A redescription of Eucyclops serrulatus (Fischer, 1851) (Crustacea: Copepoda: Cyclopoida) and some related taxa, with a phylogeny of the E-serrulatus-group

Eucyclops serrulatus (Fischer, 1851), the type species of the genus Eucyclops, is redescribed from specimens found in the St. Petersburg area, Russia (type locality) and compared with specimens from Siberia, western Europe and North Africa. A neotype is selected. Cultures were set up, and interpopulation hybridization as well as hybridization with related species was attempted. The classical description of external body morphology was combined with pore signature mapping and with DNA nuclear small subunit (18S) ribosomal gene sequence analysis. Comparisons with E. dumonti Alekseev, 2000, E. hadjebensis Kiefer, E. speratus Lilljeborg, E. turcomanus Lindberg, and E. pectinifer (Cragin, 1883) were carried out. A phylogenetic tree based on molecular information shows that E. serrulatus and E. speratus should be regarded as separate species. E. dumonti also deserves species status, but not E. hadjebensis. A cladistic tree based on the pore pattern of the cephalosome agrees well with a tree based on the sequence of the 18S rDNA gene. Cephalosome (and probably metasome) pore patterns seem useful to elucidate relationships within genera, while urosomal pore patterns better reflect the relationship between genera. E. serrulatus occurs in three morphological forms over most of its range; one of these (C) might be a rare ('recessive') morphotype, while forms A and B differ in microhabitat choice, but hybridize when living together. The same polymorphism also occurs in an American species (E. prionophorus), and therefore two hypotheses regarding its origin are advanced: either forms A and B evolved during the glacial episode (Pleistocene origin), separately on both sides of the Atlantic, or the polymorphism was already present in the ancestor of the serrulatus-group, and was later lost in some but not in all species (Pliocene origin.)