Eclipidrilus, Eisen, 1881

Rodriguez, Pilar, Vučković, Natalija & Kerovec, Mladen, 2020, New species of aquatic oligochaetes (Annelida: Clitellata) from tufa barriers in Croatia, Zootaxa 4758 (3), pp. 442-460 : 454-455

Eclipidrilus sp.

( Figure 6 )

Study material. 1 specimen from Roški slap, Croatia, 43°54’12.1”N, 15°58’31.3”E (43.90336 N, 15.975348 E) (November 6, 2013). Collector: Zlatko Mihaljević.

Description. Body consists of 61 segments, about 15 mm long. Prostomium short and pointed (250 µm long). Clitellum weakly developed, not raised, from the line of chaetae in IX to the line of chaetae in XIII. Secondary an- nuli not visible, but the specimen is very contracted due to fixation. Two chaetae per bundle, single pointed ( Fig. 6A ), 160–172 µm long, 7–8 µm thick in anterior segments, smaller in II and larger in VI, with the nodulus in distal position, at 0.3–0.4 distance from the tip. Chaetae shorter in postclitellar segments (146–151 µm), and slightly thinner (5–6 µm) than in anterior segments; in most posterior segments, chaetae 92–134 µm long, with nodulus at 0.3 distance from the tip. The ventral chaetae present and unmodified on clitellar segments. Spermathecal pores and male pores paired, surrounded by a ring of muscles, posterior to and in line with the ventral chaetae on IX and X, respectively. Female pores located in intersegment 11/12. Pharyngeal glands in segments IV to VI. Blind posterior lateral blood vessels not observed although the presence of large amounts of sediment particles in the gut makes the examination difficult. First nephridia observed in segments VII and VIII, and postclitellar from 12/13, backwards. Sperm sacs extending anteriorly to VIII and posteriorly to XV. Egg sac back to XV with eggs and yolk.

Paired testes in IX and X, ovaries paired in XI. One pair of ciliated female funnels on septum 11/12. One pair of atria in X, elongate (429 µm long, maximum diameter 97 µm) and entering segment XI; the transition of the atrial ampulla to the atrial duct is not abrupt. Atrial duct short, narrower than ampulla (88 µm long, 25–42 µm wide, thinner ectally), and devoid of prostate glands. The atrial ampulla consists of two muscle layers, the outer layer (10–12 µm thick) formed by muscular fibers that intersect in different directions and the inner layer (5–6 µm thick) arranged longitudinally; atrial epithelia 15–20 µm high. Atrial ampullae completely covered by prostate gland cells (up to 77 µm high), forming densely packed clusters ( Fig. 6B ). The atrium opens to the exterior through a pore located in a slightly elevated papilla, without a penis ( Fig. 6C ). Male duct semiprosoporous, with anterior sperm funnels not visible in the preparation; one pair of vasa deferentia per atrium (20–31 µm diameter), with a ciliated lumen, joining the atria in the basal part of the atrial ampulla, on opposite sides. The posterior vas deferens does not penetrate the ovarian segment.

One pair of voluminous spermathecae (about 750 µm long, 225 µm wide), bent, located in IX or extending into X. Ectal section of the ampullae with well-stained sperm, lining epithelium 15–18 µm high, not vacuolated; ental section of the ampullae with undetermined material and unstained sperm, lining epithelium up to 45 µm high ( Fig. 6D ). Spermathecal duct 300–350 µm long and 45–75 µm wide, covered by a thin muscle layer (4–7 µm thick), high lining cells (10–18 µm), and with a narrow lumen (4–9 µm) ( Fig. 6E ); the distal part covered by small glandular cells ( Fig. 6F ).

Remarks. We identify this specimen as a member of the genus Eclipidrilus Eisen due to the general organization of the reproductive system (semiprosoporous male duct and spermathecae in the first testicular segment), the elongate atrium and the presence of two muscular layers in the atrium, one of them with fibers arranged crosswise. Arrangement of the atrial musculature in the Croatian specimen differs from that of other Eclipidrilus species by the atrial musculature organization, since the crossed musculature is the outer layer while it is the inner layer in other species known so far (see amended diagnosis in Fend & Lenat 2012). Otherwise, the most similar Eclipidrilus species is E. fontanus Wassell, 1984 (atria paired, in X, no cuticular penis, no proboscis, paired spermathecae with simple ducts). Given the lack of more material for a detailed histological study of the male duct, this species cannot be attributed to E. fontanus or be described as a new one, since several characters, e.g. the presence of glands at the male and spermathecal pores and the structure of distal section of the atrial duct (= ejaculatory duct) must first be clarified. It is noteworthy that the genus Eclipidrilus appears to be a Nearctic genus and only E. lacustris has been reported on few occasions in Europe (Britain, Italy: Brinkhurst & Cook 1966; Cook 1971; The Netherlands: Verdonschot 1990), possibly due to recent introductions. However, E. lacustris is easily distinguished from out specimen by single, median male and spermathecal pores, and by a distinct cross-hatched atrial musculature.

The general organization of the reproductive system in the genus Eclipidrilus is similar to that of the genus Stylodrilus Claparède , as described by Cook (1971), i.e. the male duct is semi-prosoporous, although it usually shows a reduction in the anterior sperm funnel ( Fend & Lenat 2012). In the examined specimen, the anterior sperm funnels were not observed, although the anterior vas deferens has similar diameter to the posterior one.

Within Stylodrilus , some species share this type of elongate, petiolate atria: S. subcarpathicus , S. longiatriatus Dembitsky, 1976 , S. glandulosus and S. curvithecus , all with simple-pointed chaetae and distributed in South and Central-East Europe. However, all of them have sperm funnels of similar size and the atrium has a simple organization of the musculature, in one layer. Only the species S. subcarpathicus has been formerly regarded as a possible member of the genus Eclipidrilus , due to its very long atria which extend backwards to XII ( Cook 1971), but other generic characters such as the atrial musculature and the different size of anterior and posterior sperm funnels are not reported for that species, thus, they probably do not differ from the typical characters in the genus Stylodrilus (see discussion on these differences in Fend 2005).

Figures

FIGURE 6. Eclipidrilus sp. A. Chaeta. B. Atrium showing prostate glands forming clusters of cells, and inner and outer layers of muscular fibers. C. Male pore. D. Ental part of spermathecal ampulla. E. Ental part of spermathecal duct showing high lining cells. F. Ectal part of spermathecal duct showing small glandular cells (arrow) near the pore.