Pseudorhynchelmis parva ( Michaelsen, 1905 )

Martin, Patrick & Kaygorodova, Irina, 2008, A new species of Pseudorhynchelmis Hrabě, 1982 (Clitellata: Lumbriculidae) from Lake Baikal, with re-descriptions of P. parva and P. olchonensis, Zootaxa 1938, pp. 23-39 : 25-30

Pseudorhynchelmis parva ( Michaelsen, 1905) ( Figure 1 and 2)

Type reference. Lycodrilus parvus Michaelsen, 1905: 18 –20 Fig. 3, partim

Synonymy. Pseudolycodrilus parvus ( Michaelsen, 1905) . Hrabė, 1982: 185–186, Tab. IV, Figs 6–8. Rhynchelmis parva ( Michaelsen, 1905) . Semernoy, 2004: 413–415, Fig. 230.

Material examined. Zoologisches Museum Hamburg ( ZMUH), V6597. Two sexually mature specimens preserved in ethanol, from a vial labelled as “ Limnodrilus parvus ; Mich.; Korotneff; Baikal-See” containing 3 specimens and one fragment, consisting in two species.

National Museum of Prague ( NMPC), No P6j-2089. Anterior fragments of two mature specimens, longitudinally sectioned (slides Hr1724-1D, 1; Hr1724-1E, Ia, 2). Two posterior fragments (probably the remaining part of sectioned specimens), two complete immature specimens and one complete mature specimen (instead of three immature specimens mentioned in Hrabĕ, 1982); whole-mounted in Canada balsam (slide Hr 1724-1- 1 DE; I, IIw, III–V).

Lectotype. ZMUH, V6597, slide 0 5.195.0 1 (partim) and vial V6597-05.195.01 (partim). Mature specimen, incomplete (first 80 segments), anterior ends (head and sexual segments) cut along the midsagittal plane. Right side stained in alcohol carmine and mounted in Canada balsam on slide 05.195.01; left side and tail left as they were and separately preserved in ethanol after examination in glycerine (V6597-05.195.01).

Type locality. Lake Baikal, Maloye More. Exact locality unknown.

Paralectotype. ZMUH, V6597. One sexually mature specimen, incomplete (first 74 segments), anterior ends (head and sexual segments) cut along the midsagittal plane, right and left sides and tail left as they were and preserved in ethanol after examination in glycerine (vial V6597-05.195.03).

Description. Medium-sized species 12.5–16.2 mm, 74–80 segments, breadth 310–340 µm at widest (clitellum). Prostomium short and rounded ( Fig. 1D). No secondary annulation discernible, intersegmental annulation scarcely marked in postclitellar segments. Clitellum slightly noticeable, extending over ½ IX – XII. Two single-pointed setae per bundle, sigmoid, similar in dorsal and ventral bundles, 77–118 µm long, 4–5 µm thick, nodulus on the ectal third of seta ( Fig. 1C). Setal length increasing in anteclitellar segments until reaching a maximum size in X, slightly decreasing to a nearly constant length (94 – 96 µm) in postclitellar segments. In anteclitellar segments, distal nodulus, more and more median as setal length increases. Two slightly modified penial setae ( Fig. 1C, gs) per bundle in X,>114 – 122 µm long, 4 µm thick, elongate, straight, nodulus slightly proximal to the middle of shaft’s length.

Dorsal pad of pharynx extending from II to IV, pharyngeal glands in IV – VII. Anterior sperm sac confined to X; posterior sperm sacs extending to XIII ( Fig. 1A, sps). Egg sacs not seen. One pair of spermathecal pores opening somewhat posterior to the middle of VIII, on the same line as ventral setae and behind them. Slit-like spermathecal pores, invaginated into well-developed vestibulae ( Fig. 1A, spv), pore area ectally lined with a thickened cuticle ( Fig. 2C, c). Tubular spermathecal ducts ( Fig. 1A, spd), with narrow lumen, leading to voluminous ampullae ( Fig. 1A, spa) roughly divided into two pouches of different sizes, a small proximal and a large distal one. One ampulla usually filling the coelomic cavity in VIII, the other entirely pushed backwards into IX (left and right ampullae in lectotype, respectively). Sperm as an unorganized mass in ampullae, surrounding an amorphous deposit of spermatic material in large pouches of both ampullae. No communication between spermathecal ampullae and gut cavity.

Two pairs of testes in IX and X, attached ventrally to septa 8/9 and 9/10, respectively ( Fig. 1A, t). Male pores paired, in line with genital setae, opening ventrally slightly posterior to the middle of X, just behind genital setae ( Fig. 1A, mp). Two pairs of sperm funnels on septa 9/10 and 10/11, respectively ( Fig. 1A, sf), roughly of the same size, posterior ones directed towards posterior sperm sac. Two pairs of vasa deferentia ( Fig. 1A, vd). Anterior vas deferentia running on the floor of X with some loops, posterior vas deferentia making a few loops in XI before turning back to X, both penetrating atrium subapically. Vasa deferentia 21 µm wide near sperm funnel, 12 µm wide near atrium. Sacciform atria, elongate, 145 µm long, 45 µm wide, straight or (artefactually?) convoluted, with large lumen, covered with thick (7 µm) muscular layer ( Fig. 2A, m), confined to X ( Fig. 1A, a; Fig. 1B). Proximal part of atria pedunculated, terminated by well-developed, eversible, conical and elongated penes ( Fig. 1A, p) in penial sacs. Prostate glands discrete, sparsely covering atria, constituted by large cells associated in elongate bundles, separately opening onto atrium by means of short ducts ( Fig. 1A, pr). One pair of reduced small accessory copulatory glands ( Fig. 1A, cgl), surrounding penial setae near ental tip.

Female pores paired, in line with ventral setae, spermathecal and male pores. Female funnels small (61 µm tall), ciliated, attached to septum 11/12, opening onto intersegmental furrow XI/XII ( Fig. 1A, ff). Ovaries paired, attached ventrally near septum 10/11, extending to XII ( Fig. 1A, ov).

Remarks. The combination of a little modified penial setae, with slightly proximal nodulus, the slit-like spermathecal pores, with well-developed vestibulae and without tegument thickening, the sacciform atria, with large lumen and thick muscular layer, and the conical, elongated penes as a simple extension of the atrial lining cells clearly distinguishes this species among all its congeners.

Among Pseudorhynchelmis species that possess penial setae, P. p a r v a is peculiar by the progressive shift of noduli on the shaft of ventral setae, from an ectal to a median position, and a progressive increase in size, from segments II to X, until ventral setae reach their maximal size in the male segment. These morphological modifications, in association with reduced accessory copulatory glands, justify setae in the male segment being considered as genital, namely penial, setae. In this respect, P. parva is similar to P. spermatochaeta which has so-called “spermathecal” setae that only differ from somatic setae by a greater size and a so-called median nodulus. In P. semernoyi sp. n., P. alyonae and P. olchonensis (as below re-described) penial setae are thick, elongate and straight; in the latter two species, they reach their highest degree of differentiation with noduli on the lower ental third of setae and grooved ectal tips.

P. olchonensis has similar slit-like spermathecal pores but they are not lined by a thickened of cuticle and they are associated with spermathecal, tegumentary papillae. Spermathecal vestibulae are also present in P. olchonensis and P. alyonae ; in the latter species, vestibulae are modified as distinct thick bulbs. P. semernoyi sp. n. and P. minimaris are similar to P. p a r v a in their more or less sacciform atria, restricted to X, but their atria are covered by a thin muscular layer, proportionally to atrial diameter, as in all other Pseudorhynchelmis species.

Lastly, small penes are present in most species of the genus but none of them have the typical conical, elongated shape observed in P. p a r v a. In this respect, a re-examination of the specimens described by Hrabĕ (1982) as Pseudolycodrilus parvus clearly shows this typical feature ( Fig. 2B, p), as well as other distinctive characters of Pseudorhynchelmis parva as here re-described from Michaelsen’s type material, namely the peculiar genital setae ( Fig. 2A, gs), reduced copulatory glands ( Fig. 2A, cgl), sacciform atria with large lumen ( Fig. 2A, B, a), covered by thick muscular layer, and slit-like spermathecal pores lined with thickened cuticle ( Fig. 2C, c). In spite of the fact that Hrabĕ (1982) did not see the type material, his re-description of Lycodrilus parvus Michaelsen, 1905 , from his material, appears justified as both suites of specimens evidently belong to the same species.

Assuming that Michaelsen’s (1905) original description misidentified ovaries (later on confirmed by Brinkhurst, 1984), Hrabĕ created a new genus for L. parvus, Pseudolycodrilus Hrabĕ, 1982 , in order to formalize the membership of this species of the family Lumbriculidae , instead of the Tubificidae . In the same work, however, he created the new genus Pseudorhynchelmis for accommodating the lumbriculid P. olchonensis ( Burow & Koshow, 1932) , redescribed from new material. Surprisingly enough, a possible attribution of L. parvus to this new genus was not discussed despite its evident similarity to P. olchonensis . In his phylogenetic analysis of Lumbriculidae, Brinkhurst (1989) tentatively tried to maintain a distinction between both genera by virtue of pear-shaped vs. elongate atria, and median vs. ventro-lateral spermathecal pores in Pseudolycodrilus and Pseudorhynchelmis, respectively. Such a distinction is not tenable. Not only did Hrabĕ’s and Michaelsen’s type specimens prove to have their spermathecal pores on the same line as ventral setae, namely in a ventro-lateral position, but more median spermathecal pores have been documented in P. semernoyi sp. n., P. spermatochaeta , P. shamanensis and P. paraolchonensis . In addition, although most Pseudorhynchelmis species have elongate atria, short, pear-shaped to ellipsoidal atria are known in P. minimaris and P. semernoyi sp. n. Accordingly, Pseudolycodrilus and Pseudorhynchelmis must be considered synonyms. As both names were simultaneously published in the same work, the precedence of a name can be fixed by the “First Reviser”, according to art. 24.2 of the ICZN (International Commission on Zoological Nomenclature, 1999). As the genus Pseudorhynchelmis was specifically erected in order to distinguish a lumbriculid species formerly attributed to Rhynchelmis Hoffmeister, 1843 , we prefer to retain this genus name instead of Pseudolycodrilus, which refers to an originally erroneous attribution to a tubificid genus.

In his work on oligochaetes of Lake Baikal, Semernoy (2004) gave a description of one specimen assumed to correspond to Hrabĕ (1982) redescription of Pseudolycodrilus parvus . Unfortunately, distinctive morphological characters associated with spermathecal pores were not specified (vestibulae, papillae), and other characters were not described in sufficient detail (shape of setae in X, penis morphology), making impossible an unambiguous identification of the specimen, according to the given description.

Although the original description of Lycodrilus parvus by Michaelsen in 1905 was based on a series of sections, and while six type specimens were mentioned by Brinkhurst (1984) during his revision of the Tubificidae and Lycodrilidae of Lake Baikal, the extant material presently available for this study amounted to only three specimens and one fragment that proved to belong to two different species, Pseudorhynchelmis parva ( Michaelsen, 1905) and Pseudorhynchelmis semernoyi sp. n. (see description below). The sectioned material was destroyed during the Second World War and any other kind of original material has to be considered as lost forever (P. Stiewe, pers. com.). A closer scrutiny of the original description of P. parva clearly demonstrates that Michaelsen based his description of “ Lycodrilus parvus ” on a mixing of characters. Attributable to P. parva , as presently defined, are the rounded prostomium (“Kopflappen… gerundet”) vs. conical in P. semernoyi sp. n., the tilted oval, ventral-median depression associated with spermathecal pores (“… Samentaschen-Poren… in den Seitenwinkeln einer querovalen, ventralmedianen Vertiefung…”) (undifferentiated in P. semernoyi sp. n.), and the distal narrowing of vasa deferentia (32 µm thick in their “proximalen und mittleren Partie”, 15 µm thick in their “distal” part) (roughly of the same width all along their course in P. semernoyi sp. n.). In contrast, the very peculiar ventral-median round, sucker-like depressions in X, associated with the male pores, very probably correspond to the papillae associated to genital setae in P. semernoyi sp. n. Our designation of the lecto- and paralectotypes was done in order to minimize resulting taxonomical changes, taking into account the work of Hrabĕ (1982) in which new specimens were correctly attributed to the same species originally described by Michaelsen (1905).

Geographical distribution. To date, only known from Lake Baikal, Maloye More.