Phylogeography of Calanus helgolandicus and the Black Sea copepod Calanus euxinus, with notes on Pseudocalanus elongatus (Copepoda, Calanoida)

Calanus helgolandicus is a widespread epipelagic copepod species whose geographical range extends from the temperate Atlantic Ocean to the northern Mediterranean Sea. Calanus euxinus, recently designated as a distinct species though closely related to C. helgolandicus, occurs in the Black Sea. Very subtle morphological differences distinguish the two species. Pseudocalanus elongatus has a similar geographic range including North Atlantic Ocean, Mediterranean Sea and the Black Sea. In this study, population genetic variation of C. helgolandicus, C. euxinus and P. elongatus was investigated using DNA sequence variation of 540 base pair (Calanus spp.) and 575 base pair (P. elongatus) regions of mitochondrial cytochrome oxidase I (mtCOI) gene. C. helgolandicus was collected from the English Channel, the Adriatic Sea, and C. euxinus was collected from various regions of the Black Sea. P. elongatus was collected from the English Channel and the Black Sea. pecific differentiation in mtCOI was <1% for all species; mtCOI sequence variation between C. helgolandicus and C. euxinus was <0.5%. The absence of substantial genetic differentiation between C. helgolandicus and C. euxinus is particularly striking in comparison to other close species pairs in these genera. Statistically significant haplotype frequency differences were determined for different locations of the Black Sea, English Channel, and Adriatic Sea Calanus populations ( χ 2 = 3.94 , P < 0.0001 ). The haplotype diversity was high for all species: C. euxinus ( h = 0.92 ), C. helgolandicus ( h ⩽ 0.80 ), P. elongatus ( h ⩽ 0.60 ). No haplotype sharing was reported for different locations of P. elongatus, whereas the presence of haplotype sharing between C. helgolandicus and C. euxinus was remarkable. The size distribution in terms of prosome length measurements was found to be region-specific. The lack of phylogenetic differentiation between the Calanus species pair may suggest ancestral polymorphisms. The morphological and genetic similarities between C. euxinus and C. helgolandicus raise new questions about the status of C. euxinus as a different species.