Aberrantidrilus sketi Martínez-Ansemil & Sambugar, 2025

Martínez-Ansemil, Enrique & Sambugar, Beatrice, 2025, Inventory of the inland-water Phallodrilinae (Clitellata: Naididae), with descriptions of a new genus and five new species from groundwaters of southern Europe, Zootaxa 5692 (3), pp. 446-468 : 458-460

Aberrantidrilus sketi Martínez-Ansemil & Sambugar sp. nov.

( Figs 9 , 10 )

Abyssidrilus sp. 1 in Giani et al. (2011)

Holotype. MHNUSC_10144 : mature worm, incomplete (broken at segment XVII), stained in paracarmine and whole-mounted in Canada balsam. Leg. Boris Sket.

Type locality. Cave 4101 , Brje pri Koprivi, Sežana, Slovenia, 13.86220, 45.77766 ( December 1979).

Paratypes. MHNUSC_10145_1–4 : 3 mature specimens, unstained and whole-mounted in Canada balsam; 1 immature specimen, stained in paracarmine and whole-mouted in Canada balsam. All from type locality ( December 1979). Leg. Boris Sket.

Etymology. Named in honor of the late Professor Boris Sket (University of Ljubljana, Slovenia), who made outstanding contributions to the knowledge and development of biospeleology.

Description. Small worms, 3.2 mm long and 32 segments (complete paratype specimen). Diameter at segment V (slightly compressed specimens) 150–185 μm, at segment XI 175–195 μm. Clitellum hardly noticeable, extending over 1/2 X–XII. Prostomium rounded, 60–62 μm long. Secondary annulation at segments II–V. Somatic setae 32–50 μm long, 3–5(6) in the preclitellar bundles and 2–5 per bundle in clitellar and postclitellar ones; ventral setae absent in XI in mature specimens; preclitellar dorsal setae and ventral setae until about segment XXV bifid, with upper tooth slightly longer than or as long as lower ( Fig. 10A, C–D ); a mixture of single-pointed and bifid setae with upper tooth reduced in the following one or two segments; thereafter the bundles are composed exclusively of single pointed sigmoid setae ( Fig. 10B ). Penial setae absent. One pair of male pores located in two conical protuberances (porophores) in the posterior third of segment XI, in line with somatic ventral setae ( Fig. 10E–F, H [mp, po]); each porophore has a large ring of glandular cells surrounding the male pore ( Fig. 10F g ). One pair of spermathecal pores in segment XII, surrounded by a small circle of glandular cells, and placed anteriorly in the segment, in a lateral position slightly ventral with respect to the lateral lines ( Figs 9 , 10H–G [sp, g]).

Pharyngeal glands in ( III) IV– VI. Chloragogen cells beginning in VI. One pair of testes in X. Anterior and posterior sperm sacs extending to IX and XI, respectively. One pair of ovaries in XI. Egg sac not developed, one mature egg present in the ovarian segment. Vasa deferentia shorter than atria, 5.5–7 μm in diameter, entering apical end of atria ( Figs 9 , 10J [vd]). Atria tubular, comma-shaped, 205–215 μm long, 16–20 μm wide, strongly muscular (muscular layer 5–7 μm thick) ( Figs 9 , 10H, K [a]). Two prostate glands per atrium. The anterior prostate is large, compact and lobed and is attached to the apical end of the atrium ( Figs 9 , 10J [pr1]), and the posterior prostate consists in a small group of cells, hardly visible, connected by a few strands with the ectal end of the atrium (or with the porophore?) near the male pore ( Figs 9 , 10I [pr2]). Spermathecae elongated, confined to XII, each consisting of a short conical duct (about 45 μm long), curved at base, and dilating in a club-shaped ampulla filled with spermatozoids; length of the ampulla about 75 μm long, maximum width 33 μm ( Figs 9 , 10E [sd, sa]).

Distribution and habitat. Cave in the Slovenian coastal karst, Sežana, Slovenia. Alt. 290 m.

Remarks. The genus Aberrantidrilus Martin was erected to include a new groundwater species of Phallodrilinae , A. stephaniae Martin, 2015 , from the Mercantour National Park, France, and the only two groundwater species of Abyssidrilus Erséus known until then, A. cuspis ( Erséus & Dumnicka, 1988) , from Umbria, Italy and A. subterraneus ( Rodriguez & Giani, 1989) , from Cantabria, Spain ( Martin et al. 2015).The decision to erect this genus was supported by “a unique combination of characters rather than formally proposed apomorphies”. Thus, these three species share the “aberrant” position of the spermathecal pores (in the postatrial segment XII), the presence of a single penial seta per bundle oriented in anterior direction, the atria covered by a thick muscular layer and opening to the exterior on conical male protuberances (porophores), and the small size of the posterior prostate glands. A fourth species of Aberrantidrilus , A. mihaljevici Vučković, Rodriguez & Kerovec, 2020 was recently described from the tufa barrier of Roški slap, Croatia ( Rodriguez et al. 2020); the authors remark that this species shows all the abovementioned characters except the thick muscular layer of the atria and, perhaps also, the presence of posterior prostate glands (“not seen, probably due to its small size”).

The new species described here is tentatively ascribed to the genus Aberrantidrilus . The only major character by which A. sketi sp. nov. stands out in the genus is the lack of penial setae, but this is probably an unstable character in the genus as the absence of penial setae was observed also in one mature specimen of A. stephaniae ( Martin et al. 2015) and “maybe” also in the specimens of A. cf. cuspis cited from Trieste, Italy ( Sambugar et al. 1999). The tubular, narrow, comma-shaped and strongly muscular atria, the characteristically lobed anterior prostate glands and the thin and long spermathecal ducts clearly separate A. sketi sp. nov. from all the other species of the genus; the presence of glandular cells surrounding the spermathecal pores is also noteworthy, as similar structures or true glandular tissues associated or close to spermathecal pores are also present in Abyssidrilus opulentus Erséus, 1992 and Abyssidrilus potens Erséus, 1992 ( Erséus 1992a).As pointed out by Martin (in Martin et al. 2015), the specimens tentatively identify by Sambugar et al. (1999) as A. cuspis or A. cf. cuspis from Italy, France and Slovenia can be considered as a complex of cryptic species. Aberrantidrilus can be regarded now as a diversified groundwater genus of Phallodrilinae from the karst of southern Europe.

Figures

FIGURE 9. Aberrantidrilus sketi sp. nov. Genitalia, segments XI–XII.

FIGURE 10. Aberrantidrilus sketi sp. nov. A: dorsal setae of segment VII; B: dorsal setae of XVII; C: ventral setae of VI; D: ventral setae of XVII; E–K: genital organs. Scale bars: A–D and I: 5 µm, F–H and J–K: 10 µm, E: 20 µm.