Trichodrilus stygodytes Rodríguez & Fend, 2025, sp. nov.

Trichodrilus stygodytes sp. nov. (Figures 7, 8D–H, Table 3) Trichodrilus moravicus Hrabě, 1937. Hrabě 1938: 73, fig. 1–8 (partim) Holotype. MNCN 16.03/3206, one mature, mated individual, stained and whole-mounted in Canada balsam. Type locality. Omaerreka River headwaters (below a resurgence), Kortezubi, Bizkaia, Spain. 13 March 1985. Coordinates: N 43.32556, W 2.59979, 150m altitude. Paratypes. MNCN 16.03/3207, Omaerreka River source, 13 March 1985, 1 mature, mated individual, dissected, stained and mounted in Canada balsam. MNCN 16.03/3208, Okamika Cave, at the cave resurgence, 27 December 2023, 1 mature, mated individual, dissected, stained and mounted in Canada balsam. MNCN 16.03/3209, Goiketxe/ Lamina Cave, 30 July 2025, 1 mature, mated individual, stained and whole-mounted in Canada balsam. (All sampled by Pilar Rodriguez). Other material in first author’s collection. One mature, mated individual from Goiketxe/ Lamina Cave (30 July 2025), stained and whole-mounted in Canada balsam. One mature individual from Lamiñak/Las Lamiñas Cave River (5 January 1986), stained and whole-mounted in Canada balsam. Other material ascribed to the species. Hrabě’s collection in NMP under the name of Trichodrilus moravicus: Býčí Skála Cave, Blansko, Moravian Karst, Czech Republic (https://mapy.cz/s/robekerede): Hr 1007-1 a whole mount. Hr 1007–2 (I–VII) sagittal histological sections (# 88–14). Hr 1026 transverse sections (#25, 27, 31) and sagittal sections (#24, 28, 30), and one whole mounted immature individual (#29). Hr 1028 sagittal sections (#36) and 5 whole-mounted individuals (#38). Etymology. The Latin species epitheton stygodytes is derived from ancient Greek and it can be interpreted as dweller (dytes) of subterranean river (stygos, underground river known as the boundary between the worlds of the living and the dead, in Greek mythology). Noun in apposition. Description (based on material collected in Spain; for comparison with material from the Hrabě collection, see below). Very small worms, body diameter in X 0.17–0.30 mm; all incomplete (up to 40 segments), with missing posterior part of the body. Clitellum from X–XII (indistinct in some individuals). Secondary annulation from III or IV. One pair of male pores in X, behind and in line with ventral chaetae; one pair of female funnels in 11/12; two pairs of spermathecal pores, in XI and XII, behind and in line with the ventral chaetae. Male pores open on round porophores (15–20 µm high, 45–50 µm wide). One pair of chaetae per bundle, simple pointed; in the anterior body segments, dorsal chaetae c. 50–84 µm, ventral chaetae (55) 62–100 µm, dorsals 0.8–1 times smaller than ventral chaetae, nodulus distal, at 0.4 times the total length from the distal end. Pharynx in segments II and III, with a well-developed dorsal pad (Fig. 7B). Pharyngeal glands to VIII. Sperm sacs back to XI or XII, eggs sac to XII or XIII. Two pairs of testes (segments IX and X) and one pair of ovaries (segment XI). Semiprosoporous male duct. Atrium petiolate, ampulla well separated from the duct (Fig. 7A), its total length 0.3–0.5 the body diameter. Atrium total length 80–121 µm; round to oval ampulla (50–76 µm long, 50–80 µm Ø). Atrial ampulla epithelium 4–16 µm high; ampullar musculature 2–8 µm thick, up to 10 µm at the vasa deferentia junction. Prostate glands are packed clusters of glandular cells to 47–80 µm high. The atrial duct (30–40 µm long, 17–30 µm Ø) can protrude a little through the male pore forming a penis (Fig. 7E). One pair of vasa deferentia (8–13 µm Ø) join the basal part of the atrial ampulla, close to the atrial duct (Fig. 7A, C, D); the posterior vas deferens crosses into segment XI. Spermathecal ampullae round or elongate (c. 50–120 µm long, c. 50–70 µm Ø), remaining in the same segment or crossing to the next one. Short spermathecal ducts (40–70 µm long), narrow close to the ampulla (10–15 µm Ø), and with a distal widening forming a vestibule (21–30 µm Ø) (Fig. 7F). Remarks. Among the Trichodrilus species with two pairs of spermathecae, Trichodrilus stygodytes sp. nov. is morphologically similar to other species of small size with a small, spherical to oval atrial ampulla, covered by a thin muscular layer (<10 µm), with a short duct (see Annex). In particular, the new species resembles T. moravicus (see below). The main character that separates T. stygodytes from this group of species is the basal/ectal junction of the vasa deferentia to the atrial ampulla. The comparison of the measurements of the Spanish population of T. stygodytes sp. nov. with the Czech collection from Cave Býčí Skála is shown in Table 3. The Spanish worms are smaller and with smaller chaetae than the Býčí Skála population. Both populations have similar characteristics of the male duct and share the distinctive basal junction of the vasa deferentia to the atrial ampulla (compare Figs. 7A, C, D and 8D–G). The spermathecae in the Czech population are somewhat larger than in the Spanish worms, and also show an ectal vestibule in the spermathecal duct (Fig. 8H). Habitat and distribution. The habitat of the species includes cave streams and headwaters of karstic rivers. Spain and Czech Republic. Spanish populations are at more than 1600 km distance from the sites in the Czech Republic, and future molecular studies could demonstrate that they belong to different species. Among the relatively large collection from subterranean waters that we have studied in the Iberian Peninsula, we have found T. stygodytes sp. nov. only in a single karst area, while T. tenuis is a widespread species in subterranean waters, and T. moravicus (see below) has not been found, so far.

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