Rhyacodrilus palustris (Ditlevsen, 1904)

Timm, Tarmo, 2020, A literature revision of Pararhyacodrilus Snimščikova, 1986, with the erection of a new genus, Semernodrilus gen. nov. (Oligochaeta, Tubificidae, Rhyacodrilinae), Zootaxa 4786 (3), pp. 431-436 : 432-433

On Rhyacodrilus palustris and Rhyacodrilus ekmani

The systematic position of Rhyacodrilus palustris is much confused in the literature. The species was described by Ditlevsen (1904, p. 408, Taf. XVI, figs 1 & 2) under the name Ilyodrilus palustris . The description was built on comparison with the well-known species Ilyodrilus coccineus (= Rhyacodrilus coccineus ). The dorsal bifids were said to be uniform (with a much longer distal tooth, and without any connecting membrane between the teeth, as present in I. coccineus ) along the whole body. The ventral bifids were, on the contrary, similar to the dorsals only in anteclitellar bundles but different, with a shorter distal tooth, after the clitellum like in I. coccineus . The vas deferens was straight in I. palustris but coiled in I. coccineus . The atrium was prolonged and with a constriction in its middle portion in I. palustris but almost roundish in I. coccineus . The spermathecal ampulla was roundish when empty but broader than lengh when filled; nothing was said (nor shown in the author’s good drawings) about its possible connection with the intestine. The peritoneal cells on the nephridia were described as flat in I. palustris but high in I. coccineus . Perivisceral (= chloragogen) cells were very large along the whole intestine and full of clear drops. Coelomocytes were not described. I. palustris was found in Denmark, probably near Copenhagen where the Ditlensen’s study was conducted, and has never been re-described; type material does not exist.

The species ‘acquired’ the character state of connected spermathecae by the re-description of one specimen from Lake Baikal, by Michaelsen (1908) which this author considered conspecific with palustris . Michaelsen (1908, Pl. III, Fig. 7, 8) depicted a thick-walled diverticulum on the distal (not apical!—see below) portion of the spermathecal ampulla, which continued as a narrow duct and finally disappeared among the chloragogenous cells of the intestine. The actual connection with the intestinal lumen was not visible and only inferred. Later on ( Michaelsen 1909: 32), this character served to distinguish R. palustris from R. coccineus , and was accepted by all subsequent authors (see below). The proper identity of this Baikalian specimen remains unresolved. It belongs to the type series of Branchiura coccinea var. inaequalis ( Michaelsen 1905a) , a taxon that the same author had previously synonymized with Ilyodrilus palustris Ditlevsen, 1904 ( Michaelsen 1905b) .

Side note 1: The resulting name of this synonymization was Branchiura coccinea var. palustris ( Michaelsen 1905b) . In the above-mentioned redescription Michaelsen (1908) used the genus name Taupodrilu s and re-erected the variety to species rank, as Taupodrilus palustris (Ditlevsen) . One year later ( Michaelsen 1909: 31, footnote) he suggested priority of Rhyacodrilus Bretscher, 1901 over Taupodrilus Benham, 1903 , and this has been adopted since then.

Side note 2: As to ‘ inaequalis ’, this taxon has been accepted as a separate species endemic to Lake Baikal ( Čekanovskaja 1962; Semernoy 2004), as Rhyacodrilus inaequalis (Michaelsen, 1905) . Semernoy (2004: 139–144) redescribed the species and did not observe a spermathecal diverticulum. Snimščikova (1986, p. 207), apparently by mistake, accepted the synonymy of ‘ aequalis ’ and ‘ palustris ’ when including R. palustris into the identification key of Pararhyacodrilus , with the chaetal characters similar to those R. inaequalis , but not R. palustris , when referring to the original descriptions. Later on, Snischikova & Akinshina (1994) listed R. coccineus ssp. inaequalis but not R. palustris , for the Baikalian fauna.

The third record of R. palustris is from Lake Vättern in Sweden ( Ekman 1915). Piguet, who identified the specimens, took an observed spermathecal diverticulum connected with the intestine as evidence. Apparently he was misled by the erroneous synonymy of the latter with R. inaequalis as introduced by Michaelsen (1908), and he ignored that this character was not dealt with in the original description. Michaelsen’s key (1909) on aquatic oligochaetes probably served as identification guide; here R. coccineus and R. palustris were separated by the absence vs. presence of a spermathecal connection with the intestine.

Later on, however ( Piguet 1919: 32, footnote), he suggested identifying these specimens as R. coccineus , and that the diagnoses of R. coccineus should be changed to include connected spermathecae as character. Piguet apparently ignored other differences between R. coccineus and R. palustris described by Ditlevsen (1904).

Even later ( Piguet 1928, p. 96), and upon renewed investigation of the same material, he erected a new species, R. ekmani , and distinguished it from R. coccineus by differences in the bifid chaetae and, notably, in the presence vs. absence of a connection spermatheca-intestine. He further distinguished R. palustris and R. ekmani by differences in atrium shape and length of spermathecal duct, and the limitation of the dorsal hair chaetae to the anteriormost segments (up to segment V) in R. palustris . Alas, the latter character was not mentioned by Ditlevsen (1904) but apparently taken from the description of the allegedly synonymous R. inaequalis by Michaelsen (1905a, p. 10) (see above). The dorsal hair chaetae can reach down to IX–LXIX in R. ekmani . The lack of an inner duct of the sperma- thecae in the Ditlevsen’s original description of R. palustris was either unnoticed or ignored by Piguet (1919, 1928), who probably relied on Michaelsen (1908, 1909).

In spite of the above corrections by Piguet (1928), the false assumption that R. palustris occurs in Lake Vättern and that its spermathecae are connected with the intestine, has been repeated in subsequent guide-books including Čekanovskaja (1962), Brinkhurst & Jamieson (1971), Timm & Martin (2019) and others. Both R. palustris and R. ekmani are actually known only on the basis of the more than one century-old material from their respective original localities in Denmark and Sweden.

Position of the inner (ental) duct of spermathecae of R. ekmani looks different in the two available publications (but made on the same material!). In the original description ( Piguet 1928, Fig. 7B), it is depicted as attached to the distal part of the ampulla (similar to the condition in R. inaequalis sensu Michaelsen 1905b, 1908 ), whereas in one of Piguet’s original specimens studied by Brinkhurst & Kennedy (1962, Pl. VII), the inner duct is attached apically to the proximal end of ampulla.

One may speculate that the connection between the spermathecae and gut may be an intraspecific variation sometimes emerging in populations of Rhyacodrilus coccineus . If this hypothesis was true, then Pararhyacodrilus ekmani could result as a variant(?) of Rhyacodrilus coccineus . Piguet (1919) commented that this character should be added to the diagnosis of R. coccineus , but this was never made explicit by subsequent authors, himself included ( Piguet 1928). G. Milbrink (1970, p. 93) found only R. coccineus in his samples from Lake Vättern and discredited the earlier records of “ R. ekmani , and the so-called R. palustris —all very much alike”. However, he did not propose to upgrade the diagnosis of the common and easily recognizable R. coccineus with this possible variation in the spermathecae.

A taxon indirectly aligned with the modern genus Pararhyacodrilus is Rhyacodrilus ekmani var. profundalis Lastočkin (1937, p. 234) . It was found from depths of 50 to 98 m in Lake Umbozero, the Kola Peninsula in northwestern Russia. It differs from the nominate R. ekmani in its longer chaetae, a thin membrane between the teeth of dorsal bifid chaetae, and an expanded distal portion of all bifids (“humpbacked chaetae”). Lack of con- nection between the spermathecae and the digestive tract was positively noticed in the otherwise short description. Snimščikova (1986, p. 206) correctly denied any affiliation of this taxon (by default, together with the nominate R. ekmani ) to Pararhyacodrilus and raised it to species rank as Rhyacodrilus profundalis Lastočkin, 1937 . The species has never been refound. When I sampled in Lake Umbozero in 1969, at a depth of 22 m, I found several R. coccineus (Vejdovský, 1876) , but no R. profundalis ( Timm & Popčenko 1978, p. 82) .