Mesenchytraeus tschukotensis Dózsa-Farkas, 2025

Dózsa-Farkas, Klára & Makarova, Olga, 2025, A new species of the enchytraeid genus Mesenchytraeus Eisen, 1878 (Clitellata, Enchytraeidae) from the eastern Chukot Peninsula, Russian Far East, as further evidence of former trans-Beringian faunal connections, Zootaxa 5589 (1), pp. 80-90 : 83-87

Mesenchytraeus tschukotensis Dózsa-Farkas sp. nov.

( Figures 2 , 3 )

Type material. Holotype: M. 5, slide No. 3099a+b+c, adult, stained specimen. Type locality: North-East Asia, Chukot Peninsula, Lavrentiya vil. vicinity, litter in Urocytellus parryii colony, 65.59373° N, 171.02521° E, 8– 30. 07.2013, Olga Makarova leg .

Paratypes: In total 7 specimens: P.149.1. slide No. 2813a+b, P.149.2 slide No. 3096a+b, P.149.3 slide No. 3097a+b+c, P.149.4 slide No. 3098a+b, P.149.5 slide No. 3100a+b+c, P.149.6 slide No. 3101a+b, P.149.7 slide No. 3104a+b+c — adult, stained specimens, the same data as holotype .

Further material examined. No additional specimens were collected at the given site, although they were singly seen in pitfall traps.

Etymology. The species is named after the Chukot Peninsula where it was found.

Diagnosis. The new species can be recognized by the following combination of characters: (1) large worms (19–33 mm long and about 1.5–2 mm at clitellum, segments 65–78), dorsal side red-brown by pigmentation; (2) chaetae sigmoid with nodulus, maximum 8–9 per bundle in the ventral bundles, in V–IX in lateral bundles only 1–3 larger and stronger chaetae; (3) clitellum girdle-shaped; (4) five preclitellar pairs of nephridia; (5) dorsal blood vessel from XV–XX; (6) two pairs of primary and 3–4 pairs of secondary pharyngeal glands, lobed, not connected dorsally; (7) sperm funnel 875–2000 µm long and 4–10 times longer than wide, sperm duct extending backwards into XIV–XIX inside the sperm sacs, diameter 35–50 µm. Atrium 300–475µm long, maximum width 100–150 µm with 8–10 large atrial glands; (8) male copulatory organ (‘penial bulb’) is a spherical structure (diameter 270–330 µm) and consisting of two series of penial glands; (9) spermatheca free, ectal duct 500–900 µm long and the distal part thicker. Ampulla possesses two or three diverticula (300–500 µm long) with sperm in them. The ampulla thin-walled, extending to X–XII, full with sperm and not connected to the intestine.

Description. Large enchytraeid worm. The dorsal side brown and mottled by pigmentation ( Fig. 2A ). Holotype 22.2 mm long, 1.5 mm wide at clitellum, segment number 65. Body length of paratypes 19–33 mm, width 0.95– 1.4 mm at VIII and 1.4–2.0 mm at clitellum. Segments 66–78. Chaetae sigmoid with nodulus. Chaetal formula: 1,(0),2,3,4–4,5,6,(3,2): (4,5),6–9–5–8,(4,3). Chaetae unequal in size within a bundle, increasing gradually in length towards the lateral midline; size difference most pronounced in ventral and also lateral bundles of segments behind the clitellum. In II–IV mostly 3–4 chaetae per bundle laterally (about 75–100×8–10 µm) ( Fig. 2C ), in V–IX chaetae fewer in number (1–3) and enlarged (140–205×13–15 µm) ( Fig. 2D ), in X–XI the length of the chaetae slightly smaller (100–170×9–12 µm). In the ventral bundles 6–9 chaetae (80–200×9–15 µm) ( Fig. 2E ). Behind the clitellum, chaetae slightly smaller and thinner (105–120×10 µm). In caudal segments, the longest chaetae measuring 125–140 x10 µm ( Fig. 2F ). Chaetae in XII absent. Head pore a large triangular pit at one-third the distance from the anterior tip of head to 0/1. Clitellum girdle–shaped in 1/2XI–XIII, gland cells small, in reticulate pattern ( Fig. 2B ), ventrally also ( Fig. 3D ). Thickness of body wall about 30–50 µm, cuticlea 2–5 µm.

Brain incised anteriorly and truncated posteriorly, as long as wide or slightly wider than long (length about 150– 160 µm). Two pairs of primary pharyngeal glands (in 4/5–5/6), not united dorsally, and three or four pairs of secondary pharyngeal glands in V–VII or VIII; the glands lobed. Chloragocytes from IV, about 10–13 µm long. Parasites (probably Selenidiidae ?) are often found in the intestine ( Fig. 2H ). Dorsal blood vessel from XV–XX, anterior bifurcation in I. Five pairs of preclitellar nephridia from 6/7 to 10/11, anteseptale funnel only, postseptale lobed with folded canal, no interstitial tissue, efferent duct arising between the lobes ( Fig. 2I ). Oesophageal and intestinal appendages or diverticula absent. Coelomocytes small, roundish, with well-staining nucleus, about 10 µm. Sperm sac extending backwards to XXIV–XX, egg sac to XIX–XXIV. Sperm bundles 125–250 µm long, the heads clumped only at one end and 35–45 µm long ( Fig. 3E ). Sperm funnel long ( Fig. 2J ), 875–2000 µm in length and 4–10 times longer than wide. Sperm duct long, ciliated ( Fig. 3A ), loosely coiled, extending backwards into XIV–XIX inside the sperm sac, diameter 35–50 µm. Atrium ( Figs 3B, C ) 300–475 µm long, maximum width 100–150 µm, here, where the atrium is widest around the middle, joined by 8–10 large atrial glands (prostate glands), usually 200–300 µm long ( Figs 3B, C ). The atrium connects through a short ectal duct (60–100 µm long) to the male copulatory organ (‘penial bulb’) which is a spherical structure (diameter 270–330 µm) and consists of two series of closely packed individual penial glands ( Figs 3B, C ). Accessory copulatory glands and subneural glands absent. Spermatheca ( Figs 3F–L ) free, the ectal duct 500–900 µm long. The distal part of duct 140–200 µm long and 75–100 µm wide, later only 60–80 µm wide and slightly tapering, at the entrance to the ampulla 45–65 µm wide only. On the contrary, the width of canal of ectal duct widens from 8 µm to 12 µm from the distal end in the direction of the ampulla ( Figs 3I, J ). Ampulla with two or three diverticula (300–500 µm long and 80–110 µm wide) with sperm in them and each have separate insertions ( Figs 3F–H ). The ampulla thin-walled, extending to X–XII ( Figs 3G, K ). In each somite there is a bulging out of ampulla, each such sac-like part being separated from the one in the adjoining segment by the constriction caused by septum (the widest sac about 400–900 µm wide). No connection with intestine. In two paratypes, they cross each other before they extend backwards. The ampulla also full with sperm ( Fig. 3L ). Mature egg was not seen in the egg sac.

Differential diagnosis. Among the previously described species of Mesenchytraeus , eleven species have free spermathecae and two diverticula ( M. harrimani Eisen, 1904 ; M. hydrius Welch, 1919 ; M. gelidus Welch, 1916 ; M. maculatus Eisen, 1904 ; M. chromophorus Altman, 1936 ; M. setchelli Eisen, 1904 ; M. obscurus Eisen, 1904 ; M. franciscanus Eisen, 1904 ; M. antaeus Rota & Brinkhurst, 2000 ; M. arcticus Bell, 1962 ; M. melanocephalus Christensen & Dózsa-Farkas, 1999 ) ( Eisen, 1904; Welch, 1916; Altman, 1936; Bell, 1962; Christensen & Dózsa-Farkas, 1999; Rota & Brinkhurst, 2000). Most of these species are large or very large worms. From all these species, Mesenchytraeus tschukotensis sp. nov. differs by having two or three spermathecal diverticula, whereas the other species have always only two ones. M. harrimani is also pigmented but can be distinguished by the following features: M. harrimani is larger,> 60 mm long, with>100 segments (after Eisen, 1904), 38–50 mm long, with 90– 113 segments (Dózsa-Farkas, unpublished observations on new material) whereas M. tschukotensis sp. nov. is 19– 33 mm long, with 65–78 segments. The sperm funnel is much longer, extending backwards to segments XX–XXIII in the sperm sac (vs located only in segments XI–XII in M. tschukotensis sp. nov.). The sperm sac and egg-sac both extend also more backwards, at maximum to segments XLIII–XLVI (vs XX–XXIV), the atrium and atrial glands are longer, 500–800 µm and 600–800 µm long, respectively (vs 300–475 µm and 200–300 µm long, respectively). The diameter of the penial bulb is larger (700–900 µm, vs 270–330 µm) and has a muscle layer around the penial glands, but in M. tschukotensis sp. nov. this layer is absent. The spermathecal ectal duct is shorter (250–400 µm, vs 500–980 µm).Another species with a similar shape of spermatheca, M. hydrius , is not pigmented, in the lateral chaetal bundles there are more chaetae (4–7, vs 1–3 in M. tschukotensis sp. nov.), and these are not enlarged. M. hydrius possesses only 5 atrial glands (vs 8–10), and the spermathecal ectal duct is regularly thin (vs thicker ectally). The body of M. gelidus is almost of the same size, but more strongly pigmented, dark reddish brown or black; its lateral chaetal bundles have more chaetae (4–5, vs 1–3) and are not enlarged; the sperm funnel is smaller, with a collar and more atrial glands (12, vs 8–10); the spermathecal ectal duct is shorter with unicellular glands at the orifice. M. maculatus is larger, 45–60 mm long with 93 segments. The sperm duct is coiled in XII, not extending backwards, it has 12 or more atrial glands, and a large white shield around the spermathecal pores; the spermathecal diverticula are thinner and the ampulla extends only to VII–VIII (vs X–XII). An interesting agreement is noted between the new species and both M. chromophorus and M. antaeus in that the longest chaetae in the ventral and lateral bundles are also located laterally, in contrast to some of the other species listed above (the character is not described in all species) where in the ventral bundles the largest chaetae are located towards the ventral midline and the largest lateral chaetae in a bundle are found towards the dorsal midline. However, in M. chromophorus there are only four atrial glands and the dorsal vessel begins from XIII–XIV (vs XV–X in M. tschukotensis sp. nov.). M. antaeus is much larger (33–61 mm with 91–127 segments) and characteristically has two sets of large accessory glands. M. franciscanus is also a large species (20–30 mm with 78 segments), yellowish in colour, without pigment, it possesses a large accessory gland and the spermathecal diverticula are long and narrow. In M. obscurus , the spermathecal diverticula are long and thin, and the atrium is with 16–20 glands. M. setchelli is slightly smaller (only 12 mm long) and white in colour; the atrium is with 5 glands and the spermathecal diverticula are ovoid with a thick epithelium. M. arcticus is smaller (20 mm, with 70 segments); in the lateral chaetal bundles there are more chaetae (3–6, not enlarged); it possesses 7 atrial glands and one „accessory apparatus”; the spermatheca extends backwards only into VII or VIII, its ectal duct and diverticula are shorter (both 200 µm long). Finally, M. melanocephalus is pigmented, but smaller (14–22 mm with 44–58 segments); in the lateral bundles of segments VI–VIII it has only one typical enlarged chaeta; the sperm funnel is very short (140–340 µm long), with 5–6 atrial glands and two accessory glands; the spermatheca extends only into V–VI and both ectal duct and ampulla are also shorter (200–240 µm long). It should be noted that all species compared here with M tschukotensis sp. nov. are known morphologically only from the respective original descriptions, M. melanocephalus excepted (see Christensen & Dózsa-Farkas, 1999).

Distribution. Until now, this species is only known from the type locality, in the very east of the Chukot Peninsula.

Figures

FIGURE 2. Micrograph of Mesenchytraeus tschukotensis sp. nov. A. Pigmented forepart of the body, dorsal view. B. Clitellar glands, dorsal view. C. Lateral chaetae in IV. D. Larger lateral chaetae in VIII. E. Ventral chaetae in VIII. F. Ventral chaetae in LVI. G. Chloragogen cells (dorsal vessel marked with arrow). H. Parasites (Selenidiidae) in intestine. I. Nephridium. J. Dissected sperm funnel. Scale bars A–H 50 μm, I–J 200 μm. The micrographs were taken from the opened worms or the dissected organs.

FIGURE 3. Micrograph of Mesenchytraeus tschukotensis sp. nov. A. Sperm duct (the cilia well visible). B–C. Male copulatory organs (a=atrium, atrial glands marked with white arrows and penial glands, marked with black arrows). D. Clitellar glands (bursal slits, marked with arrows) ventral view from outside. E. Sperm bundle. F. Ectal part of a spermatheca with two diverticula (ectal duct marked with white arrow, diverticula marked with black arrows). G. Prepared spermatheca of holotype (ectal duct marked with white arrow, diverticula marked with black arrows, a=ampulla). H. Spermathecal ectal part of holotype with three diverticula (marking is the same as in G). I. Distal part of spermathecal ectal duct (thicker part marked with white arrow. the canal marked with black arrow). J. Proximal part of spermathecal ectal duct only (canal marked with arrow, far wider than in distal part). K. Spermathecae in V–XI. L. Sperm in ampulla. C, G–J from holotype. Scale bars A–F, H–J, L 50 μm, G 200 μm, K 500 μm. The micrographs were taken from the opened worms or the dissected organs.