Aulodrilus aestivus, Ohtaka, 2021

Ohtaka, Akifumi, 2021, Taxonomical study of Japanese Aulodrilus Bretscher (Annelida, Clitellata Tubificinae) with descriptions of two new species, Zootaxa 4952 (1), pp. 1-32 : 16-18

Aulodrilus aestivus sp. nov.

( Figures 11 , 12 )

Holotype. NSMT-An 628, a mature specimen, 20 July 1984, the anterior part of body sagittally sectioned and mounted on one slide and the posterior one whole-mounted on another slide.

Type locality. Paddy field in Takasaki City, Gunma Prefecture, Japan (36.361N, 138.956E) .

Paratypes. Paratypes. NSMT-An 629, a whole-mounted specimen; NSMT-An 630, a cross-sectioned specimen; locality and date the same as for the holotype.

Other material examined. Japan: 10 mature and 2 immature specimens, paddy field in Takasaki City , Gunma Prefecture (type locality), 20 July 1984 . 7 mature and 5 immature specimens, paddy field in Gunma Town , Gunma Prefecture, 20 July 1984 . 1 mature and 7 immature specimens, paddy field in Tomioka City , Gunma Prefecture, 16 Aug. 1985, 13 Aug. 2017 . One mature specimen, paddy field in Izumo City , Shimane Prefecture, 24 July 2015, coll. N. Kado.

Etymology. The specific epithet refers to the summers during which all the specimens examined were collected.

Description. Fixed mature specimens: 8–15 mm length, 0.20–0.35 mm width, and 45–80 segments. Posterior end of 0.2–0.5 mm in length without septa, devoid of chaetae. Prostomium almost absent ( Fig. 11B, C ). Epidermis 6–10 µm thick, with glandular cells dense in anterior part. Pharynx in II and III, pharyngeal glands weakly developed ( Fig. 11C ). Alimentary canal abruptly widening in VIII or IX. Chloragogen cells from VII on, covering gut thinly. Intestinal wall thin in middle and posterior segments. Transverse vessels form loops in II–VI; thick in VII and VIII as hearts ( Fig. 11B ). Living animals in loose mud tube with mucus and foreign matter.

Dorsal chaetal bundles consist of short hairs and bifid crotchets. Dorsal hairs beginning in either of IV–VIII, smooth and weakly curved ( Fig. 11D ); 2–6 per bundle, 60–90 µm long; the length less than twice that of crotchets in the same respective bundles. Dorsal crotchets in anterior segments ( Fig. 11E ) 4–13 per bundle and 42–62 µm long, with nodulus slightly distal and with parallel teeth, with the upper tooth 2/3–1/2 times longer and much thinner than the lower ( Fig. 11H, I ); those in posterior segments 2–5 per bundle and 33–44 µm long, more strongly curved, with nodulus situated at 1/3–1/4 from distal end, and lower teeth thickened more than those in anterior segments. Lower tooth of posterior dorsal crotchets sometimes rounded or divided into some short teeth ( Fig. 11J, K ). Ventral chaetae ( Fig. 11F, G ) all bifid crotchets 5–12 per bundle, 44–62 µm long in anterior segments; 3–7 per bundle, 36–42 µm long in posterior ones; distal shape and position of nodulus not different from dorsal crotchets. Number of ventral chaetae decreased in genital segments. No modified genital chaetae. In mature specimens, ventral chaetae in segment with male pore reduced in number or lost completely. No lateral expansions on chaetal shafts in dorsal or ventral chaetae.

Clitellum conspicuous, usually extending from chaetal line of IX to slightly behind chaetal line of XI ( Fig. 11A ), but often shifted anteriorly from 1/2VI to VIII in accordance with shift of other genital organs. Testes and ovaries paired, usually in IX and X ( Fig. 12A ), sometimes in VI and VII, respectively. Male funnels small, 15–20 µm in diameter. Vasa deferentia 150–200 µm long, hardly winding, connected with atrium apically ( Fig. 12A ). Atria crescent-shaped, 120 µm long and 36 µm wide, with high and glandular inner epithelium ( Fig. 12B ). Prostate glands about as large as atria, connected to concave side of atria through short stalks. Penes long when protruded, opening close to each other within a large median male bursa situated in middle to posterior 1/3 of X (or VII) ( Figs. 11A , 12A,E ). Spermathecae ( Fig. 12C ) small, separately opening on small papillae anterior to the chaetae of IX (or VIII) and close to ventral nerve cord ( Fig. 12D ). Ampullae ovoid in shape, 35 µm long by 28 µm wide, with the wall 2–5 µm thick. Spermathecal ducts 30 µm long with thick wall. Loose sperm in the ampullae. Sperm sac extending anteriorly as far as V, with egg sac as far back as XII, both weakly developed.

Variation. Two cases were found in the segmental position of genital organs among specimens examined ( Fig. 17 ): one has testes and spermathecae in IX with ovaries and male pores in X (see description), and the other has testes and spermathecae in VI with ovaries and male pores in VII. Among specimens examined from the type locality (n =12), the former case (9 individuals) was more frequent than the latter (three individuals). No intermediate conditions between the two cases above have been found there. The clitellum always covered the segments having spermathecae and male pores, but varied in the segmental position in accordance with the position of genital organs. The beginning of dorsal hairs also varied in accordance with the position of the genital organs: The dorsal hairs began in IV, V, or VI in specimens having male pores in VII, and they began in VI, VII or VIII in those having male pores in X.

Distribution. The present species has hitherto been collected in summer seasons from several inundated paddy fields in central and western parts of Japan. A congener, A. limnobius , has been collected in these localities together with another tubificine, Limnodrilus hoffmeisteri Claparède.

Remarks. Aulodrilus aestivus sp. nov. is most similar to A. pigueti and A. acutus by the following characteristics: 1) male efferent ducts open into a median male bursa; and 2) dorsal hairs are absent in some anterior segments. However, the present species has neither oar-shaped chaetae nor blade-shaped chaetae in the dorsal bundles, which are characteristic, respectively, of A. pigueti and A. acutus . The crescent-shaped atrium of the present species is also different from the globular atria of A. pigueti and A. acutus . In contrast, the present species resembles A. pluriseta in having hair chaetae and bifid crotchets in dorsal bundles, and crescent-shaped atria. However, in the latter species, dorsal hair chaetae begin in II and male ducts open in line with the ventral chaetae, thus differing from the present species in which hair chaetae begin in IV–VIII and male ducts open into a median male bursa.

Figures

FIGURE 11. Aulodrilus aestivus sp. nov. from Takasaki, Japan. A, ventral view of anterior segments; B, parasagittal section of anterior segments showing locations of vascular and reproductive systems; C, sagittal section of anterior segments with alimentary canal; D, dorsal hair chaeta in a mid-segment; E, dorsal chaeta in VII; F, ventral chaeta in IV; G, the same in a posterior segment; H and I, distal ends of dorsal chaetae in anterior segments; J and K, variation of distal ends of dorsal chaetae in posterior segments.

FIGURE 12. Genital organs of Aulodrilus aestivus sp. nov. from Takasaki, Japan. A, parasagittal section of genital segments; B, longitudinal section of atrium; C, longitudinal section of spermatheca; D, cross section at the spermathecal pores in IX; E, the same at the male pores in X.

FIGURE 17. Arrangements and segmental positions of genital organs in the Aulodrilus species hitherto reported. Segmental numbers are shown in Arabic numerals.