Gianius parvogenitalis Martinez-Ansemil 2025, sp. nov.

Gianius parvogenitalis Martínez-Ansemil sp. nov. (Figs 5, 6) Holotype. MHNUSC_10148: mature worm stained in paracarmine and whole-mounted in Canada balsam. Leg. Ana I. Camacho. Type locality. CAN072 Upwelling of the Clarión stream, Cantabria, Spain, –3,54130, 43.34713 (13 Mars 2003). Etymology. The specific name refers to the small (Latin parvus) size of the genital organs (Latin genitalia). Description. Small worm, 1.8 mm long. Number of segments 26. Diameter at segment V 125 μm, at segment XI 175 μm. Clitellum 1/2 X–XII. Prostomium rounded, 45 μm long. Secondary annulation up to VI. Somatic setae bifid, 18–32 μm long, with upper tooth equal or slightly thinner and longer than lower (Fig. 6B); 3–5 setae per bundle in segments II–VII and 2 thereafter. Penial setae bifid, with a minute upper tooth, 2–3 setae per bundle, about 22 μm long and 1.5 μm thick (Figs 5A–B, 6D [ps]). One pair of male pores located in the posterior third of segment XI, in line with somatic ventral setae. One pair of spermathecal pores in segment X, placed anteriorly in the segment, slightly lateral with respect to the alignment of the ventral somatic setae. Pharynx with a well-developed pharyngeal pad. Pharyngeal glands in segments IV–VII. Chloragogen cells beginning in VI. Oesophageal appendages and diverticula absent. One pair of testes in segment X. One pair of ovaries in segment XI. Sperm sacs not observed. Egg sac extending to 12/13. Vasa deferentia coiled, 4–5 μm wide, about as long as the atria (Figs 5A, 6C, E [vd]). Atria cylindrical and curved (C-shaped), about 50 μm long and 7–8 μm in diameter, with a thin muscular outer layer (about 1 μm thick); ectally, the atrium ends directly in the tegument, surrounded by a thin circular muscular layer (Figs 5A, 6C–E [a]). Two prostate glands per atrium, the anterior one placed near the entrance of the vas deferens, and the posterior one at the ectal part of the atrium, close to the penial setae (Figs 5A, 6C–E [pr1, pr2]). Spermathecae composed of a short duct (22 μm long, 11 μm wide) and a thin-walled, pyriform, slender ampulla (48 µm long, up to 14 µm wide), filled with spermatozoa (Figs 5A, 6F [sd, sa]). Distribution and habitat. Upwelling of the Clarión stream, Cantabria, Spain. Alt. 57 m. Remarks. Following Erséus (1992a), the presence of elongated and curved atria, each with two prostate glands attached to proximal and distal ends, respectively, vasa deferentia entering apical ends of atria, slender spermathecae with short ducts, and small straight penial setae allocate the new species described above in the genus Gianius Erséus. The narrow cylindrical C-shaped atria, only slightly tapering towards the ends, its small size (only 50 μm long and 7–8 μm in diameter), and the presence of bifid penial setae in low number (only 2–3 per bundle) are distinctive characters of Gianius parvogenitalis sp. nov. with respect to all the other known species in the genus.