At present, the genus Tasserkidrilus Holmquist, 1985 includes 14 freshwater species, all in the Holarctic Region (Table 1). Of all the species, 11 were recorded in the Palearctic Region, of which nine species are endemic to Lake Baikal and one species is endemic to the Kamchatka Peninsula. Despite high endemism of the genus, there are two species, T. americanus (Brinkhurst & Cook, 1966) and T. kessleri (Hrabě, 1962), which have a more scattered distributional pattern, occurring in several countries in both Eurasia and North America.

The first documentation regarding the genus Tasserkidrilus is Tubifex kessleri, which was described by Hrabě (1962), was based on an single, incomplete specimen found in Lake Onega (Russia). Morphological features make this species easily recognisable from other tubificines – some anterior ventral chaetae with intermediate teeth and the funnel- or cone-shaped, chitinous penis sheaths, shown in Fig. 4 below. At present, the latter is considered a diagnostic feature for the identification of the genus Tasserkidrilus, which was later accepted as the correct genus assignment for many species previously classified in Tubifex (Holmquist 1985). Moreover, the taxonTubifex kessleri has included several subspecies in the past, which are currently considered as valid species of Tasserkidrilus (Table 1).

Figure 1.  

Map showing the locality of the Tešmak swamp in Slovakia and a photograph of the channel with the documented population of Tasserkidrilus cf. americanus.

Figure 2.  

Photographs of a-b anterior ventral chaetae c posterior ventral chaeta and d pectinate chaetae of T. cf. americanus; the black arrow indicates the intermediate teeth.

Figure 3.  

Photograph of the penis sheath of Tasserkidrilus cf. americanus from Slovakia.

Figure 4.  

Drawings of the penial sheaths presenting the differences in shape and length between the North American and European populations of T. (cf.) americanus and T. kessleri. Drawings maintaining proportionality and shape of the penis sheath, based on photographs by van Haaren and Soors (2013) and Holmquist (1985), are free interpretations by the author.

Brinkhurst and Cook (1966) found worms with similar features to Hrabě's specimen in the Great Lakes of North America, although they differed by the presence of long hair chaetae in all anterior segments. The penis sheaths had the same shape and dimensions; however, they were not always as broad as in Hrabě's specimen. The material was described by Brinkhurst & Cook (1966) as a subspecies, Tubifex kessleri americanus, which was subsequently elevated to species and selected as the type species of a new genus,Tasserkidrilus, by Holmquist (1985). The information provided by Reynolds and Wetzel (2021), who represent Aulodrilus americanus Brinkhurst & Cook, 1966 as the type species of Tasserkidrilus, is incorrect.

All North American records of T. americanus date back to studies carried out more than 30 years ago (see GBIF Secretariat (2021)). At present, new records outside the main distribution area are very rare; for example, van Haaren and Soors (2013) documented its occurrence in Europe for the first time. On the basis of the different morphology of the penis sheaths and the environmental requirements of the European population, when compared with North American populations, those authors stated that they were observing a new “form”. The main goal of the study is to present a possible new record of the species T. cf. americanus from Central Europe and to compare the new information with briefly updated literature data about the species from elsewhere in the world.

A macroinvertebrate sample was taken semi-quantitatively using a standardised hydrobiological mesh from a muddy bottom substrate, submerged plants and organic detritus in the channel flowing from the Olvár stream that supplies water to the Tešmak swamp in the southern part of Slovakia in April 2020 (Fig. 1, Suppl. material 1). The swamp and channel, which contribute water from the Olvár stream, are of a eutrophic character with seasonal water level fluctuations. The sampled material was fixed in a 4% formaldehyde solution in plastic sample containers. In the laboratory, the macroinvertebrates were sorted and preserved in 70% ethyl alcohol. Prior to identification, aquatic oligochaetes were processed through ethyl alcohol and clove oil to dehydrate and clear their bodies in order to study their internal tissues and organs. Subsequently, they were fixed in Canada Balsam as permanent mounts and kept in a dryer set at 40°C for four days. Species were identified using the binocular microscope Leica DMLB and the determination keys by Hrabě (1979), Kasprzak (1981), Timm (2009) and van Haaren and Soors (2013).

Altogether, 63 specimens, of which 30 were sexually mature, were identified as Tasserkidrilus cf. americanus. The density of the population reached 648 individuals per square metre. The species was diagnosed on the basis of its characteristic, external, morphological features. Ventral chaetae from segments II to V (or VI) have a longer upper tooth and several are equipped with an intermediate tooth, which can sometimes be doubled (Fig. 2a and b). From segment VII, the length of the upper and lower teeth equalises. Penial ventral chaetae in XI are thicker and shorter than anterior chaetae and in a reduced number or are completely absent. Posterior ventral chaetae have teeth equal in length, with the upper tooth 1 – 1.5 x thinner than the lower one and some of them may have an intermediate tooth (Fig. 2c). Hair chaetae present in anterior and posterior segments are up to 440 µm long. The species has pectinate chaetae similar to Tubifex tubifex (Müller, 1774) (Fig. 2d). All chaetae are shorter in the posterior part of the body when compared with the anterior part. The length and number of chaetae are presented separately for each study describing the morphological features of the species in Table 2. Adult specimens have penis sheaths situated in segment XI (Fig. 3). Fig. 4 and Table 2 indicate the difference in shape and length of penis sheaths between populations of T. americanus and T. kessleri.

In Europe, frequent records of T. cf. americanus in streams have been documented from Belgium and The Netherlands (van Haaren and Soors 2013). The present finding of the species from Slovakia has reopened the discussion about the origin, distribution and taxonomy of the European population. The penis sheaths of individuals from Slovakia, Belgium and The Netherlands are proportionally larger and wider when compared to individuals occurring in North America, whereas no comparison can be made with the population from Russia due to lack of information. In Tubificinae, the proportions and specific shapes of penis sheaths are essential features for identification of many species of one same genus when the external features are similar. Therefore, the differences observed within T. americanus “forms” must be carefully considered. Actually, the issue of whether T. cf. americanus in Europe is the same as T. americanus in North America cannot be definitely resolved without genetic information for both forms. It is possible, in fact, that we are dealing with a cryptic species or subspecies. van Haaren and Soors (2013) preferred to name their material "Tasserkidrilus near americanus", because they did not have enough evidence on the identity of that species. Moreover, the authors mentioned that the record of Ilyodrilus templetoni (Southern, 1909) from the Estuary of the Elbe River (Germany), described by Pfannkuche (1977), was most likely the American form of T. americanus, based on the length and shape of the penis sheaths (Table 2). However, recent studies, which took place in the Elbe Estuary, did not confirm that the species really occurred there and neither did I. templetoni (see Wetzel et al. 2012).

The ecology of the discussed forms seems to be different as well, although the environmental requirements of North American populations of T. americanus showed a consistent pattern (Brinkhurst and Cook 1966, Hiltunen 1967). The species was found mostly in large, oligotrophic lakes, although Timm (2009), Spencer and Hudson (2003), and Soors et al. (2013) observed that the habitat of T. americanus includes rivers and coastal intertidal and subtidal waters as well. In contrast, the European populations of T. cf. americanus seem to be more tolerant to organic pollution, since they were found in eutrophic waters, marshes and, in the case of Slovakia, in the small channel with slow-flowing water near the swamp. van Haaren and Soors (2013) added that the European worms are tolerant to desiccation as well.

Another question that arises from the occurrence of T. cf. americanus in Slovakia is its origin: is it a non-indigenous species that was introduced from its natural area (North America), as stated by Soors et al. (2013), or is it native to Europe, but too rare to record? Moreover, van Haaren (2002) added that the identification of oligochaete species was not popular in Belgium in the past and, thus, the species may have been overlooked. This may also be the case of its current record in the channel in Slovakia, for which the latter has not undergone any research of its macroinvertebrate community or monitoring of its water quality by state institutions (Makovinská et al. 2015). Moreover, the Tešmak swamp belongs to The Poiplie Special Protected Area under the Ramsar Convention and Natura 2000. Therefore, the introduction of alien species by human activity is highly unlikely due to its protection status, although it cannot be ruled out. Even the Ipeľ River, which is a tributary of the Danube and flows near the swamp, cannot be considered a potential vector of the spreading of T. americanus, since it is a medium-sized river inappropriate for navigation and the species has not been documented there at all. Based on those arguments, the presence of T. cf. americanus in Slovakia remains inexplicable and requires further investigation.

Support for this research was provided by the project LIFE15 NAT/SK/000861 Wetland habitat restoration and bird protection of Poiplie, Horna Orava and Senianske rybniky SPAs in Slovakia. I am very grateful to two reviewers and associate editor for their constructive feedback, recommendations and comments that substantially helped to improve the manuscript.

LIFE15 NAT/SK/000861 Wetland habitat restoration and bird protection of Poiplie, Horna Orava and Senianske rybniky SPAs in Slovakia.

Institute of Zoology, Slovak Academy of Sciences, Bratislava, Slovakia.

All applicable national and institutional guidelines and permissions for the sampling and preserving of macroinvertebrates were followed.

The author has declared that no competing interests exist.