Prosthiostomum huidongense Liu, 2024

Liu, Hai-Long, Lin, Da-Hao, Wang, An-Tai, Hu, Zhang-Li & Zhang, Yu, 2024, Description of a new marine flatworm of Prosthiostomum (Platyhelminthes, Polycladida, Prosthiostomidae) from the South China Sea, Zoosystematics and Evolution 100 (2), pp. 437-445 : 437

Prosthiostomum huidongense Liu sp. nov.

Material examined.

Holotype: MBM287880, Huidong , 22°44.95'N, 114°45.05'E, Guangdong Province, China; under rocks at the intertidal zone, 21 April 2023, coll. Hai-Long Liu; sagittal sections on 10 slides, deposited at MBMCAS . Paratypes: MBM287881, sagittal sections on 11 slides, same data as for holotype. Molecular voucher specimens: 20230421A1, material of the following GenBank accession numbers have been deposited at the Shenzhen Key Laboratory of Marine Bioresource and Eco-environmental Science, GenBank: OR680085 (COI), OR680128 (16S), and OR680129 (28S). Collection data are the same as for the holotype and paratype .

Diagnosis.

Body oval-elongated; dorsal surface cream-colored with numerous yellowish-brown maculae; a pair of cerebral eyes clusters forming an approximately inverted “V” shape; band of marginal eyes extending backwards to behind brain, few eyes scattered between the marginal band and the cerebral eyes; a pair of prostatic vesicles distinctly separated; male atrium shallow, inner wall slightly ruffled; sucker located at two-thirds of body length, removed from the female gonopore by twice the distance between the male and female gonopores.

Description.

Body oval-elongated with rounded ends, 10.8-12.1 mm long and 5.6-7.0 mm wide at its widest in living state (n = 3) (Fig. 2A ). Tentacles absent. Dorsal surface smooth, cream-colored, uniformly covered with numerous yellowish-brown maculae, pharynx slightly bulging (Fig. 2A ). Ventral surface translucent, without color pattern. Cerebral region without pigmentation and provided with a pair of clusters of cerebral eyes, located at approximately 1 mm behind the anterior margin, each cluster comprising 15-20 eyes (Fig. 2B, C ). About 140 marginal eyes irregularly scattered along the anterior body margin, extending backwards to a level just behind the brain; few eyes scattered between the marginal band and the cerebral eyes (Fig. 2B, C ). Ventral eyes not observed. Frontal branch of the main intestine extending anteriorly to the brain (Fig. 3B ). Tubular pharynx in a pharyngeal pocket, positioned in the anterior half of the body. Mouth opening ventrally, located shortly behind the brain (Fig. 3B ). Male and female gonopores closely set at the body center, distance between male and female gonopores being 0.6-0.8 mm; sucker situated 1.4-1.8 mm posterior to the female gonopore, twice distance between the male and female gonopores (Fig. 3A, E ). Male copulatory apparatus comprising a large seminal vesicle, a pair of prostatic vesicles, and an armed penis papilla, located immediately posterior to the pharyngeal pocket (Fig. 3A, C, E ). A pair of spherical prostatic vesicles (0.05-0.07 mm in diameter) with a thin muscular wall (0.009-0.012 mm thick), located at both sides of the ejaculatory duct (Fig. 3A, D ). Seminal vesicle oval, about 0.2 mm in diameter, with a muscular wall about 0.04 mm thick (Fig. 3A, E ). Spermiducal vesicle separately open into the anterior portion of the seminal vesicle (Fig. 3A, D ). Penis papilla armed with pointed tubular stylet (0.14-0.17 mm in length), enclosed in penis pouch, protruding into male atrium (Fig. 3C ). Shallow male atrium with slightly ruffled inner wall, positioned approximately perpendicular to the body wall (Fig. 3A, E ). Female reproductive system consisting of vagina, cement pouches, and uteri. Vagina short about 0.1 mm long, lined with a ciliated epithelium; the dorsal portion of the vagina enlarged and anteriorly curved, connecting to the uteri (Fig. 3A, E ). Cement glands numerous, concentrated around the vagina (Fig. 3A, E ). Pair of oviducts converging before joining proximal end of vagina. Ovaries not discerned.

Etymology.

The name of the new species is originated from the name of Huidong City, Guangdong Province, China.

Distribution.

So far only known from Huidong, Guangdong, China.

Habitat.

Intertidal, under rocks.

Molecular phylogeny and genetic distances.

The BI and ML trees based on the partial 28S sequences (904 bp) are almost identical in their general topology, and we show only the ML tree (Fig. 4 ). In the phylogenetic tree, all Prosthiostomum species form a clade with high support values (BP = 81%, PP = 1). Prosthiostomum huidongense sp. nov. is sister to the clade formed by all other species of the genus, excluding P. lobatum , with high support values (BP = 78%, PP = 1). The uncorrected p-distances of the partial COI sequences (658 bp) of the 11 Prosthiostomum species/molecular entities is 5.7-24.3% (Suppl. material 1: table S2).

Figures

Figure 2. Prosthiostomum huidongense sp. nov. A. Photograph of entire body, 20230421 A 1 (left, living specimen in dorsal view), MBM 287881 (middle, preserved specimen in dorsal view, paratype), MBM 287880 (right, preserved specimen in ventral view, holotype); B. Schematic diagram of head, showing arrangement of the eyes; C. Magnification of head, dorsal view (holotype); D. Magnification of genital pore and sucker (paratype). Abbreviations: fg - female gonopore; mg - male gonopore; ph - pharynx; su - sucker. Scale bars: 1 mm.

Figure 3. Prosthiostomum huidongense sp. nov., MBM 287880 (holotype), schematic diagram (A) and photomicrographs of sagittal sections (B-E), anterior to the left. A. Copulatory complex and sucker; B. Anterior end of the body; C. Penis stylet; D. Male copulatory apparatus; E. Copulatory complex and sucker. Abbreviations: ab - anterior branch of main intestine; br - brain; cg - cement glands; cp - cement pouch; fg - female gonopore; it - intestine; m - mouths; ma - male atrium; mg - male gonopore; ph - pharynx; pv - prostatic vesicle; spv - spermiducal vesicle; st - stylet; su - sucker; sv - seminal vesicle; va - vagina; ut - uterus. Scale bars: 0.5 mm (A, E); 200 μm (B); 50 μm (C, D).

Figure 4. Maximum likelihood phylogenetic tree of species of the family Prosthiostomidae based on 28 S rDNA. For clades that received either ≥ 70 % ML bootstrap or ≥ 0.80 BI posterior probability, the support values are presented at the nodes.