Cryptic species within the freshwater copepod genus Hemidiaptomus (calanoida, diaptomidae).

The copepod genus Hemidiaptomus is composed by about 17 large-bodied species inhabiting the freshwater temporary water bodies of the whole Palaearctic region. It currently comprises three subgenera, which are mostly parapatric and present only limited co-occurrence areas. There is no consensus on the taxonomical arrangement of the entire group, and to date no molecular studies have been carried out to test it. Accordingly, a thorough revision of the entire genus using a combined morphological and molecular approach is strongly advisable. We have thus decided to investigate the pattern of morphological and molecular diversity of three species of the genus Hemidiaptomus belonging to the three different subgenera (i.e. Hemidiaptomus (Gigantodiaptomus) amblyodon, H. (Hemidiaptomus) gurneyi s.l., and H. (Occidodiaptomus) ingens) with the aim of checking the reliability of the morphological characters currently used for species identification, and the possible presence of cryptic species within the genus. The three studied species present a sharply different molecular structuring. H. amblyodon shows a remarkable molecular and morphological constancy throughout its distribution range; conversely, observed distances between presumed conspecific clades of H. ingens and H. gurneyi suggest that under these binomens are in fact included complexes of cryptic, or currently just unrecognized, species, which should have began to diverge during the Miocene or even the Oligocene. However, the possibility that mtDNA clock in diaptomids could tick at a faster pace when compared with that of other crustaceans should be also taken in consideration, and further studies aimed at investigating the natural history of diaptomid copepods are needed in order to test this hypothesis. Finally, our results gives further evidences of the high potential for copepod speciation with no or little morphological changes, and stress the need of a revision of the most controversial Palaearctic diaptomid genera, like Hemidiaptomus and Arctodiaptomus, with a combined molecular and morphological approach.