Dugesia aconcinna Chen & Dong, 2025

Dugesia aconcinna Chen & Dong sp. nov.

Figs 1 View Figure 1 , 2 View Figure 2 , 7 View Figure 7 , 8 View Figure 8 , 9 View Figure 9

Collection site, habitat, and reproduction.

On 4 January 2019, the specimens were collected from a stream in the Qingfeng village, Leizhou Peninsula (Figs 1 View Figure 1 , 7 A, B View Figure 7 ), which is a volcanic spring at an altitude of 56 m a. s. l.; air temperature was 22 ° C and water temperature was 19 ° C. With respect to the D. aconcinna population, six mature worms and five asexual worms were collected in the field. After ~ 5 months under laboratory conditions, all of the immature worms sexualised, although none of the worms (sexual in the field and the sexualised ones in the laboratory) produced any cocoons.

Material examined.

Holotype • ZMHNU -TPYC 5 , Qingfeng village (21°14'33"N, 110°9'49"E), alt. 56 m a. s. l., Suixi County, Guangdong Province, China, 4 January 2019, coll. Z-M Dong, L Wang and J-Z Chen, sagittal sections on 14 slides GoogleMaps . Paratypes • RMNH.VER.22250.1 , ibid., sagittal sections on 11 slides GoogleMaps • RMNH.VER.22250.2 , ibid., sagittal sections on 10 slides GoogleMaps • ZMHNU -TPYC 1 -3, 6, 7, 11, ibid., sagittal sections on 14, 29, 15, 5, 21, 14 slides GoogleMaps • ZMHNU -TPYC 9 , ibid., horizontal sections on 23 slides GoogleMaps • ZMHNU -TPYC 8 , ibid., transverse sections on 19 slides GoogleMaps .

Diagnosis.

Dugesia aconcinna is characterised by the presence of the following features: live, mature animals rather small; asymmetrical openings of the oviducts into the common atrium; vasa deferentia separately opening into the posterior portion of the seminal vesicle; penis papilla of a very characteristic shape, with the part housing the connecting duct, diaphragm, and ejaculatory duct being a cylindrical structure with a blunt tip, while at its right-hand side sits a large penial fold that attaches to the base of the penis papilla; ejaculatory duct following a ventrally displaced course through the penis papilla, after which it opens at the tip of the papilla; presence of a duct between diaphragm and seminal vesicle.

Description.

Body of both asexual and sexual live specimens is quite small, with the sexual worms being only 6–9 mm in length and 1.0– 1.2 mm in width. The low-triangular head is provided with two blunt auricles and two eyes, which are placed in pigment-free spots. Each pigmented eyecup houses numerous photoreceptor cells. The dorsal surface is yellow-brown, with many brown pigment granules and pale blotches all over the dorsal surface; the ventral surface is paler than the dorsal body surface (Fig. 7 C View Figure 7 ).

Pharynx situated in the mid-region of the body, measuring ~ 1 / 6 of the body length. Mouth opening located at the posterior end of the pharyngeal pocket. Outer pharyngeal musculature is composed of a thin, subepithelial layer of longitudinal muscles, followed by a thin layer of circular muscles; no extra inner layer of longitudinal muscles was observed. The inner pharyngeal musculature consists of a thick, subepithelial layer of circular muscle, followed by a thin layer of longitudinal muscle.

In those specimens in which we were able to examine the ovaries, most of the gonads were not hyperplasic (specimens TPYC 3, 5, 7, 8, and RMNH.VER.22250.1 ), excepting specimens TPYC 6, and 11, and RMNH.VER.22250.2 . In general, the oval ovaries are situated at 1 / 3–1 / 4 of the distance between the brain and the root of the pharynx, occupying ~ 1 / 4 of the dorso-ventral space. The oviducts arise from the dorsal wall of the ovaries, then turn to the ventral side and run in a caudal direction to the level of the genital pore. Subsequently, the left oviduct bends dorsally to open into the common atrium, while the right oviduct exhibits a much more pronounced curvature towards the dorsal body surface, after which it recurves in antero-ventral direction to open into the most ventral, proximal portion of the bursal canal, at the point where the latter communicates with the common atrium (Figs 8 A View Figure 8 , 9 A View Figure 9 ). Thus, the right oviducal branch opens dorsally to the left one. The oviducts are lined with a columnar, infranucleated epithelium.

The sac-shaped copulatory bursa lies immediately behind the pharyngeal pocket and may occupy the entire dorso-ventral space or, at least, a considerable portion of it. The bursa is lined with a columnar, vacuolated epithelium with basal nuclei and is devoid of any surrounding musculature (Figs 8 B – D, F View Figure 8 , 9 View Figure 9 ). Near its communication with the postero-dorsal section of the bursa, the bursal canal is rather narrow and may occupy ~ 1 / 8 of the dorso-ventral space (Figs 8 F View Figure 8 , 9 A View Figure 9 ). From thereon, the canal expands somewhat in diameter while it runs in a caudal direction to the left side of the male copulatory apparatus. At the level of the gonopore, the posterior section of the canal exhibits a rather abrupt, ventrally directed bend, after which it opens into the common atrium (Fig. 9 A View Figure 9 ). The bursal canal is lined with cylindrical, infranucleated, ciliated cells and is surrounded by a subepithelial layer of longitudinal muscles, followed by a layer of circular muscle; an extra outer layer of longitudinal musculature, forming the ectal reinforcement, extends from the copulatory bursa to the atrium. Erythrophil shell glands open into the vaginal region of the bursal canal, near the oviducal openings.

The numerous, well-developed testes are situated dorsally and provided with mature spermatozoa. On either side of the midline of the body, testicular follicles are arranged in eight or nine longitudinal zones and extend from the posterior level of the ovaries to almost the posterior end of the body.

At the level of the pharyngeal pocket, the vasa deferentia expand to form spermiducal vesicles, which are packed with mature spermatozoa (Fig. 9 View Figure 9 ). Upon reaching the large penis bulb, the vasa deferentia turn dorso-medially and quickly decrease very much in diameter while penetrating the wall of the bulb. Subsequently, the sperm ducts open separately and symmetrically into the mid-posterior section of the seminal vesicle, near the point where it communicates with the connecting duct that leads to the diaphragm. The sperm ducts are lined with a nucleated epithelium and surrounded by a layer of circular muscle.

The voluminous, oval seminal vesicle is lined by a flat, nucleated epithelium and is surrounded by a layer of intermingled muscle fibres. The seminal vesicle occupies ~ 2 / 5 of the dorso-ventral space and is located in the ventral portion of the penis bulb, close to the ventral body surface (Figs 8 B – D View Figure 8 , 9 View Figure 9 ). Although the penis bulb is rather shallow, it is, nevertheless, a large structure, occupying almost the entire dorso-ventral space (Figs 8 B – D View Figure 8 , 9 View Figure 9 ). A relatively long and broad duct connects the seminal vesicle with a small diaphragm, the latter leading to the ejaculatory duct (Figs 8 C, D View Figure 8 , 9 View Figure 9 ). This interconnecting duct is lined by an infranucleated epithelium and is surrounded by a layer of intermingled muscle fibres. The small diaphragm is located at the level of the root of the penis papilla and receives the abundant secretion of erythrophil penis glands (Figs 8 C, D View Figure 8 , 9 View Figure 9 ). Both the connecting duct and the ejaculatory duct run a ventrally displaced course through the penis papilla, with the relatively broad ejaculatory duct opening at the tip of the papilla (Figs 8 D, F View Figure 8 , 9 View Figure 9 ). The ejaculatory duct is lined with a cuboidal, infranucleated epithelium and is devoid of any surrounding musculature.

The penis papilla has a very characteristic shape. The part that houses the connecting duct, diaphragm, and ejaculatory duct is a cylindrical structure with a blunt tip. This seems to be a rather symmetrical portion of the papilla but it should be noted that it concerns a lateral, left-hand part of the penis papilla. The right-hand part of the papilla develops a large penial fold (Figs 8 E View Figure 8 , 9 B View Figure 9 ) that attaches to the base of the penis papilla. Penis papilla and penial fold are covered with a nucleated epithelium, which is underlain by a subepithelial layer of circular muscle, followed by a layer of longitudinal muscle fibres (Figs 8 B – F View Figure 8 , 9 View Figure 9 ).

The genital atrium is divided into a common atrium and male atrium. The common atrium communicates with a gonoduct, which leads to the ventral gonopore; the gonoduct is lined by a columnar epithelium and receives the openings of cement glands (Fig. 9 View Figure 9 ).

Etymology.

The specific epithet is derived from the Latin adjective aconcinna , asymmetrical, and alludes to the asymmetrical penial fold as well as the asymmetrical oviducal openings into the bursal canal.

Discussion.

A good number of Dugesia species possesses so-called penial annexes in the form of penial folds, which sometimes were indicated by the term adenodactyl. However, the term adenodactyl should not be applied to these penial annexes ( Stocchino et al. 2017). Penial folds are located at the base of the penial papilla and are usually covered by the musculature of the penis bulb; folds may be of the parenchymatic-muscular type or of the parenchymatic-musculo-glandular type ( Stocchino et al. 2017). Furthermore, penial folds may be located at both the ventral and dorsal side of the penis papilla, albeit the ventral fold may be smaller than the dorsal one, or a single fold may be restricted to the dorsal, dorso-lateral, or lateral portion of the papilla. Such a single fold is present in ~ 20 species of Dugesia . For the present comparative discussion, it suffices to concentrate on those species that exhibit a dorso-lateral or lateral penial fold more or less comparable to that of D. aconcinna , viz., D. arcadia de Vries, 1988 , D. benazzii Lepori, 1951 , D. golanica Bromley & Benazzi, 1991 , D. hoidi Dols-Serrate, Stocchino & Riutort, 2024 , D. iranica Livanov, 1951 , D. libanica Bromley & Benazzi, 1991 , D. mariae Stocchino, Dols-Serrate & Riutort, 2023 , D. minotauros de Vries, 1984 . However, all of these species differ from D. aconcinna in the absence of a connecting duct between the seminal vesicle and the diaphragm, perhaps excepting D. izuensis Katô, 1943 (cf. Sluys et al. 1998: table II; Dols-Serrate et al. 2024). For D. izuensis a diaphragm was not described but it is presumed that the abundant openings of eosinophilic penial glands approximately halfway into the ejaculatory duct ( Kato 1950; Kawakatsu 1983) coincides with the location of the diaphragm, which is presumably very small. However, in other aspects D. izuensis is rather different from D. aconcinna . For example, in D. izuensis the penis papilla is a massive and plump structure, whereas the papilla in D. aconcinna is cylindrical. Other differences concern the penial fold. In D. izuensis the fold has the shape of a conical papilla with the central part filled with cyanophilic secretion, in contrast to the flap-like fold of D. aconcinna that lacks any secretions.

With respect to the shape and position of its penial fold, D. aconcinna resembles D. benazzii , D. hoidi , and D. mariae , all of which possess a flap-like penial fold that extends dorso-laterally of the penis papilla, which holds true also for D. minotauros ( de Vries 1984; Dols-Serrate et al. 2024). However, in all of these species the fold is situated on the left side of the penis papilla, in contrast to D. aconcinna in which the fold extends over the right side of the papilla. But there are also other differences. In D. benazzii and D. hoidi , the two vasa deferentia follow highly asymmetrical trajectories before opening, equally asymmetrically, into the seminal vesicle. In contrast, in both D. mariae and D. aconcinna the sperm ducts follow symmetrical trajectories. In D. iranica the penial fold sits also on the left side of the penis papilla, albeit it is not a flap-like fold, but a conical structure of variable size, which sometimes may be as large as the penis papilla; it is of the musculo-glandular type. All of this is different from the situation in D. aconcinna . Similarly to D. aconcinna , D. libanica possesses also a relatively long, cylindrical penis papilla with a blunt tip, with at its right-hand side a well-developed penial fold. In contrast to D. aconcinna , the fold of D. libanica is not flap-like but consists of a pear-shaped papilla, located dorsally to the right of the penis papilla, that may reach a length of ~ 3 / 4 of the penial papilla ( Bromley and Benazzii 1991). Notably, none of above-mentioned species belongs to the same clade as D. aconcinna . Although, these species ( D. adunca , D. ancoraria , D. bengalensis , and D. notogaea ) are closely related molecularly, they can be easily distinguished from D. aconcinna by anatomical features.