Dugesia patula Chen & Dong, 2025

Dugesia patula Chen & Dong sp. nov.

Habitat and reproduction.

The specimens were collected from Yulong County, at the foot of Yulong Snow Mountain, at an elevation of 1730 m. The ambient temperature was 24.8 ° C, with the water temperature being 16.4 ° C. The Jinsha Jiang river is a broad and fast-flowing course, with murky water and a bottom rich in mud and sand (Fig. 3 A View Figure 3 ). In the field, flatworms could be seen freely crawling on the surface of the mud and sand. All observed flatworms were asexual individuals. Under laboratory conditions and after two months of cultivation, the asexual worms were fissiparous, of which 8 ex-fissiparous specimens sexualized (Fig. 3 B View Figure 3 ). However, after two years of breeding in the laboratory, no cocoons were observed.

Material examined.

Holotype: ZMHNU –LMYL 4 , Yulong County, Lijiang City , Yunnan Province, China, 27°9'26"N, 99°48'25"E, 17 June 2023, coll. G-W Chen, Z-M Dong, and coworkers, sagittal sections on 28 slides GoogleMaps .

Paratypes: ZMHNU –LMYL 2 , 3, and 7 ibid., sagittal sections on 60, 49, and 27 slides, respectively GoogleMaps ; RMNH.VER.22274.1 , ibid., sagittal sections on 30 slides GoogleMaps ; RMNH.VER.22274.2 , ibid., sagittal sections on 20 slides GoogleMaps ; ZMHNU –LMYL 5 , ibid., transverse sections on 42 slides GoogleMaps ; ZMHNU –LMYL 6 , ibid., horizontal sections on 22 slides GoogleMaps .

Etymology.

The specific epithet is derived from the Latin adjective patulus, meaning “ spread out, ” “ broad, ” or “ wide open, ” and alludes to the very broad ejaculatory duct and its wide opening at the tip of the broad and blunt penis papilla.

Diagnosis.

Dugesia patula is characterized by the following features: symmetrical openings of the oviducts into the bursal canal; vasa deferentia opening symmetrically into the mid-lateral portion of the seminal vesicle; a large and pointed diaphragm; a short duct between the seminal vesicle and diaphragm; a very broad and short ejaculatory duct opening at the tip of the penis papilla.

Description.

Living sexual animals measured 13.8–18.7 mm in length and 2.1–3.3 mm in width; asexual worms measured 7.3–12.4 mm in length and 1.37–1.72 mm in width. The head is bluntly triangular and provided with two blunt auricles and two eyes located within pigment-free patches. The dorsal body surface is brown, with lighter brown body margins (Fig. 3 B View Figure 3 ); the ventral body surface is light brown.

The pharynx is located in the middle of the body, measuring approximately one-sixth of the body length (Fig. 3 B View Figure 3 ). The mouth opening is located on the ventral side at the posterior end of the pharyngeal pocket. The outer pharyngeal musculature is composed of a subepithelial layer of longitudinal muscles, followed by a thin layer of circular muscles. The inner pharyngeal musculature consists of a thick subepithelial layer of circular muscle, followed by a thin layer of longitudinal muscle.

The ovaries are located at a short distance behind the brain, i. e., between 1 / 10 th and 1 / 14 th of the distance from the brain to the root of the pharynx. The oval-shaped ovaries occupy about one-third of the dorso-ventral space. From the dorsal sides of the ovaries, the oviducts extend posteriorly along the ventral side up to the level of the gonopore, after which they curve dorso-medially to open separately and symmetrically into the ventro-lateral portion of the bursal canal, at the point where the canal communicates with the common genital atrium (Figs 4 A View Figure 4 , 5 View Figure 5 ). Cyanophil shell glands discharge their secretion into the vaginal region of the bursal canal, immediately ventrally of the oviducal openings (Figs 4 A View Figure 4 , 5 View Figure 5 ).

The large, sac-shaped copulatory bursa is located far behind the pharynx (at approximately 400–600 μm behind the pharyngeal pocket) and occupies the entire dorso-ventral space. Its length is about twice the dorso-ventral diameter of the body, i. e., measuring approximately 600–800 μm (Fig. 4 B View Figure 4 ). Furthermore, there is a considerable distance between the bursa and the penis bulb (Fig. 5 View Figure 5 ), with the consequence that the bursal canal is rather long. The bursa is lined by a stratified, columnar, vacuolated epithelium provided with basal nuclei and is devoid of any surrounding musculature (Fig. 4 B View Figure 4 ). The rather broad bursal canal arises from the mid-posterior wall of the copulatory bursa, from where it runs posteriad, curving downwards at the level of the gonopore to open into the common atrium (Figs 4 C, D View Figure 4 , 5 View Figure 5 ). The bursal canal is lined with cylindrical, nucleated, ciliated cells and surrounded by a thin, subepithelial layer of longitudinal muscles, followed by a thicker — at least on the ventral side of the canal — layer of circular muscle. There is an extra outer layer of longitudinal musculature extending from the atrium to halfway along the bursal canal, forming the ectal reinforcement.

The dorsal testes occupy about 1 / 3 of the dorso-ventral space and extend from the rear of the ovaries to the posterior end of the body; the testes house numerous mature spermatozoa.

At the posterior end of the copulatory bursa, the vasa deferentia begin to expand to form spermiducal vesicles, which are packed with sperm (Fig. 5 View Figure 5 ). At the anterior level of the penis bulb, the spermiducal vesicles diminish abruptly in diameter upon curving dorso-medially and, subsequently, penetrating the penis bulb to open symmetrically into the mid-lateral portion of the seminal vesicle (Fig. 5 View Figure 5 ). These narrow portions of the vasa deferentia are lined with cuboidal, nucleated cells and surrounded by a thick layer of circular muscles.

The penis bulb is positioned in the central portion of the body and measures almost half of the dorso-ventral space (Figs 4 C – F View Figure 4 , 5 View Figure 5 ). A large, sac-shaped seminal vesicle occupies a major portion of the penis bulb. The vesicle is lined by a flat, nucleated epithelium and is surrounded by a thick layer of irregularly crosswise arranged muscle fibers that fills a major portion of the penis bulb (Fig. 4 E, F View Figure 4 ). The postero-ventral portion of the seminal vesicle narrows to form a short duct that arises from the posterior wall of the seminal vesicle and opens into a large and pointed diaphragm (Figs 4 C – F View Figure 4 , 5 View Figure 5 ). This connecting duct is lined with a nucleated epithelium and surrounded by a thin layer of circular muscles. The diaphragm receives the abundant secretion of orange-staining penis glands (Fig. 4 C, D View Figure 4 ). The diaphragm points into a very broad and short ejaculatory duct, which forms a large cavity. The ejaculatory cavity lined with a cuboidal, infranucleated epithelium and is devoid of any well-discernible musculature. The ejaculatory cavity immediately gives rise to a very broad opening at the blunt tip of the penis papilla (Figs 4 C – F View Figure 4 , 5 View Figure 5 ).

The penis papilla is a barrel-shaped structure, covered with a nucleated epithelium, which is underlain by a subepithelial layer of circular muscle, followed by a layer of longitudinal muscle fibers. A second type of erythrophilic penis glands discharge their sections through the blunt tip of the papilla. The glandular elements of this second type of penial gland lie far outside of the penis bulb (Figs 4 C, D, F View Figure 4 , 5 View Figure 5 ) and produce a more finely granular secretion than those opening into the diaphragm.

The rather spacious male atrium narrows to form a short canal that opens into the common atrium, the latter leading to a gonoduct, which opens through the ventral gonopore; the gonoduct is lined by a tall, columnar, nucleated epithelium and receives the openings of abundant, erythrophilic cement glands (Figs 4 C – F View Figure 4 , 5 View Figure 5 ).

In several specimens, a sclerotic spermatophore is present in the ejaculatory duct or in the copulatory bursa (Fig. 4 B, D View Figure 4 ).

Discussion.

The presence of an extremely wide-mouthed opening of the ejaculatory duct at the tip of the penis papilla sets Dugesia patula immediately apart from its congeners, as for none of the presently known species has such a wide opening been reported. There are only two species that have relatively wide openings of their ejaculatory ducts, albeit considerably less wide than in D. patula , viz., D. afromontana Stocchino & Sluys, 2012 and D. musculosa Chen & Dong, 2024 ( Stocchino et al. 2012; Wang et al. 2024). However, in other characters, D. patula is rather different from either of these two congeners. For example, in D. afromontana the oviducts open asymmetrically into the bursal canal, the seminal vesicle is elongated, and the diaphragm is stubby, contrasting with conditions in D. patula . The latter is here reported from Yunnan Province, while D. musculosa is known from an adjacent province in China, Guangxi Province. Despite this geographic proximity, there are considerable anatomical differences between both species. Notably, D. musculosa exhibits (a) a highly muscular bursal canal, (b) a large, rounded seminal vesicle that receives the asymmetrical openings of the vasa deferentia, and (c) a stubby diaphragm, features that are all absent in D. patula . In D. musculosa the copulatory bursa is also rather large, as in D. patula , but in contrast to the latter, its bursa is situated directly in front of the penis bulb. It is noteworthy that a large copulatory bursa is present also in D. bursagrossa Harrath & Sluys, 2025 , from Saudi Arabia. But in this species the bursa is extremely large, while it is also located immediately anterior to the penis bulb (cf. Harrath et al. 2025, Fig. 7 View Figure 7 ). Furthermore, in D. bursagrossa the ejaculatory duct opens subterminally at the penis papilla, contrasting with the wide, terminal opening in D. patula .

The available molecular data also support the conclusion that D. patula is different from D. afromontana and D. musculosa (Fig. 2 View Figure 2 , Suppl. materials 2, 3).

Another noteworthy feature of D. patula concerns its large and pointed diaphragm. Until recently, such a pointed diaphragm was known only from Dugesia species from the Western Palearctic region: D. cretica (Meixner, 1928) , D. gonocephala (Dugès, 1830) , D. liguriensis De Vries, 1988 , D. minotauros De Vries, 1984 , D. praecaucasica , D. transcaucasica , and D. leporii Pala, Stocchino, Corso & Casu, 2000 ( Stocchino et al. 2017). However, it turned out to be present also in D. crassimentula Sluys & Stocchino, 2024 , and D. insolita Stocchino & Sluys, 2024 , from Madagascar ( Stocchino et al. 2024), albeit in the last-mentioned species, the diaphragm has taken over the function of the penis papilla. Thus, D. patula is the first species from the Oriental Region for which a pointed diaphragm is reported.