Duplominona pusilla Curini-Galletti & Stocchino & Norenburg 2019, n. sp.

Duplominona pusilla Curini-Galletti n. sp.

(Fig. 3 A–E)

Holotype. Panama: Caribbean Sea: Bastimento Is., off Wild Cane Key (Lat. 9.350423, Lon. -82.165980), mediumcoarse sand at - 7 m, in channels between coral heads, June 2010: one whole mount (USNM 1569253).

Paratypes. Same data as holotype, two specimens sagittally sectioned (USNM 1569254–1569255); one specimen studied karyologically.

Other material. Bastimento Is., off Wild Cane Key (Lat. 9.349746, Lon. -82.166838), medium sand at - 4.5 m, June 2010: two specimens studied karyologically.

Etymology. The name refers to the diminutive size of the species (lat. pusillus: small).

Description. A slender, comparatively very small monocelid (Fig. 3 A); longest fixed specimen about 750 μm long. Rod-shaped rhabdoids, about 2.5 μm long. Statocyst inside brain, abutting its anterior edge. Subepidermal musculature strong. Small pharynx at about mid-body, about 60 μm long. Oesophagus about 1/4 the total length of pharynx.

Male genital system. With 8–10 testes in one irregular row. With a copulatory organ (Figs 3 C, E) about 50 μm long in sections. Outer muscular layer about 2 μm thick. Prostatic tissue poorly developed. The cirrus is about 30 μm in squeezed mounts, with at least 20 rows of densely and regularly packed spines (Figs 3 B, E). Proximal spines scale-like, about 2–3 μm high and 2–3 μm broad at basis, with a curved distal tip; medially, spines become progressively smaller, to 1.5 μm in length; distalmost spines are slender, with straight apex, up to 2.5–3 μm long (Fig. 3 B).

With an elongate accessory organ, about 30 μm long in sections, lined by a strong muscular coating, up to 4 μm thick, and provided with a comparatively broad stylet, about 18 μm long (Figs 3 D, E).

Female genital system. Ovaria and vitellaria as in previous species. With a small bursa, about 20 μm across, just in front of the copulatory organ. With a long vaginal duct, surrounded by heavily pigmented (glandular?) tissue; vaginal pore very close to male pore (Figs 3 C, E). Female duct running posteriorly to the bursa, lined by an irregular, vacuolated epithelium. The female duct opens to the outside very close to the pore of prostatoid organ. In living specimens, the two pores appear to be separated, very close to each other. In the sectioned, fully mature specimen, a single pore, where both female duct and prostatoid organ converge, was instead detected (Figs 3 D, E). However, the presence of a ciliated epithelium, similar to the outer body epithelium, between the two ducts, suggests the possibility that this pore fusion may be an artefact due to a strong contraction of the specimen sectioned.

Diagnosis. A small Duplominona species with up to 10 testes in one row. Cirrus cylindrical, with about 20 rows of scale-like spines proximally, up to 2.5 –3 μm long and broad, with curved apex; medially, spines are similar but smaller, about 1.5 μm long; distal spines are longer, to 3 μm, straight, and more slender. With a small bursa and an external vagina opening very close to male pore. With a prostatoid organ provided with a broad stylet 18 μm long. Female duct and prostatoid organ open to the outside in close proximity—may be fused. Pore indices: a:b:c: d = ≈12: 1: 4: (?).