Mesenchytraeus spermatoglomeratus

Description. Small worms, active, twisting when submerged in water, pale in vivo, body wall thin, transparent. Body length 6.5 – 11.5 mm (holotype 8.0 mm), 7.0 – 12.5 mm in vivo; body width 270 – 355 µm at V (330 – 450 µm in vivo), 470 – 575 µm at clitellum (320 – 625 µm in vivo). Segments 28 – 46 (holotype 34). Head pore transverse slit near the apex of prostomium (Fig. 2 A). Epidermal gland cells inconspicuous, almost absent. Chaetal formula: 2,3 - 3,4: 2 – 5 (6) - 2 – 5 (6), up to 6 per bundle in the last segments. Chaetae sigmoid, nodulated, distally thinner and single-pointed. Lateral chaetae 44 – 100 µm in length and 3.5 – 5 µm in maximal width, shortest in II (44 – 48 µm), gradually increasing in size from III backwards, maximal size in IX (80 – 100 µm) (Table 1). Ventral chaetae, except those in V, VI and XI, 63 – 120 µm in length and 4 – 7.5 µm in maximal width (Fig. 1 A, D). Ventral chaetae in V, VI and XI reduced to 2 per bundle (occasionally 3) (Fig. 1 B, C, E). Ventral chaetae in V and VI longer and stouter than the others even in immature specimens, 126 – 158 µm in length and 7.5 – 11 µm in maximal thickness (Fig. 1 B, C). Ventral chaetae in XI shorter and thinner than in V and VI, but thicker than the ordinary ventral ones, 6 – 8.5 µm in maximal thickness (Fig. 1 E). Chaetae of XII lacking in fully mature specimens. Clitellum extending from 1 / 2 XI to XIII, elevated conspicuously. Hyalocytes (12 – 24 µm in diameter) more abundant than granulocytes, irregularly arranged and in contact with each other. Granulocytes (13 – 22 µm in diameter) irregular in shape, having some connection with each other and forming a loose net (Fig. 2 B, F). Male pores distinct in the ventral middle of XII. Spermathecal pores paired, in the lateral line at IV / V (Fig. 2 K). Brain in I-II (Figs. 1 J, 2 E), trapezoidal, deeply concave anteriorly and slightly incised posteriorly, 137 – 170 µm long and 105 – 125 µm in maximal width. Blood colourless or slightly yellowish. Dorsal vessel arising from XIV – XVI. Two pairs of primary pharyngeal glands in IV and V, attached to septa of 4 / 5 and 5 / 6 respectively, and both not connected dorsally. Three pairs of secondary lobes ventrally in V – VII, largest in VI. No oesophageal appendages. Intestinal diverticula absent. Chloragogen cells developed, originating from VIII, 12 – 45 µm high, absent in clitellum. Coelomocytes abundant, disc-shaped with conspicuous nuclei, 7.5 – 15.0 µm in diameter, with uneven refractile granules (Fig. 2 C). Five pairs of nephridia in preclitellar segments from VI / VII to X / XI. Anteseptale stalked, containing nephrostome only; postseptale bilobed and with fewer interstitial tissue. Efferent duct arising between the two lobes (Figs. 1 F, 2 G). Sperm funnels cylindrical, occupying 1 / 3 – 1 / 2 body cavity in XI, 390 – 440 µm in length and 122 – 200 µm in maximal width. Their ental openings much narrower than funnel body, 43 – 73 µm wide. The heads of spermatozoa 17 – 20 µm in length (Figs. 1 I, 2 I). Vasa deferentia with conspicuous cilia in the canal, wound in irregular spirals in coelom from XII and posteriorly into XV – XXII, 18 – 30 µm in width, lumen of vasa deferentia 14 – 24 µm wide (Fig. 2 L). The transition between vas deferens and atrium gradual. Atria in XII – XIII, cylindrical, remarkably wider than vasa deferentia, 415 – 650 µm long and 65 – 85 µm in maximal width. No cilia observed in atrial canal. Each atrium possessing 4 – 6 tubular prostate-like glands (each 117 – 195 µm in length and 50 – 68 µm in width) (Figs. 1 H, 2 I, J). Each atrium divided into two distinct parts, an ental part (length: 217 – 400 µm; width: 27.5 – 58 µm) with thin muscle layer and thick epithelium, and an ectal part (length: 200 – 350 µm; width: 25 – 44 µm) with a muscular duct connecting with the penial bulb. Penial bulb large, compact in ventral of XII, consisting of muscle strands and surrounded by masses of accessory glands of different size (Figs. 1 H, 2 J). A pair of sperm sacs large from XI – XIV in different size, and backwards extending to XV – XXIV, each containing numerous ball-shaped sperm bundles (diameter: 50 – 110 µm), a large number of spermatozoa coiled inside in vivo but much more loose in fixed condition (Fig. 2 D, H). Egg sacs paired, large from XIV – XXII, continuing caudad to XVII – XXXIII, usually containing 2 – 11 mature eggs at a time. One pair of spermathecae in V – VII, without ectal glands. Ectal duct 215 – 385 µm long and 37 – 50 µm wide, projecting into ampulla. Ampulla spherical, 83 – 102 µm in diameter. No diverticula visible. Ampulla filled with spermatozoa in fully mature specimens. Ental ducts stout, 195 – 244 µm long and 62 – 67 µm wide, connecting to the dorsal side of oesophagus separately in the posterior of VI or anterior of VII (Figs. 1 G, 2 K).

Diagnosis. This new species is recognized by the following characters, diagnostic in combination: 1) enlarged ventral chaetae in segments V – VI and XI, 2 per bundle; 2) five pairs of preclitellar nephridia in VI / VII to X / XI; 3) sperm sacs asymmetrically paired from XI – XIV to XV – XXIV, containing curly ball-shaped sperm bundles; 4) spermathecae without diverticula, ental ducts connected with oesophagus in VI – VII; 5) atrium tubular, with 4 – 6 developed prostate glands, distinct from vas deferens; 6) vas deferens very long, extending backwards to XV – XXII.

Differential diagnosis (Table 3). This new species is morphologically most similar to the co-occurring species M. gigachaetus Xie, 2012, M. anisodiverticulus Shen et al., 2012 and M. monodiverticulus Shen et al., 2012 and to a further terrestrial species, M. longiductus Christensen & Dózsa-Farkas, 2012, collected in Korea, which also have enlarged ventral chaetae in V – VI, and atria, egg sacs, and spermathecae attached to oesophagus. However, Mesenchytraeus spermatoglomeratus sp. nov. possesses enlarged ventral chaetae in XI and ball-shaped sperm bundles in the sperm sacs, two characteristics absent in the latter four species. Besides, M. gigachaetus differs from M. spermatoglomeratus by a higher number per bundle of enlarged ventral chaetae (3 – 4), no sperm sac visible and no atrial prostate gland. Mesenchytraeus anisodiverticulus is distinct from this new species by its absence of atrial prostate glands and the presence of two asymmetrical spermathecal diverticula. Mesenchytraeus monodiverticulus is different from M. spermatoglomeratus by its higher number per bundle of enlarged ventral chaetae (4 or 5), absence of atrial prostate glands, and spermatheca with one diverticulum. In Mesenchytraeus longiductus, atrial prostate glands are less numerous (only 2) and fewer accessory glands surround the penial bulb (only 4 – 5). Five other species (M. crenobius Timm, 1994, M. kontrimavichusi Piper et al., 1982, M. tetrapodus Timm, 1978, M. monochaetus Bretscher, 1900 and M. calyx Dózsa-Farkas et al., 2015) differ from M. spermatoglomeratus in terms of location and number of enlarged chaetae, chaetae formula, spermatheca, origin of dorsal vessel and habitat requirement. Unlike the new species, M. crenobius, M. tetrapodus and M. monochaetus are aquatic enchytraids. They also have 1 – 2 spermathecal diverticula. The terrestrial M. kontrimavichusi differs from this new species in: spermatheca with one diverticulum, not attached to oesophagus, atria absent, dorsal vessel arising from XV – XVIII. The other terrestrial member, M. calyx, is different from M. spermatoglomeratus by its larger body size (20 – 39 mm), the location of enlarged chaetae (VII – IX) and spermatheca with 7 – 9 pointed diverticula.

Etymology. Named after the shape of sperm bundles: " glomeratus " refers to " condensed into a ball ". " spermatoglomeratus " refers to the sperm bundles of the new species, rolled into a round ball shape.

Holotype. Fully mature, whole-mounted specimen, stained, JLO 201412005. Type locality. Mt. Changbai, Jilin Province, hardwood forest, 42 ° 180.607 '' N, 127 ° 50.453 '' E, 1034 m asl, dark brown forest soil under Quercus mongolica, coll. Z. J. Piao, Dec. 2014. Paratypes. JLO 201412003 – JLO 201412004, two whole mounted adult specimens from Korean pine broadleaved mixed forest, 42 ° 20.150 '' N, 127 ° 53.056 '' E, 1084 m asl, dark brown forest soil under Pinus koraiensis, 10 December 2014, coll. Z. J. Piao; JLO 201412006, one whole mounted adult specimen from type locality, same data as holotype. JLO 201503001 – JLO 201503002, two whole mounted specimens, fully mature, from secondary Betula platyphylla forest, 42 ° 22.719 '' N, 128 ° 01.020 '' E, 786 m asl, dark brown forest soil under Betula platyphylla, 2 April 2015, coll. Z. J. Piao.