Plagiostomum aestuarium Wang & A. Wang, sp. nov.

Material examined.

Holotype: PLA-Pl 030, sagittal sections on two slides. Paratypes: PLA-Pl 031, sagittal sections on three slides; PLA-Pl 032, sagittal sections on three slides; PLA-Pl 033, transverse sections on six slides; PLA-Pl 034, horizontal sections on three slides. The type specimens were deposited in IZCAS (Institute of Zoology, Chinese Academy of Sciences).

Specimen habitat.

All specimens were collected by Si-Qi Wang from the intertidal zone in Huidong County, Huizhou City, Guangdong Province, China, 22.72038°N, 114.95644°E, 30 March 2024 (Fig. 2). The specimens were collected from Cladophora algae. The water temperature was 24.4 ° C, salinity was 25.9 ‰, and pH was 8.14.

Etymology.

The Latin word aestuarium means “ estuary, ” and the habitat of the new species is located in an intertidal estuary.

Diagnosis.

Plagiostomum aestuarium is distinguished by the following characters: a blunt anterior end with tentacle-like structure; a dark-brown patch covering the anterior end; a pair of black eyespots; a pair of testes and a pair of ovaries are separated, the ovaries are located anterior to the testes; the vitellaria are separated at the anterior end, merged at the dorsal part of the intestine, and separated again at the posterior end on both sides of the intestine; the penis is encased in a well-developed penis sheath; the proximal part of the penis is curling up in coiling form within the distal sac, the distal part of penis is relatively short.

Description.

The length of the new species is 2.05 ± 0.62 mm (mean ± SD, n = 9, the same as below), and the middle body is cylindrical in cross section, 0.41 ± 0.08 mm in width (Fig. 3 A). The anterior end of the body is curved and has a slight tentacle-like structure. A triangular dark-brown patch is present at the anterior end (Fig. 3 A, B). A pair of black eyespots is laterally symmetrical and is located at the edge of the dark-brown patch (Figs 3 B, 4 B, 7 E). The pharynx is about 1 / 3 of the body width and 1 / 7 of the body length, located behind the dark brown patch. (Fig. 3 A, B). The ovary of the new species is located in the middle 2 / 5 of the body and on both sides of the body (Fig. 3 A, D). The caudal end is conical (Fig. 3 C, E), and five to seven clearly observable long ellipsoidal cement glands with fine tubes converge at the genital pore (Fig. 3 C). The penis is attached at the posterior end of the seminal vesicle (Fig. 3 E). The genital pore opens at the 1 / 10 near the posterior end (Fig. 3 A, C, E).

The surface of the body is richly ciliated, with cilia 2 / 3 as long as the thickness of the epidermis (Fig. 4 B-G). The basal lamina of the ventral epidermis is thick, with secretory gland cells (Fig. 4 A). The anterior end has two types of frontal gland cells; the ones that are stained red by histological staining (fg - 2) are distributed within the sub-anterior epidermis, and the ones that are stained mauve-colored by the histological staining (fg - 1) are filled in between the fg - 2 and the brain. Both types of glands have a dense glandular duct leading to the anterior epidermis at the ventral side (Fig. 4 A, F). The brain is located inferior to the eyespots and anterior to the pharynx (Fig. 4 A, F). The muscular pharynx opens ventrally in the anterior region and connects to a long sac-like intestine surrounded by abundant pharynx gland cells, and the epidermis of the pharynx cavity is covered by cilia (Fig. 4 A, B). The male copulatory organ is located caudally, occupying about 1 / 4 of the body length, and opens jointly with the female genital pore into the common genital cavity (Fig. 4 A, C). The genital pore is surrounded by the dark-red stained shell glands (Fig. 4 A, C, E, G).

Female reproductive organs consist of vitellaria, ovaries, oviducts, a female genital pore, shell glands, and cement glands. A pair of vitellaria extend anteriorly to the pharynx, posteriorly to the male copulatory organ, and bilaterally above the ovary, taking up 4 / 5 of the body length. The left and right vitellaria anterior to the ovary are separated, join at the back of the intestine, and are distributed on the two sides of the intestine behind the ovary (Figs 4 A, 5 A, C, E). The paired ovaries are located in the middle of the body on both sides of the intestine, proximal to the ventral side of the vitellaria, without obvious enveloping membranes (Figs 4 D, E, 5 A). The oviducts, about 8 μm in diameter, consist of only a layer of connective tissue and stretch from the posterior end of the ovary (Fig. 4 D, E, as indicated by the arrow). Branches of vitelline ducts converge into the oviducts (Fig. 4 D, E) and then connect the egg-forming cavity. The egg-forming cavity is surrounded by abundant purple-stained shell glands. The vagina opens dorsally in the middle part of the common genital cavity (Figs 4 A, D, E, 5 F).

Male reproductive organs consist of testis, seminal vesicle, prostatic glands, penis, distal sac, and penis sheath. The paired testes, about 1 / 5 of the body length, are located posterior to the ovaries and ventral to the intestine (Figs 4 A, 5 C). The purple-red spindle-shaped spermatozoa are present between the spermatogonia (Fig. 4 A). The hemispherical seminal vesicle is located directly behind the posterior end of the testes, with no vas deferens observed (Fig. 4 A, C). Several striated prostatic glands are attached to the posterior wall of the seminal vesicle, encasing the spermatogonia, and there is no separate prostatic membrane (Figs 4 A, C, G, 5 E). The distal sac is attached posteriorly to the seminal vesicle, a tubular structure with a diameter of 10 μm, and shows no sperm storage during non-mating period (Figs 4 A, C, 5 B). The penis sheath is attached to the seminal vesicle. From outer to inner, the penis sheath consists of the outer ring muscle, radial muscle, and sleeve-shaped inner ring muscle that surrounds the penis (Figs 4 A, C, 5 D). The penis musculature, which terminates in a papilla-like structure, is located in the common genital cavity and is encased in the penis sheath. The penis sheath is “ w ” - shaped in the sagittal section and concentric in the transverse section (Figs 4 A, C, 5 D). The seminal vesicle, distal sac, penis, and penis sheath are encased in a layer of epithelial cells that can move together. The common genital cavity opens on the ventral side of the caudal end and is surrounded by abundant cement glands that converge posteriorly towards the common genital pore (Fig. 4 A, C, F).