Duplominona terdigitata Curini-Galletti, 2019

Curini-Galletti, Marco, Stocchino, Giacinta A. & Norenburg, Jon L., 2019, New species of Duplominona Karling, 1966 and Pseudominona Karling, 1978 (Platyhelminthes: Proseriata) from the Caribbean, Zootaxa 4657 (1), pp. 127-147 : 131-132

Duplominona terdigitata Curini-Galletti n. sp.

( Fig. 2 )

Holotype. Panama: Caribbean Sea: Bastimento Is. off Hospital Point (Lat. 9.350423, Lon., -82.218656), silty fine sand, at - 13 m, June 2010: one whole mount ( USNM 1569246 ).

Paratypes. Same data as holotype, four specimens sagittally sectioned ( USNM 1569249–1569252 ); one specimen studied karyologically .

Other material. Bastimento Is., off Wild Cane Key (Lat. 9.350423, Lon. -82.165980), medium-coarse sand at - 7 m, in channels between coral heads, June 2010: three specimens observed alive, two of which sagittally sectioned as deposited as whole mounts ( USNM 1569247; USNM 1569248). Bastimento Is., off Wild Cane Key (Lat. 9.349746, Lon. -82.166838), medium sand at - 4.5 m, June 2010: one specimen studied karyologically.

Etymology. The name refers to the morphology of the caudal end of the species, split into three even-sized ‘fingers’ (lat. ter: three; digitus: finger).

Description. Sturdy, comparatively thick monocelid; longest fixed specimen about 1.6 mm long. Anterior tip rounded; caudal tip tripartite, provided with numerous adhesive glands ( Figs 2 A, C ). With needle-shaped rhabdoids, 4–6 μm long. Statocyst as in the previous species. With strong subepidermal musculature ventrally. With a very short pharynx at mid-body. Oesophagus less than 1/4 the total length of pharynx.

Male genital system. With about 20 testes in two regular rows, running from immediately behind brain to in front of pharynx. With a copulatory organ about 50–70 μm long and 30–40 μm high in sections. The outer muscular layer is about 2.5 μm thick. With a nearly spherical seminal vesicle, separated by a muscular septum from the pros- tatic vesicle ( Figs 2 D, F ).

The spiny cirrus is lined by the inner muscular layer. When inverted, the cirrus is about 33 μm long, with at least 15 rows of densely packed spines. Observations on one specimen with everted cirrus and on sectioned specimens with inverted cirrus revealed a pattern of proximal rows of long (up to 9 μm), needle-like spines, which narrow abruptly above a basis 2.5–3.5 μm wide. These proximal spines are followed distally by an area with few, very small spines, and by a few distalmost rows of triangular, slightly curved, larger spines, up to 4 μm ( Figs 2 B, D, F ). In other specimens with inverted cirrus, spines are packed to a degree that observations are particularly difficult.

With an ovoid-elongate accessory organ, about 30 μm across in sectioned specimens ( Fig. 2 E ). It is lined by a muscular coating, about 3 μm thick, and provided with a very slender, pen-nib shaped stylet 23–25 μm long. The accessory organ opens to the outside through an own pore, just in front of the female pore. Due to their closeness, in living specimens it is difficult to discern whether these pores are separated or fused.

Female genital system. Ovaria and vitellaria as in previous species. With a large pre-penial bursa, up to 80 μm long, surrounded by several vacuoles, lined by a resorbiens-type tissue, and abutting the gut. With a short, wide vaginal duct, surrounded by few circular muscles, opening to the outside through a vaginal pore ( Fig. 2 F ). The tract of female duct posterior to the bursa is lined by an irregular epithelium. Female pore surrounded by many female glands.

Karyotype. With n=3, and basic karyotype for the Monocelididae ( Curini-Galletti & Martens, 1990) . Karyotype formula: FN=5; Chromosome I: 43; 47.76 (m); Chromosome II: 37.53; 48.41(m); Chromosome III: 19.43; 4.55 (a) (based on one plate).

Diagnosis. Species of Duplominona with tripartite tail. Proximal spines of cirrus needle-shaped, up to 9 μm long; medially, spines are sparse and short, about 1.5 μm long; distal spines are 3.5–4 μm long. With a large bursa in front of the copulatory organ, and an external vagina. With a prostatoid organ provided with a slender stylet 23–25 μm long. Female duct and prostatoid organ open to the outside through very close, yet distinct pores. Karyo- type with two pairs of metacentric and one pair of smaller heterobrachial chromosomes. Pore indices: a:b:c:d = ≈12: 6: 6: 1.

Figures

FIGURE 2. Duplominona terdigitata n. sp. (A) Habitus of a living animal; see Fig. 1 for interpretation of anatomical features (B) Reconstruction of the everted cirrus showing the sclerotised copulatory structures. (C) Micrograph of a living animal. (D, E) Micrographs of sagittal sections of copulatory (E) and accessory (E) organs. (F) Reconstruction of the genital organs from sagittal sections. (Scale bars = 10 m).