Trichodrilus benati, Rodríguez & Fend, 2025

Trichodrilus benati View in CoL sp. nov.

( Figures 1 View FIGURE 1 , 2 View FIGURE 2 , Table 2)

Holotype: MNCN 16.03 About MNCN /3201, 1 mature, mated individual, dissected, stained in haematoxylin and mounted in Canada balsam. 8 December 1987.

Paratypes: MNCN 16.03 About MNCN /3202, 1 mature, dissected individual (sperm in sperm funnels, but spermathecal ampullae filled with undetermined mass) . MNCN 16.03 About MNCN /3203, one mated and dissected individual . MNCN 16.03 About MNCN /3204, one mated, whole-mounted individual. All from the type locality. 8 December, 1987 .

Type locality: Mairulegorreta Cave , Gorbeia ( Itxina Karst), Zigoitia, Alava, Spain ( P. Rodriguez leg.). Coordinates N43.022706, W2.756745, 900 m altitude. Sampled at the headwaters of the cave river ( Table 1) GoogleMaps .

Etymology: For Beñat Zaldibar, in appreciation for his expert collaboration on several occasions in the histology of the species.

Other material (in first author’s collection):

— Mairulegorreta Cave , one mature, mated, whole-mounted individual, from type locality. 8 December 1987, at the resurgence .

—Azoleta Cave, 1 mature, mated individual, dissected. 10 March 1985. (P. Rodriguez leg.).

— Arandilla R. source, 5 individuals, mated, whole mounts, 10 September 2023 ( P. Rodriguez leg.) .

Description (based on type locality population): Specimens incomplete, without posterior end, 15–25 segments, body diameter 0.26–0.35 mm in segment X. Prostomium conical. Secondary annulation from segment III. Anterior ventral chaetae shorter in II (70–89 µm), and gradually longer (to 96–128 µm), most posterior chaetae down to 90 µm; nodulus at 0.36–0.44 distance from the tip relative to total chaeta length. Anterior dorsal chaetae 0.8–1 the length of ventral chaetae in the same segment. Clitellum from (posterior part of IX) X to XII (or anterior part of XIII); cells do not show any particular order ( Fig. 1B View FIGURE 1 ). Male pores open in conical porophores, located very posterior in segment X, in line with ventral chaetae and directed backwards ( Fig. 1 D, E View FIGURE 1 ). Spermathecal pores in segments XI and XII, behind and in line with ventral chaetae. Female pores in 11/12.

Pharynx well developed dorsally in II and III ( Fig. 1A View FIGURE 1 ). Pharyngeal glands from IV (only dorsally) to VII or VIII (only ventrally). First nephridia in segment VII, absent in IX to XI and present again from segment XII ( Fig. 1C View FIGURE 1 ). Sperm sac back to XII (no egg sac formed).

Paired testes in IX and X, one pair of ovaries in XI. Semiprosoporous male duct. Atrium oval, gradually narrowing toward the pore, continuous with the conical duct, which, however, is well separated histologically from the ampulla ( Figs. 1E View FIGURE 1 , 2 View FIGURE 2 ). Total length of atrium 120–144 µm (c. 0.5 the body diameter), maximum diameter 56–68 µm, atrial duct 55–58 µm long and 16–24 µm wide, forming a short penis with the ectal end lined by cuticle ( Fig. 1D, E View FIGURE 1 ). Muscular layer of atrial ampulla 5–10 µm thick; glandular atrial epithelium compact (12–18 µm) with nuclei, and a narrow lumen; prostate cells organized in clusters to 54–73 µm high. One pair of vasa deferentia (10–22 µm Ø) joins the apical part of each atrial ampulla; the posterior one crosses into segment XI.

Paired spermathecae in segments XI and XII, sac-like ampulla (90–280 µm long x 45–74 Ø) and a short duct (40–92 µm long x 10–21 µm Ø) with a small ectal vestibule (17–34 µm Ø) ( Fig. 1F View FIGURE 1 ). First spermathecal ampulla larger than the second one; ampullae can cross into following segment.

The Azoleta Cave specimen differs from those of the type collection mainly by the larger dimensions of the worm and, in particular, of the atrium (see Table 2). The Arandilla River population has been provisionally ascribed to T. benati sp. nov. due to the general morphology of the male duct ( Fig. 1G, H View FIGURE 1 ), the body diameter ( 0.24–0.27 mm), and the atrium total dimensions (105–135 µm long x 54–72 µm Ø), although the duct is shorter. The main differences are: smaller chaetae (37–67 µm long) and a thinner atrial musculature (2–4 µm).

Remarks: The apical junction of the vasa deferentia to the atrium is one of the most distinct characteristics of the new species. This character is also known in T. intermedius ( Fauvel, 1903) from a well in northwestern France, a species that is much larger than T. benati n.sp., with posterior lateral blood vessels, a tubular atrium and large, partially muscular vasa deferentia (for comparison, see Table 2). Hrabě (1937) re-examined a specimen of Trichodrilus intermedius , probably a syntype from Fauvel’s type collection; it was histologically sectioned and deposited in the National Museum of Prague.

The species T. benati sp. nov. is most similar to T. medius Hrabě, 1960 . Atria of both species have a similar shape, and in both cases the ampulla gradually narrows toward the duct, i.e., the atrium is not petiolate, but the duct is well distinguished histologically from the ampulla. The most peculiar character in the description of T. medius was the very apical junction of the posterior vas deferens, while the anterior one joins at “ a quite considerable distance ” ( sic). The study of the type species from the NMP has allowed us to do new measurements ( Table 2) and photos. The transverse section in Hrabě´s (1960) fig. 24 (see Fig. 3A View FIGURE 3 ) is found on the slide Hr 1517-30 (#9), and we interpret it as misleading, since that atrial ampulla section does not correspond with the middle section of the atrium, but it is somewhat lateral. In fact, the sagittal sections in the slide Hr1517-9 (#1, #2)) show the vasa deferentia joining subapically to the atrial ampulla ( Fig. 3B, E View FIGURE 3 ). The duct in T. medius is longer than the ampulla, and some individuals have a salient penis, formed by the inner cells of the duct (type-2 penis: Rodriguez & Giani, 1994) ( Fig. 3C, D View FIGURE 3 ) while T. benati sp. nov. the duct is shorter and the ampulla and has a type-1 penis, i.e., formed by the protrusion of the atrial duct. Spermathecal ampullae in T. medius are larger than in T. benati ( Fig. 3F View FIGURE 3 ).

Another five Trichodrilus species with 2 pairs of spermathecae and without lateral posterior blood vessels have been described as having apical or subapical vasa deferentia junctions. Among these species, T. tacensis Hrabě, 1963 clearly differs from T. benati sp. nov. by the tubular atrial ampulla, and the posterior vasa deferentia not penetrating the post-atrial segment. In the other four species, the atrial ampulla is oval or spherical, but they are mainly distinguished from T. benati sp. nov. by the following characters: T. angelieri Giani & Rodriguez, 1994 has very distinct epidermal glands and the atrium has a simple, cylindrical ejaculatory duct; T. claparedei Hrabě, 1938 and T. seirei Timm, 1979 (based on the holotype) are mainly distinguished by the spherical shape of the atrial ampulla and by the thick atrial musculature (up to more than 23 µm), and T. hrabei Cook, 1967 has a small (80 µm long) atrium, with an unusual, laterally-compressed shape, and without a well-defined atrial duct.

Habitat and distribution. Found in cave streams and karst springs in northern Spain.