Aulodrilus americanus Brinkhurst & Cook, 1966

Ohtaka, Akifumi, 2021, Taxonomical study of Japanese Aulodrilus Bretscher (Annelida, Clitellata Tubificinae) with descriptions of two new species, Zootaxa 4952 (1), pp. 1-32 : 5-7

Aulodrilus americanus Brinkhurst & Cook, 1966

( Figures 3 , 4 )

Aulodrilus americanus Brinkhurst & Cook, 1966: 19 , fig 7R, S.

Aulodrilus americanus Brinkhurst & Cook : Brinkhurst 1971: 528, fig 8.23 A, B; 1986: 193.

Material examined. Japan: 1 mature specimen, a spring-fed stream in the Hokkaido National Agricultural Experiment Station, Tsukisamu, Sapporo City , Hokkaido, 9 July 1983. Two immature specimens, spring-fed Ushiwatari stream on Mt. Chokai , Yuza Town , Yamagata Prefecture, 29 July 2015. 5 immature specimens, a brook in Ozegahara Mire, Gunma Prefecture, 8 May 1998. North America : 3 immature specimens, Coeur d’Alene Lake , Idaho, 24 June 2004, coll. J. Kuwabara (S. V. Fend collection) .

Description of Japanese material. A live (incomplete) individual 20 mm long and 0.38 mm wide in preclitellar segments, segments 42+. Body light red when living. Prostomium conical ( Fig. 3F ). No distinct secondary annulations. Pharynx in II–III, covered with a thin layer of pharyngeal glands. Chloragogen cells from VI on, thinly covering gut. No stomachal dilatation. Transverse blood vessels forming loops in II–VII, thick in VIII and IX. Individuals living in soft mud tube with adherent foreign matter.

Chaetae all crotchets, with form, size and number not different between dorsal and ventral bundles. Those in anterior segments (II–V or II–VI, Fig. 3A–C ) slightly longer than the following ones, 7–9 per bundle, 80–94 µm long, with distal nodulus and with rudimentary upper tooth or simple pointed distal end; those from VI or VII on ( Fig. 3D,E ) 4–6 per bundle, 48–62 µm long. Nodulus at 1/3 from distal end, and distal end brush-like with a series of teeth gradually decreasing in size from lower to upper ( Fig. 4A,B ). No lateral expansions in the chaetal shafts in any segments. No modified genital chaetae.

Clitellum surround the segments from beginning of X to end of XII, with the wall 16–30 µm thick. Gonads and copulatory organs paired ( Fig. 3G ). Testes and ovaries attached respectively to posterior sides of septa 9/10 and 10/11. Male funnels large. Vasa deferentia short and not winding, 240 µm long and 20 µm wide, connected to apical end of atrium. Atria bean-shaped, 150 µm long and 80 µm in maximum diameter. Prostate glands as large as atria. Ejaculatory ducts not detected. Penes 90 µm long, 20 µm wide, two-layered with ordinary cuticular covering, and separate openings lateral to ventral chaetal line at middle of XI. Spermathecae in X, small ( Fig. 3G ); ampullae ovoid in shape, 70 µm long and 45 µm wide; ducts stout and well-marked off from the ampullae, about 100 µm long, 20–25 µm wide, opening laterally at middle of X. Sperm sac in X. Egg sac observed to extend as far back as XII.

Remarks. All essential characteristics of the present Japanese material agree well with the original description ( Brinkhurst & Cook 1966), the only account prior to this paper (subsequent accounts summarize data of the original description). Spermathecae were "not observed" ( Brinkhurst & Cook 1966: 19) in the original material, whereas the single sexually mature Japanese specimen available to us had one pair of spermathecae. The spermathecal ampullae of the present material were small, but they contained sperm masses ( Fig. 3G ) and hence can be considered as completely formed. Occasional absence of spermathecae has been observed in other tubificines, for example, in Tubifex tubifex , Ilyodrilus templetoni ( Brinkhurst 1971) and in Teneridrilus mastix ( Brinkhurst 1978) , and absence of spermathecae may be an intraspecific variation also in this species. Furthermore, Brinkhurst & Cook (1966) described eversible pseudopenes in the original description of Aulodrilus americanus , however, the present new material has true penes as defined by Baker and Brinkhurst (1981), because they are two-layered, solid structures, composed of the wall of the penial sac and end of the atrial wall, and have cuticular coverings

The segmental position of the genital organs in A. americanus was not originally described, but the present specimen has genital organs located int the normal position of Tubificinae (male pores and spermathecal pores in XI and X, respectively). Unlike many other Aulodrilus species, extra testes were not found in the present specimen.

Liang et al. (1998) showed no split of distal ends of chaetae and depicted a membrane between short upper and long lower teeth of chaetae from X on in their Aulodrilus americanus from China. The brush-like form of chaetae in A. americanus can be recognized in observation without high magnifications, and such chaetae appear from VI or VII. A. americanus reported by Liang et al. (1998) might not be A. americanus . The taxonomic position of their A. americanus should be considered provisional until the segmental location and accurate structure of the different types of chaetae has been reinvestigated in the reference material.

Distribution. This species has been recorded in North America ( Brinkhurst & Cook 1966; Brinkhurst 1967, 1978; Hiltunen 1967, 1969; Stimpson et al. 1975; Spencer 1980) and in Japan (Ohtaka, unpublished data in Timm 1999a), showing circum-Pacific distribution ( Timm 1999a). These records are formally the first of this species in Japan. The Japanese habitats were cool waters, including spring-fed streams in Sapporo (water temperature 10.5°C), Ushiwatari stream in Yuza Town (12.3 °C), and a brook in a sphagnum bog of Ozegahara Mire, Gunma Prefecture. Differing from many other Aulodrilus species living in warm waters, the preference of A. americanus for cool-water habitats is apparently unique.

Figures

FIGURE 3. Aulodrilus americanus Brinkhurst & Cook from Sapporo, Japan. A, dorsal chaeta in III; B and C variation of distal ends of dorsal chaetae in anterior segments; D, dorsal chaeta in a mid-segment; E, the same in a posterior segment; F, sagittal section of anterior segments; G, parasagittal section of genital segments.

FIGURE 4. Aulodrilus americanus Brinkhurst & Cook, from Sapporo, Japan.SEM photomicrographs of dorsal chaetal bundle (A) and two chaetae (B) in a mid- segment.