Mesenchytraeus liberothecus

Description. Medium sized, body wall thin and transparent, twisting when submerged in water. Body length 13.5 – 19.5 mm (holotype 17.2 mm) after fixation, 12.0 – 20.0 mm in vivo; body width 470 – 700 µm at V (430 – 550 µm in vivo), 520 – 750 µm at clitellum (490 – 625 µm in vivo). Segment number 59 – 77 (holotype 73). Head pore near the top of prostomium, transverse slit. Epidermal gland cells yellowish but inconspicuous, slightly increased in anterior segments, only one transverse row at level of chaetae in each segment. Chaetal formula: 2 – 4 - 3,4: 2 – 4 - 3 – 5 (6), 6 per bundle in posterior segments (Table 2). Chaetae distinctly sigmoid and nodulated, distally thinner and single-pointed. Lateral chaetae 66 – 100 µm in length, 4.5 – 7 µm in maximal width, shortest in II and III (66 – 85 µm), gradually longer from III on, maximal length in V and VI (73 – 100 µm) (Fig. 3 A – D, Table 2). Ventral chaetae, except those in V and VI, 90 – 166 µm in length and 4.5 – 9.5 µm in maximal width. Ventral chaetae in V and VI reduced to 2 – 3 per bundle, 155 – 190 µm in length and 12 – 13 µm in maximal thickness (Fig. 3 B, C). Chaetae of XII absent in fully mature specimens. Clitellum in XI – XIII, elevated inconspicuously. Hyalocytes (11 – 17 µm in diameter) irregularly arranged, more abundant and larger than granulocytes (2 – 5 µm in diameter), often in contact with each other, interspersed with granulocytes (Fig. 4 B). Paired male pore distinct and separate, located in the ventral middle of XII. Paired spermathecal pores in the lateral line of VI / V. Brain small, trapezoidal, incised anteriorly, truncate or weakly concave posteriorly, 150 – 180 µm long and 120 – 170 µm in maximal width (Figs. 3 F, 4 A). Blood colourless. Dorsal vessel arising from XXII. Two pairs of primary pharyngeal glands in IV and V, both not connected dorsally and attached to respective posterior septa. Three pairs of secondary lobes ventrally in V – VII, in V smallest. No oesophageal appendages. Intestinal diverticula absent. Chloragogen cells developed, originating from VI, 26 – 47 µm high, absent in clitellar region. Coelomocytes elliptic with conspicuous nuclei, densely distributed in anterior body cavity, 15 – 17 µm in length and 20 – 24 µm in width, evenly granulated (Fig. 4 F). Preclitellar nephridia 5 pairs from VI / VII to X / XI. Nephridial anteseptale containing nephrostome only, and postseptale bilobed with undeveloped interstitial tissue. Efferent duct arising between the two lobes. Sperm funnel small, trumpet-shaped, in segment XI, 150 – 200 µm in length and 60 – 100 µm in maximal width. Opening of sperm funnel much wider than funnel body, 150 – 250 µm wide. Heads of spermatozoa 32 – 61 µm long (Figs. 3 E, 4 C). Vasa deferentia short, with conspicuous cilia in the canal, wound in irregular spirals in coelom from XI to XII, 22 – 27 µm in width, lumen of vasa deferentia 15 – 19 µm wide, merging with atria gradually. Atria in XII, cylindrical, conspicuously wider than vasa deferentia, 220 – 320 µm long and 37 – 55 µm in maximal width, with 4 – 5 prostate-like glands (length: 117 – 122 µm; width: 51 – 56 µm) (Figs. 3 G, 4 D). Atrium connected to penial bulb in XII. Penial bulb compact ventrally in XII, surrounded by several accessory glands. One pair of asymmetrical sperm sacs, large from XIV to XX – XXXIV, containing numerous flame-shaped sperm bundles (Fig. 4 G). Each bundle 97 – 118 µm long, with spermatozoal heads clumped at one end (20 – 24 µm wide and wider than tails) (Fig. 4 H). A pair of egg sacs present, extending into XXI – XXVIII, usually containing 3 – 13 mature eggs at a time. Spermathecae one pair in V, no ectal glands. Ectal duct 215 – 325 µm long and 37 – 64 µm wide, projecting into ampulla. Ampulla cup-shaped, maximal diameter 83 – 88 µm, carrying two oval-shaped diverticula (85 – 97 µm in length and 61 – 73 µm in width). Ampulla filled with spermatozoa in fully mature specimens. Spermathecae without ental duct, free in the body cavity of segment V (Fig. 4 E).

Diagnosis. This new species is characterized by the following combination of diagnostic traits: 1) enlarged ventral chaetae in segments V – VI, 2 – 3 per bundle; 2) five pairs of nephridia from VI / VII to X / XI in preclitellar segments; 3) asymmetrical sperm sacs paired, from XIV to XX – XXXIV, containing flame-shaped sperm bundles; 4) spermatheca with two small diverticula and free from oesophagus; 5) atrium cylindrical, with 4 – 5 large prostate glands; 6) vasa deferentia short, only from XI to XII.

Differential diagnosis (Table 3). The terrestrial species M. kontrimavichusi, inhabiting the region nearby Chaun Bay in the Magadan Region, Russia, resembles M. liberothecus most by its spermathecae detached from oesophagus and the presence of both sperm sacs and egg sacs. However, M. kontrimavichusi differs from this new species by the location of enlarged chaetae (V – IX), spermathecae with one diverticulum and reaching backwards to VII and the absence of an atrium. All remaining species with enlarged ventral chaetae differ from this new species in the attachment of spermathecae to the oesophagus. In addition, the aquatic members of M. crenobius, M. tetrapodus and M. monochaetus are different from this new species by the location and number of enlarged chaetae, and by the origin of the dorsal vessel. The co-occurring species of M. gigachaetus, M. anisodiverticulus and M. monodiverticulus are distinct from this new species by their higher number of enlarged chaetae per bundle, the origin of the dorsal vessel, the structure of the spermathecae, and in details of the sperm sacs and atrial prostate glands. The main characters that discriminate M. longiductus from this new species are: spermatheca with no diverticula and atrium with lower number of prostate glands (only 2). The main differences of M. calyx from M. liberothecus are as follows: large body length (20 – 39 mm), location of enlarged chaetae and spermatheca with 7 – 9 pointed diverticula. The major differences between M. liberothecus and M. spermatoglomeratus sp. n. are the locations of enlarged chaetae, the origin of dorsal vessel and the structure of the spermatheca.

Etymology. From the Latin adjective " liber ", meaning " free ", referring to the spermathecae of the new species that are free and not connected via ental ducts with the oesophagus.

Holotype. Fully mature, whole-mounted specimen collected in April 2015, stained, JLO 201504005. Type locality. Mt. Changbai, Jilin Province: Coniferous forest (42 ° 08.177 '' N, 128 ° 12.739 '' E; 1285 m asl), dark brown forest soil under Picea asperata, coll. Z. J. Piao. Paratypes. JLO 201504006, JLO 201504007, two whole-mounted fully mature specimens from type locality, same data as holotype. JLO 201504015 – JLO 201504017, three whole mounted specimens, all fully mature, from Pure larch forest (42 ° 06.391 '' N, 128 ° 13.078 '' E; 1389 m asl), dark brown forest soil under Larix gmelinii, 2 April 2015, coll. Z. J. Piao.