Varichaetadrilus salinus, Ohtaka, 2024

Ohtaka, Akifumi, 2024, Three new tubificine species (Annelida: Clitellata: Naididae) from Japan, Zootaxa 5529 (1), pp. 186-200 : 193-196

Varichaetadrilus salinus sp. nov.

( Figures 8–9 )

Holotype. NSMT-An 1929 , a whole-mounted mature and complete specimen, 22 September 2012, coll. A. Ohtaka .

Type locality. Offshore of an oligohaline Lake Ushiro-gata on the Tsugaru Peninsula , Goshogawara City, Aomori Prefecture, Honshu, Japan (41.02097° N, 140.32534° E), 0.9 m deep with muddy bottom.

Paratypes. NSMT-An 1930–1931 , two whole-mounted mature specimens; one is complete, the other is incomplete posteriorly; locality, date and collector the same as for the holotype .

Other material examined. Four mature and five immature specimens mounted on slides; locality, date and collector the same as for the holotype . One whole-mounted mature specimen from the type locality, 3 September 2022 . Two mature and six immature whole-mounted specimens from offshore Lake Mae-kata (41.02623°N, 140.3500°E), located north of the type locality, Ushiro-gata, 1.2 m deep with muddy bottom .

Etymology. The specific name refers to the environments of the brackish lakes from which the specimens were collected.

Diagnosis. Body 5.5–11.0 mm long in preserved state. Chaetae all bifid with shorter upper teeth; 2–3 in preclitellar bundles, 0–1 in postclitellar bundles. No modified genital chaetae. Vasa deferentia long and tubular. Atria tubular with solid prostate glands at the ental portion. Penes large, opening at XI ventrally. Penial sheaths thin and finger cot-shaped. Spermathecae absent.

Description. Live worms reddish anteriorly, pinkish posteriorly. Body 8–15 mm long in living state, 5.5–11.0 mm long in preserved state. Width 0.20–0.32 mm at middle of body. Segments 48–68. Prostomium pointed conical ( Fig. 8A ); length 55–64 µm, almost as long as basal diameter. Segments II to VI biannulate ( Fig. 8A ), each with short anterior and long, chaetae-bearing posterior annuli. Segment from IX on elongate, longer than diameter. Posterior end of 0.5–1.5 mm tapering with many transverse furrows. Body wall smooth and thin, epidermis 4–8 μm thick. Clitellum from 2/3 X to end of XII, 9–15 μm thick, glandular. Coelomocytes not found throughout the body. Pharynx in II and III, dorsal wall thicker than ventral wall ( Fig. 8B ). Pharyngeal glands attached on pharynx dorsally in II–IV. Chloragogen cells beginning in VI, thinly covering the alimentary canal. Transverse vessels forming complicated loops in I–VII, thick in VII and VIII as hearts ( Fig. 8B ). In middle and posterior segments dorsal vessel 17–26 μm thick, thicker than ventral vessel (9–16 μm thick); a simple transverse vessel connecting dorsal and ventral vessels in the posterior third of each segment ( Fig. 8C ). Nephridia not observed in any segments.

Chaetae all bifid crotchets; the form, size and number not different between dorsal and ventral bundles. In preclitellar segments, chaetae 2–3, rarely 4 per bundle, 45–60 μm long; nodulus situated 1/3–1/4 from distal end, and distal teeth slightly diverging ( Fig. 8D ): upper tooth half to 2/3 as long as, and thinner than lower one ( Fig. 8E, F ). In clitellar segments (X–XII) chaetae 1–2 per bundle, 40–55 μm long; segment XI devoid of ventral chaetae. In postclitellar segments, chaetae 0–1 per bundle, 40–50 μm long ( Fig. 8G ); nodulus situated slightly distally to the middle, and upper tooth slightly shorter and much thinner than curved lower tooth ( Fig. 8H ).

Gonads and copulatory organs paired. Testes in X, ovaries in XI. Male funnels on ventral part of anterior face of 10/11, 75–96 µm in diameter ( Fig. 9A ). Vasa deferentia not coiled up, about 400 µm long, 11–14 µm wide throughout course, with thin wall of 1.5 µm, ciliated, entering atrium apically. Atria tubular, about 300 µm long, 24–37 µm wide, with thick (5–10 µm) and glandular inner epithelium and with thin muscular cover ( Fig. 9B ). A solid prostate gland, about 40 µm wide, connected with ental portion of atrium through a short stalk. Ejaculatory ducts not detected. Large penes, each set in deep and narrow penial pouch opening at posterior 2/3 of XI ventrally. Penial sheaths thin and finger cot-shaped ( Fig. 9 C–E ), 32–40 µm long, 18–23 µm wide at base, sometimes with weak wrinkles at the ental part. Female funnels on ventral part of anterior face of 11/12, about 50 µm in diameter. Spermathecae absent in all ten mature specimens examined. Sperm sac in X–XI. Egg sac in XI–XII.

Distribution and habitat. The present new species was collected from muddy bottoms in shallow (0.9–1.2 m deep) Lake Ushiro-gata and the neighboring L. Mae-kata, both in the Tsugaru Peninsula, Aomori Prefecture, Japan. Temperature of bottom water in Lake Ushiro-gata was 24.5°C, with a pH of 9.0 (22nd September 2012). Lakes Ushiro-gata and Mae-kata are oligohaline, with electrical conductivity ranging from 10 to 15 mS cm-1 and salinity ranging from 0.8 to 1.2%.

Remarks. The genus Varichaetadrilus was originally erected by Brinkhurst (1981) as Varichaeta and renamed by Brinkhurst & Kathman (1983) due to the existence of the preoccupied dipteran genus name Varichaeta Speiser. This genus was initially defined by the presence of hair and pectinate chaetae, long tubular atria with a small subapically attached prostate gland, distinct penes with sheaths, lack of modified spermathecal chaetae, and lack or only slightly modification of penial chaetae ( Brinkhurst & Kathman 1983). Hair and pectinate chaetae, and small-sized prostate glands should be removed from the genus diagnosis because several species without hair and/ or pectinate chaetae [ V. psammophilus ( Loden, 1977) , V. fulleri Brinkhurst & Kathman, 1983 , V. angustipenis ( Brinkhurst & Cook, 1966) , V. vestibulatus Cui & Wang, 2009 ] and species with a large prostate gland [ V. harmani ( Loden, 1979) and V. bizkaiensis Rodriguez & Giani, 1984 ] have been incorporated in the genus ( Timm 2006). Absence of modified spermathecal chaetae is also canceled from the generic criteria because conspicuously modified spermathecal chaetae have been reported in V. vestibulatus . The spermathecal chaetae of V. vestibulatus are typical of Isochaetides , suggesting a close relationship between Varichaetadrilus and Isochaetides .

Long and tubular atria and penes with symmetrical penial sheaths as found in the present new species meet the criteria for the genus Varichaetadrilus . Among the ten valid species of Varichaetadrilus so far known ( Timm 2006; Cui & Wang 2009), three North American species, V. psammophilus , V. fulleri and V. angustipenis have bifid chaetae only as in V. salinus sp. nov. The upper teeth of the bifid chaetae are longer than the lower teeth in most of the anterior segments of V. fulleri and V. psammophilus , so they are different from those in V. salinus sp. nov. where the upper teeth are distinctly shorter than the lower teeth. The penial sheaths of V. psammophilus are widened at both the proximal and distal ends and with round hood ( Loden 1977), those in V. angustipenis are long and cylindrical with hood ( Brinkhurst & Cook 1966), and those in V. fulleri have a large hood ( Brinkhurst & Kathman 1983). They are hence clearly different from the finger cot-shaped penial sheath without hood in V. salinus sp. nov. Absence of spermathecae in V. salinus sp. nov. is unique among congeners, suggesting parthenogenetic reproduction. Asexual reproduction by architomy was demonstrated in the introduced population of V. harmani from North America into the Netherlands ( Timm 2006).

Absence of hair chaetae, tubular atria and cylindrical penial sheath found in the present species are shared with Isochaetides Hrabě, 1966 . However, Isochaetides species have modified spermathecal chaetae ( Rodriguez & Achurra 2010) which has not been observed in the present species. Furthermore, absence of genital chaetae, tubular atria receiving vas deferens apically, and absence of an ejaculatory duct such as found in the present species fit the definition of Tasserkidrilus Holmquist, 1985 , emended by Timm (1989). However, other generic characters of Tasserkidrilus such as vas deferens with different thickness and asymmetrical funnel-shaped penial sheath disagree with those of the present new species.

Figures

FIGURE 8. Varichaetadrils salinus sp. nov. from Lake Ushiro-gata, Aomori Prefecture, Japan. A. Lateral view of anterior segments (holotype). B. Parasagittal section of anterior segments, showing vascular system and alimentary canal. C. The same in posterior segments, anterior body end to the left. D. Ventral chaeta in an anterior segment. E, F. The same,variations of the distal ends. G. Ventral chaeta in a posterior segment. H. The same, distal end.

FIGURE 9. Male reproductive organs in Varichaetadrilus salinus sp. nov. from Lake Ushiro-gata, Aomori Prefecture, Japan. A. Male duct. B. Cross section of atrium. C, D, E. Variations of penial sheaths.