The existence of more species than those currently recognised or estimated is suggested by the abundance of morphologically unrecognised or cryptic species, even in well-known taxa (Barratt et al., 1997). Due to the lack of taxonomically useful morphological characters and widespread dispersal capabilities, many freshwater invertebrates were traditionally believed to constitute single, cosmopolitan species (Suatoni et al., 2006). With the advent of molecular systematics, however, this view has now changed and previously undetected genetic diversity has demonstrated the existence of cryptic species complexes in many aquatic invertebrates (De Meester et al., 2002). The number of cryptic species hidden in various taxonomic groups of freshwater animals is unknown and it seems to be an important element of biological diversity in freshwater environments. The presence of species, which are not distinguished on the basis of morphology but isolated in terms of reproduction, has been studied intensively in recent years because of the recognition of biodiversity protection as a priority for nature conservation.
Bickford et al. (2006) considered two or more species to be ‘cryptic’ if they are, or have been, classified as a single nominal species because of their at least superficially morphological indistinguishability. Two categories of cryptic species may be distinguished: (i) populations of very similar individuals or even individuals impossible to identify morphologically with strong reproduction barriers between them, (ii) populations previously recognized as a part of a single, highly variable morphologically or ecologically divergent species, sometimes treated as its forms (varietas), in cases where strong reproduction barriers between them have been found.
Species richness in different taxonomic groups of freshwater invertebrates is still growing as it is demonstrated that numerous common species are in fact cryptic complexes. Examples were presented among molluscs (Korinkova et al., 2006), and many groups of crustaceans (King et al., 1998; Nilssen et al., 2007; Belyaeva and Taylor, 2009) and insects (Williams et al., 2006; Pfenninger et al., 2007; Paredes-Esquivel et al., 2009). Cryptic species were found also among various groups of Clitellata, e.g. Tubificidae (Beauchamp et al., 2001), Lumbriculidae (Gustafsson et al., 2009) and numerous Hirudinea. The number of leech species has been growing in recent years mainly as a result of the acceptance of numerous cases, previously regarded as morphological forms as valid species. It can be illustrated by the cases of the morphological forms of certain common, highly divergent leech species occurring in Europe and North-America such as Glossiphonia complanata (Linnaeus, 1758), Alboglossiphonia heteroclita (Linnaeus, 1761), Erpobdella testacea (Savigny, 1822), Haemopis sanguisuga (Linnaeus, 1758), Piscicola geometra (Linnaeus, 1758), Dina parva Moore 1912, and Erpobdella punctata (Leidy, 1870), which were finally determined to be valid species (Agapow and Bielecki, 1992; Govedich et al., 1999; Neubert and Nesemann, 1999;Verovnik et al., 1999; Grosser, 2004; Hovingh, 2004; Jueg et al. 2004). Morphological characters, especially the structure of reproductive organs and colouring patterns were mainly used in the past to distinguish between these new species and in the majority of cases specific methods of DNA analysis confirmed the taxonomic arrangements based on morphology (e.g. Utevski and Trontelj, 2004; Hovingh, 2004). Petrauskiene et al. (2009) presented hybrid offspring of interbreeding parents of the genus Hirudo having typical colouration pattern similar to one of the parents and suggested weakness of the reproductive isolation between species.
Erpobdella octoculata (L., 1758) is one of the most common leech species in European freshwaters. It typically dominates the leech assemblages in Central-European lowlands in lake littoral, streams and rivers as well as in urban park ponds (Koperski, 2006). Contrary to other European species of the genus, E. octoculata is an extremely polymorphic species - typically at least 4-5 forms or morphotypes, differing in colour, were distinguished. They were traditionally called f. vulgaris, atomaria, typica, localis, pallida - all as described by Pawłowski (1936). The rank of these forms in modern taxonomy is not clear - such terms are illegal under International Code of Zoological Nomenclature (1999, article 45.6.4). The most important differences in their colouring patterns are relative area of not-pigmented spots around papillae and background colouring of the body. Individuals of E. octoculata seem to be divergent also in terms of their ecological preferences as well as in terms of their life-history. They were presented by Maltby and Calow (1986) as coexisting semelparous and iteroparous populations.
Partial or complete reproduction barriers between co-habiting forms would explain great variability inside the species. The main aim of this study was to reveal any reproduction barriers and potentially cryptic radiation inside the groups of individuals and populations of E. octoculata. The differences in reproductive success of the morphological forms were compared experimentally. The preliminary results of the study were presented briefly in Koperski et al. (2008). The data presented here are based on the results of field sampling, experimental breeding of selected individuals and analysis of DNA sequences.