclopskolensis Lilljeborg, 1901 Cyclopsstrenuus (Fischer, 1851 ) Cyclopsscutifer Sars, 1863 Cyclopsvicinus Uljanin, 1875 - Cyclopsinsignis Claus, 1857 ' Cyclopsbohater Kozminski, 1933 Cyclopslacustris Sars, 1863 Cyclops furciferClaus, 1857 Cyclopsohridanus Kiefer, 1932. Fig. 1. Coxa of the 4th swimming legs of Cyclopsspec., posterior aspect. General scheme with groups of spines indicated. ' DESCRIPTIVE NOTES Cyclops strenuus . C. strenuus (s. str.) shows a very different pattern of chromatin-diminution than do other species in the genus Cyclops, as the eliminated heterochromatic par- ticles are located behind the anaphase nuclei. It can also be recognized mor- phologically by the shape of its 4th thoracic somite, which reaches its maximal breadth in the middle of the segment. This species can be confused with C. abyssorum and some seasonal types of C. furcifer. The coxa (fig. 2) normally has spines belonging to groups A and C, and in- frequently group D. The coupler occasionally lacks spines, but in most cases it .