The morphology of the cephalothoracic appendages is compared among 12 taxa of Cyclops O. F. Müller, 1776: C. strenuus strenuus Fischer, 1851, C. strenuus landei Koźmiński, 1933, C. abyssorum Sars, 1863, C. furcifer Claus, 1857, C. heberti Einsle, 1996, C. singularis Einsle, 1996, C. scutifer Sars, 1863, C. kolensis Lilljeborg, 1901, C. insignis Claus, 1857, C. canadensis Einsle, 1988, C. columbianus Lindberg, 1956, and C. alaskaensis Lindberg, 1956. Diagnostic microcharacters found to be new or rarely mentioned in the earlier descriptions are: the relative length of the aesthetascs on the ancestral XXI antennular segment; the spinule ornamentation on the caudal surface of the antennal coxobasis; spinule ornamentation of the maxillular palp, and setulation of the maxillular palp setae; spinule ornamentation on the frontal surface of the maxilliped syncoxopodite; setulation of the medial spine and spinule ornamentation on the frontal surface of leg 1 basipodite. Hypothetical roles of the elongate aesthetascs on the ancestral XXI antennular segment in the pelagic Cyclops species, and long pinnate setae on the maxillular palp are discussed. C. strenuus landei is synonymized with C. strenuus strenuus, and C. strenuus strenuus sensu Koźmiński, 1933 is found to be conspecific with C. heberti. Refined morphological delimitation of the taxa resulted in obtaining new data on the geographical distribution of C. heberti (southern France, central Italy, northern Germany and Poland) and C. singularis (Hungary and Slovakia). Nearctic occurence of C. strenuus has not been confirmed.
Crustacea, Copepoda, Cyclops, morphology, cephalothoracic appendages, microcharacters, adaptive value, geographical distribution.