Limnoplana obscuriviridis Liu sp. nov.

Figs 3, 4

Etymology.

The specific name is a compound of the Latin adjective obscure (dark) and virid (green), referring to the dark-green dorsal coloration of the body.

Material examined.

Holotype: China • hermaphrodite; Guangdong Province, Shenzhen; 22°31.33'N, 113°57.06'E; 6 Dec. 2024; Bing-Bing He leg.; underneath stones in the mangroves; sagittal sections on 19 slides; GenBank accession: PZ 111957 (COI), PZ 111960 (16 S), PZ 112131 (18 S) and PZ 112136 (28 S); MBM 288503. Paratypes: China • 1 hermaphrodite; same data as for holotype; sagittal sections on 18 slides; GenBank accession: PZ 111961 (16 S), PZ 112132 (18 S) and PZ 112137 (28 S); MBM 288504 • 1 hermaphrodite; same data as for holotype; sagittal sections on 18 slides; GenBank accession: PZ 111962 (16 S), PZ 112133 (18 S) and PZ 112138 (28 S); MBM 288505.

Description.

The body is thin and elongate oval, with the posterior end slightly narrowed (Fig. 3 A). Anesthetized specimens are 16–35 mm long and 4–7 mm wide, while mature adults are approximately 30 mm long and 7 mm wide. The dorsal surface is dark green or khaki greenish, with two darker stripes running along the midline (Fig. 3 A, B). The ventral surface is lighter than the dorsal. Some of the smaller specimens (approximately 12 mm long and 4 mm wide) are somewhat reddish. The extreme margin of the body is colorless, and while the heart-shaped brain region is reddish (Fig. 3 B, D). Tentacles are absent. Tentacular eyespots are arranged in two long clusters, each consisting of 18–35 eyespots (Fig. 3 D). Cerebral eyespots are scattered between tentacular eyespots, totaling approximately 30 eyespots (Fig. 3 D). Marginal eyespots encircle the entire body, with more than one layers in the anterior 1 / 4 of the body (Fig. 3 D), while only one sparse layer in the remaining regions of the body.

A ruffled pharynx is located in the anterior half of the body, measuring 6–10 mm in length (Fig. 3 C). The mouth is located in the middle region of the pharynx, approximately at the anterior one-third of the body. (Fig. 3 C). Genital pores are situated at the posterior end of the body: the male pore is located approximately 0.3 mm from the posterior end, and the female pore opens about 0.1 mm behind the male pore (Fig. 4 B, E).

The male copulatory apparatus is comprised of a true seminal vesicle, a free prostatic vesicle, and an unarmed penis. A pair of vasa deferntia run ventrally, enter the proximal end of the elongate tubular seminal vesicle separately (Fig. 4 C, F). The seminal vesicle (0.5–0.7 mm in its long axis) has a distinct muscular wall, and is situated beneath the prostatic vesicle (Fig. 4 E). Its distal end gradually narrows to form the ejaculatory duct, which runs obliquely backward and upward for a short distance, then bends sharply before opening at the tip of the penis (Fig. 4 E, F). The prostatic vesicle (0.7–1 mm in its long axis) is an elongate tube running in a wavy manner, with a strong muscular wall and is lined with smooth epithelium (Fig. 4 E, F). The prostatic secretion is strongly stained by acid fuchsin, taking on a somewhat dirty red hue (Fig. 4 E). The prostatic duct receives the ejaculatory duct at the base of penis (Fig. 4 B). A conical penis without stylet projects vertically into the male atrium (Fig. 4 B, E, F).

The ovaries are situated dorsally. The distal ends of a pair of uteri fuse at the midline to form a common oviduct, which runs upward to enter the vagina (Fig. 4 A, F). From this point, Lang’s duct with numerous radial folds runs postero-ventrally, and leads to Lang’s vesicle (Fig. 4 A, F). Lang’s vesicle is “ U ” shaped, with both ends swollen into a spherical form (Fig. 3 E), and lined with a thin columnar epithelium. Lang’s vesicle contains granular secretion that stain blue with aniline blue (Fig. 4 A). The vagina extends anterior for a short distance, then curves postero-ventrally, running along ventral side to near the proximal end of the prostatic vesicle; then vagina runs obliquely upward and backward, extending for some distance close to the dorsal side, eventually turning ventrally to open at the female gonopore (Fig. 4 A, D, E, F). The vagina is lined with both ciliated and smooth epithelium. From the common oviduct to the proximal end of the prostatic vesicle, the vagina is surrounded by cement glands, which are stained with acid fuchsin (Fig. 4 A, F).

Distribution.

The species is known from Shenzhen, Guangdong, China.

Habitat.

The specimens were collected from the estuary of the Dasha River, where the salinity is approximately 4 ‰ and mangroves are present. They were found intertidally underneath stones in the mangroves.

Molecular phylogeny.

Limnoplana obscuriviridis sp. nov was positioned inside the Stylochoidea superfamily with high support (96 / 0.94), clustering with all other stylochoids except Callioplana marginata (Fig. 5; Suppl. material 1: fig. S 1).

Remarks.

There are two recognized species in this genus: Limnoplana annardalei (Kaburaki, 1918) and Limnoplana amara (Kaburaki, 1918), which were described from Koh Yaw Island, Talé Sap and Singgora, Thailand, respectively. Limnoplana obscuriviridis sp. nov. can be distinguished from two congeners by three key morphological characters: i) presence of a true seminal vesicle (absent in L. annardalei and L. amara); ii) the two swollen ends of Lang’s vesicle (not illustrated as swollen in L. annardalei; Kaburaki 1918: plate VIII, fig. 1; L. amara described as similar to L. annardalei; this character was possibly overlooked or not explicitly described by Kaburaki); and iii) a distinct dorsal pattern characterized by two dark longitudinal stripes along the midline (uniform olive-greenish without any marking in L. annardalei and L. amara) (Kaburaki 1918). In addition, the new species differs from L. annardalei by the position of the genital organ (the male pore is situated at the posterior tenth of the body, about 1 mm from the posterior end in L. annardalei; while in L. obscuriviridis sp. nov, it is closer to the posterior end of the body, approximately 0.3 mm from the posterior end); and the shape of the brain (kidney-shaped in L. annardalei vs. heart- shaped in L. obscuriviridis sp. nov.) (Kaburaki 1918). Notably, the tentacular and cerebral eyespots blend together in L. amara, which distinguishes it from L. obscuriviridis sp. nov. (Kaburaki 1918).