Certain new species have been found in recent years as genetically separated groups of individuals inside former species (complexes) of leeches (e.g. Govedich et al., 1999; Trontelj and Sket, 2000; Hovingh, 2004; Utevsky and Trontelj, 2005) and other common freshwater invertebrates (e.g. Korinkova et al., 2006; Gustafsson et al., 2009) and then confirmed with the results of COI and ITS analysis. The results of the analysis of DNA sequences presented in our study clearly suggest the lack of reproductive barriers between the analysed morphological forms. On the basis of these results, Erpobdella octoculata should be treated as one, valid species, in spite of its great morphological variability. However, these conclusions do not necessarily imply that there are no cryptic species in this taxon over the entire range. The presence of differences in DNA sequences between individuals, found with the use of the AP-PCR method, seems to be related to geographical distance between sub-populations. However, experimentally confirmed diversified reproductive success of individuals belonging to different morphological forms seems to show the presence of weak reproductive barriers between them. The rise of isolated groups of individuals divergent in reproductive success and, consequently, new species inside group of co-habiting individuals (or the sympatric speciation) has been supported under specific circumstances (Munday et al., 2004; Barluenga et al., 2006a, b; Schliewen et al., 2006). Sympatric speciation has not been studied in leeches.
There is a visible incongruity between the above results of DNA sequences and the results of experimental study, which is not easy to explain. Results of experimental breeding could be recognized as an effect of ‘assortative mating’ similar to those described in other aquatic invertebrates (Hull, 1996; Hermann et al., 2009). Presented incongruity can also be an effect of impaired reproduction of individuals collected in particular water body, as a result of their disease or weakness. This question definitely needs further studies. It must be emphasized that breeding of E. octoculata in laboratory is difficult and laborious, and these animals reproduce only once a year (those collected in the field as well as those hatched in laboratory), which makes the experimental procedures highly time-consuming.
The results of preliminary experiments on mating of E. octoculata (Koperski, unpublished data) show the lack of any differences in colour pattern of offspring when parents bred on the stone bottom differing in colour (white, black and brown). The different percentage of morphological forms recorded in field samples suggests an adaptive value of divergent colour patterns in different habitats. Such differences may be a result of diverse mortality of individuals differing in colour dwelling on different bottom substrates. The pressure of visual predators like benthivorous fish may be one of the most important factors determining such mortality. The behavioural defence of erpobdellid leeches induced by the presence of freshwater fish was found in experiments by Mr Paweł Prus (Koperski, 2002). It must be added that E. octoculata was found to be a dominant species especially in environments inhabited by benthivorous fish (Koperski, 2006), which suggests a great role of induced, anti-predator defence of this species.
Numerous scientists highlight the study of cryptic species as extremely important in global biodiversity research (Bickford et al., 2006; Cook et al., 2008). The presented results seem to constitute a rarely presented example of genetic uniformity among widely occurring species, highly variable in terms of ecology and morphology. The case of Erpobdella octoculata is an exception when compared with other common, widely distributed and morphologically variable, old-established complexes of European leech species as Erpobdella testacea (one former species divided into 3 species), Glossiphonia complanata (4 species), Glossiphonia (at presence Alboglossiphonia) heteroclita (3 species), Piscicola geometra (ca. 10 species) or Haemopis sanguisuga (2 species) (Grosser - Egel Europas). It stands in opposition especially with the results of Govedich et al. (1998, 1999) and Hovingh (2004) concerning North-American erpobdellid species, describing, on the basis of morphology and genetic analysis, a few new species and genera.