Molecular and morphological patterns across Acanthocyclops vernalis-robustus species complex (Copepoda, Cyclopoida)

Morphological traits within Acanthocyclops (Kiefer, 1927) are highly variable, and morphology is too constrained to give complete information of phylogenetic relationships. This study combined morphological and molecular techniques to investigate the taxonomic and phylogenetic relationships of three species of Acanthocyclops (Acanthocyclops trajani, Acanthocyclops einslei and Acanthocyclops vernalis) inhabiting continental Europe. Morphological indices subjected to principal component analysis (PCA) separated sample populations into three distinct clusters corresponding with the taxonomic status of the species analysed. In addition, the taxonomy status of A. trajani and A. einslei was in agreement with molecular data; however, the intraspecific variation in sequences of 12S rRNA was lower in contrast to specimens morphologically determined as A. vernalis, which were divided into two deeply divergent clades, based on mtDNA sequence divergences. Moreover, high sequence divergence (26%) between these clades indicated the existence of another species that may not be a sister taxon of A. vernalis s.s. Results point to the need for further taxonomic work on Acanthocyclops.