Species delineation in Cyclops Müller, 1776 (Copepoda: Cyclopoida: Cyclopidae) based on morphology, DNA sequences, genome size, and chromatin diminution

A combination of traditional and new kinds of characters is used to delineate three cold-water cyclopoid copepod species, Cyclops gracilipes Sars, 1903, C. floessneri nom. nov., and C. abyssorum Sars, 1863, which are distributed in southern Siberia, Mongolia, and Europe, respectively. The species name Cyclops glacialis Flössner, 2001, a junior primary homonym of Cyclops glacialis Brady, 1910, is replaced herein by a new name, Cyclops floessneri nom. nov. Morphological analysis revealed one trait in males, the number of aesthetascs on the first antennulary segment, that could be used alone for species discrimination. A few morphological characters differing in frequency of variable states suggest that C. gracilipes and C. floessneri nom. nov. are more closely related to one another than either is to C. abyssorum, which is consistent with the single-base-pair difference in the 18S ribosomal RNA barcode between C. abyssorum and the other two Cyclops species in contrast to no difference between C. gracilipes and C. floessneri nom. nov. The timing of DNA excision occurs at the 4th cleavage division in C. gracilipes and the fifth in C. abyssorum (undetermined in C. floessneri nom. nov.). The amounts of excised DNA are large, vary interspecifically, ∼24–48 pg DNA of germline DNA content, and account for more than half of the germline-limited DNA in each species. Nonetheless, the reduced somatic genome sizes of these three species remain large (∼6–11 pg) compared to the somatic genomes of warm-water cyclopoids. Chromatin diminution may possess several adaptive roles likely to be co-opted exaptations. In closely related taxa that undergo chromatin diminution and in which differences in morphology or DNA barcodes are minor or absent, chromatin diminution may provide a clear marker of speciation events.