Interest in cryptic species has increased significantly with current progress in genetic methods. The large number of cryptic species suggests that the resolution of traditional morphological techniques may be insufficient for taxonomical research. However, some species now considered to be cryptic may, in fact, be designated pseudocryptic after close morphological examination. Thus the “cryptic or pseudocryptic” dilemma speaks to the resolution of morphological analysis and its utility for identifying species. We address this dilemma first by systematically reviewing data published from 1980 to 2013 on cryptic species of Copepoda and then by performing an in‐depth morphological study of the formerEurytemora affiniscomplex of cryptic species. Analyzing the published data showed that, in 5 of 24 revisions eligible for systematic review, cryptic species assignment was based solely on the genetic variation of forms without detailed morphological analysis to confirm the assignment. Therefore, some newly described cryptic species might be designated pseudocryptic under more detailed morphological analysis as happened withEurytemora affiniscomplex. Recent genetic analyses of the complex found high levels of heterogeneity without morphological differences; it is argued to be cryptic. However, next detailed morphological analyses allowed to describe a number of valid species. Our study, using deep statistical analyses usually not applied for new species describing, of this species complex confirmed considerable differences between former cryptic species. In particular, fluctuating asymmetry (FA), the random variation of left and right structures, was significantly different between forms and provided independent information about their status. Our work showed that multivariate statistical approaches, such as principal component analysis, can be powerful techniques for the morphological discrimination of cryptic taxons. Despite increasing cryptic species designations, morphological techniques have great potential in determining copepod taxonomy.

Numerous studies have revealed 17 species and subspecies in the species complex E. serrulatus (Fischer, 1851). As a result, it is clear now that the former cosmopolitan species in fact represent a group of closely related species. Some of them are possibly cryptic taxa and need to be redescribed. The objective of this study was to analyse the broadly distributed E. serrulatus populations in Europe and Asia: in Saint Petersburg (the type locality, Russia), Central Russia, Odessa region and Zakarpattia region – Dniester and Danube river drainage basins accordingly (Ukraine), Valencia (Spain), Oslo (Norway), Paris (France) and in Taiwan. Mitochondrial gene (CO1) and morphological analyses revealed a significant heterogeneity between and within the above-mentioned populations. Populations from Ukraine displayed 27.1% genetic differences, supported by qualitative and quantitative morphological distinctions. The heterogeneity in the CO1 gene was shown even in E. serrulatus from the Orlov Pond (Russia) – terra typica for this species. Morphological analysis did not confirm this heterogeneity. Valencian (Spain) and Taiwanese E. cf. serrulatus separate out from other populations. As a result, a new species Eucyclops taiwanensis sp. n. from Taiwan is described. http://www.zoobank.org/urn:lsid:zoobank.org:pub:237198AE-4549-4E55-A836-AAE8A039CB51

The Baltic Sea is one of the world's most intensive centres for maritime traffic. This sea has become an important recipient of alien species transported with ballast water from all over the world. Sibling species are a special group of invasive animals that are hardly distinguishable from relative but often ecologically different local species. In this study, a molecular genetic diagnosis was performed, based on the analysis of DNA nucleotide sequences (barcoding) in the populations of a dominant circumpolar species Eurytemora affinis (Poppe, 1880) from the Gulf of Finland and the Atlantic coast of European estuaries and of Eurytemora carolleeae Alekseev and Souissi, 2011 Alekseev, VR and Souissi, A. 2011. A new species within the Eurytemora affinis complex (Copepoda: Calanoida) from the Atlantic Coast of USA, with observations on eight morphologically different European populations. Zootaxa, 2767: 41–56. [Google Scholar] , a new North American suspected invasive form. The results of the diagnosis were completed by morphological analyses. Considering the key role of E. affinis in the Baltic Sea zooplankton community, the invasion of E. carolleeae might have consequences for biodiversity, biogeography, conservation and ecosystem management in this area.

Comparative phylogeography has become a powerful approach in exploring hidden or cryptic diversity within widespread species and understanding how historical and biogeographical factors shape the modern patterns of their distribution. Most comparative phylogeographic studies so far focus on terrestrial and vertebrate taxa, while aquatic invertebrates (and especially freshwater invertebrates) remain unstudied. In this article, we explore and compare the patterns of molecular diversity and phylogeographic structure of four widespread freshwater copepod crustaceans in European water bodies: the harpacticoids Attheyella crassa, Canthocamptus staphylinus and Nitokra hibernica, and the cyclopoid Eucyclops serrulatus, using sequence data from mtDNA COI and nuclear ITS/18S rRNA genes. The three taxa A. crassa, C. staphylinus and E. serrulatus each consist of deeply diverged clusters and are deemed to represent complexes of species with largely (but not completely) non-overlapping distributions, while in N. hibernica only little differentiation was found, which may however reflect the geographically more restricted sampling. However, the geographical patterns of subdivision differ. The divisions in A. crassa and E. serrulatus follow an east–west pattern in Northern Europe whereas that in C. staphylinus has more of a north–south pattern, with a distinct Fennoscandian clade. The deep mitochondrial splits among populations of A. crassa, C. staphylinus and E. serrulatus (model-corrected distances 26–36%) suggest that divergence of the lineages predate the Pleistocene glaciations. This study provides an insight into cryptic diversity and biogeographic distribution of freshwater copepods.