Dugesia circumcisa Chen & Dong, 2021

Wang, Lei, Chen, Jin-zi, Dong, Zi-mei, Chen, Guang-wen, Sluys, Ronald & Liu, De-zeng, 2021, Two new species of Dugesia (Platyhelminthes, Tricladida, Dugesiidae) from the tropical monsoon forest in southern China, ZooKeys 1059, pp. 89-116 : 89

publication ID	https://dx.doi.org/10.3897/zookeys.1059.65633
publication LSID	lsid:zoobank.org:pub:731F8C31-26B3-4F81-8FE8-3595E360BC58
persistent identifier	https://treatment.plazi.org/id/292AFA17-03F6-4153-BC1D-61F0212F207A
taxon LSID	lsid:zoobank.org:act:292AFA17-03F6-4153-BC1D-61F0212F207A
treatment provided by	ZooKeys by Pensoft
scientific name	Dugesia circumcisa Chen & Dong
status	sp. nov.

Treatment

Dugesia circumcisa Chen & Dong sp. nov.

Material examined.

Holotype. ZMHNU-YWSZ2, Shiwan Dashan Mountain National Natural Reserve (21°54'34"N, 107°54'52"E), Shangsi County, Guangxi Province, China, alt. 245 m above sea level (a.s.l.), 1 January 2019, coll. G-W Chen, D-Z Dong and co-workers, sagittal sections on 14 slides GoogleMaps . Paratypes. ZMHNU-YWSZ1, ibid., sagittal sections on 28 slides; RMNH VER. 19974.a, ibid., sagittal sections on 26 slides; ZMHNU-YWSZ5, ibid., horizontal sections on 15 slides; ZMHNU-YWSZ6, ibid., transverse sections on 30 slides; ZMHNU-YWSZ8, ibid., sagittal sections on 19 slides; ZMHNU-YWSZ9, ibid., sagittal sections on 15 slides; RMNH VER. 19974.b, ibid., sagittal sections on 21 slides; ZMHNU-YWSZ11, ibid., transverse sections on 60 slides; ZMHNU-YWSZ12, ibid., horizontal sections on 19 slides.

Diagnosis.

Dugesia circumcisa is characterised by the presence of the following features: right vas deferens opening at anterior portion of the seminal vesicle, and the left one opening at lateral portion of the vesicle, with the left sperm duct opening dorsally to the right one; two diaphragms in ejaculatory duct, the distal one receiving secretion of penial glands; ejaculatory duct with ventral course through penis papilla and with terminal opening; small nozzle at tip of penis papilla; wide duct connecting male and common atrium; asymmetrical openings of the oviducts into the bursal canal; chromosome complement triploid, with 24 metacentric chromosomes.

Etymology.

The specific epithet is derived from the Latin adjective circumcisus, 'pruned of excess, sheared on all sides’, and alludes to the appearance of the tip of the penis papilla.

Habitat and reproduction.

Approximately 20 animals were collected from a freshwater stream on the Shiwan Dashan Mountain (Fig. 3A, B View Figure 3 ) at an altitude of 245 m a.s.l. (air temperature 8.6 °C, water temperature 12 °C). The Shiwan Dashan Mountain lies in the tropical monsoon forest, which forms a discontinuous system in tropical areas of Asia, Africa, and South America. The most typical tropical monsoon rainforests are distributed in Southeast Asia, with those of mainland China being located mainly near the Tropic of Cancer, thus including parts of the provinces of Hainan, Guangdong, Guangxi, Yunnan, and Tibet. Within this monsoon rainforest of mainland China, Shiwan Dashan Mountain forms its southernmost part. None of the animals was sexually mature at collection. During the first period of 150 days (January to May) in the laboratory culture, the worms only showed asexual reproduction by means of fission. However, during the following days, seven individuals sexualised, while eventually 3/5 of the animals sexualised, although thus far they have not produced any cocoons.

Karyology.

Each of the five, randomly selected specimens exhibited triploid chromosome complements. In a total of 100 metaphase plates examined, 86 chromosome complements were triploid with 2n = 3 × = 24 chromosomes, with all chromosomes being metacentric (Fig. 4 View Figure 4 ); chromosome complements of the remaining 14 plates could not be determined, due to either lack of well-dispersed chromosomes or over-dispersed sets of chromosomes. Karyotype parameters, including relative length, arm ratio, and centromeric index, are given in Table 2 View Table 2 . The first pair of chromosomes is clearly larger than the others, being 2.17 times larger than the shortest chromosome.

Description.

In asexual living specimens, body 8-15 mm in length and 0.8-1.5 mm in width, while in sexualised specimens the body measures 15-22 mm in length and 1.5-2.3 mm in width. Head of low triangular shape and provided with two auricles, as well as two eyes located in pigment-free patches (Fig. 3C View Figure 3 ). Dorsal surface dark brown, excepting pale body margin and a fuzzy, pale mid-dorsal stripe; accumulations of pigment follow the outline of the pharyngeal pocket. Ventral surface light brown.

Pharynx situated in the mid-region of the body, measuring ca. 1/5th of the body length (Fig. 3C View Figure 3 ); mouth opening located at the posterior end of the pharyngeal pocket. Outer pharyngeal musculature composed of a subepidermal layer of longitudinal muscles, followed by a layer of circular muscles (Fig. 5A View Figure 5 ). Inner pharyngeal musculature composed of a thick subepithelial layer of circular muscles, followed by a thin layer of longitudinal muscles (Fig. 5A View Figure 5 ).

The ventral ovaries are located at a short distance behind the brain and dorso-medially to the ventral nerve cords. The development of the ovaries differs greatly between specimens. In some animals the ovaries are rather small or even poorly developed (Fig. 5B View Figure 5 ), while in others the gonads are clearly hyperplasic (Fig. 5C, D View Figure 5 ). The oviducts arise from the dorsal surface of the ovaries and run dorsally to the ventral nerve cords in a caudal direction. At approximately the level of the gonopore, the oviducts curve dorsad to open separately and asymmetrically into the vaginal portion of the bursal canal, with the right oviduct opening dorsally to the left one (Figs 5E View Figure 5 , 7 View Figure 7 , 8 View Figure 8 ).

The large sac-shaped copulatory bursa, which occupies the entire dorso-ventral space, is lined by a vacuolated epithelium with basal nuclei and is almost devoid of any surrounding musculature (Figs 6A View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ). From the postero-dorsal wall of the bursa, the rather narrow bursal canal runs in a caudal direction to the left side of the copulatory apparatus, after which it curves ventrally and opens into the common atrium (Figs 6A View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ). The bursal canal is lined with columnar, nucleated, ciliated cells and surrounded by a thin subepithelial layer of longitudinal muscles, followed by a slightly thicker layer of circular muscle (Fig. 5F View Figure 5 ). An ectal reinforcement layer of longitudinal muscles runs from the vaginal region to ca. halfway along the bursal canal. Shell glands discharge their erythrophil secretion into the vaginal region of the bursal canal, near the oviducal openings.

The small, dorsally located testes are poorly developed and provided with immature spermatozoa (Fig. 5C View Figure 5 ). Testicular follicles are arranged on either side of the midline of the body in three or four longitudinal zones, extending from the posterior level of the ovaries to almost the posterior end of the body. Spermatozoa are absent also from the vasa deferentia, which upon reaching the level of the penis bulb curve dorso-mediad and asymmetrically penetrate the wall of the penis bulb. The right sperm duct penetrates the antero-lateral wall of the penis bulb and opens into the anterior portion of the seminal vesicle (Figs 6B, D View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ). The left sperm duct penetrates the lateral wall of the penis bulb and opens through the mid-lateral wall of the seminal vesicle (Figs 6C View Figure 6 , 7 View Figure 7 ). Furthermore, the left sperm duct opens dorsally to the right one. The sperm ducts are lined with nucleated cells and are surrounded by a layer of circular muscles.

The large and well-developed penis bulb occupies the major part of the dorso-ventral space and is composed of intermingled muscle fibres (Figs 6B-F View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ). The penis bulb is asymmetrical in that a major portion expands dorsally to well beyond the midline of the body or even almost extends to the dorsal epidermis. This dorsal portion houses the oblate seminal vesicle, which is lined with a flat, nucleated epithelium. Via a valve-like diaphragm in its ventral wall, the seminal vesicle communicates with an expansion or vesicle in the proximal portion of the ejaculatory duct, which has a more or less vertical orientation. Subsequently, this expansion communicates via a large, blunt, valve-like diaphragm with a much larger expansion of the ejaculatory duct (Figs 6B View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ), which is lined by a nucleated epithelium. The second diaphragm receives the openings of erythrophil penis glands, in contrast to the proximal, first diaphragm, which does not receive any secretion.

From the point of the large expansion, the ejaculatory duct changes its vertical orientation and starts to run more or less parallel to the body surface or attains an oblique, ventro-caudal orientation, thus basically conforming to the particular orientation of the penis papilla. The narrow section of the ejaculatory duct that runs between the large expansion and the tip of the penis papilla is lined by an infranucleated epithelium and follows a ventral course through the papilla, opening terminally at its tip (Figs 6F-H View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ). In point of fact, the ejaculatory duct opens at a small nozzle located at the otherwise blunt tip of the penis papilla (Figs 6F, G View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ). This nozzle can either be extended (Figs 6G View Figure 6 , 8 View Figure 8 ) or be withdrawn to greater or lesser extent (Fig. 6H View Figure 6 ).

The asymmetrical penis papilla is covered by an infranucleated epithelium, which is underlain by a subepithelial layer of circular muscle, followed by a layer of longitudinal muscle fibres. The penis papilla almost completely occupies the small male atrium, the latter communicating with the common atrium via a wide duct (Figs 6G View Figure 6 , 7 View Figure 7 , 8 View Figure 8 ). The common atrium opens to the exterior via a gonoduct, which is lined by a columnar epithelium and receives the openings of abundant cement glands.

Discussion.

Generally, there is only one diaphragm present in the ejaculatory duct of species of Dugesia , and only a few species exhibit two diaphragms, such as D. bijuga Harrath & Sluys, 2019, D. machadoi de Beauchamp, 1952, D. mirabilis de Vries, 1988, D. maghrebiana Stocchino et al., 2009, D. didiaphragma de Vries, 1988, and D. semiglobosa ( De Vries 1988; Stocchino et al. 2009; Harrath et al. 2019; Wang et al. 2020). However, in the three first-mentioned species, the ejaculatory duct runs a central course through the penis papilla, in contrast to the ventral trajectories in D. semiglobosa , D. maghrebiana , D. didiaphragma , and the new species D. circumcisa . The copulatory bursa of the latter lacks the complex stratified epithelium, which projects through an opening of the bursa that is present in D. semiglobosa , while it also lacks the large seminal vesicle enclosed by a highly muscularised, elongated penis bulb as present in D. didiaphragma and the knob-like extension on the penis papilla of D. maghrebiana .

In species with two diaphragms, the proximal diaphragm is usually small and basically formed by a non-glandular constriction of the seminal vesicle, while the true diaphragm is a larger structure that receives the secretion of penial glands, as is usual for the diaphragm in species of Dugesia . The situation in D. circumcisa is slightly different in that the proximal diaphragm is not small but consists of a well-developed valve.

Generally, in species of Dugesia the openings of the left and right sperm ducts into the intrabulbar seminal vesicle are located at ca. the same level. However, in D. circumcisa the vasa deferentia open asymmetrically into the seminal vesicle. Such asymmetrical openings have been reported explicitly for D. bifida Stocchino & Sluys, 2014, in which the sperm ducts open halfway into the vesicle, with the right duct opening dorsally to the left one. This contrasts with the situation in D. circumcisa , in which the right sperm duct opens into the anterior portion of the seminal vesicle and the left duct opens through the mid-lateral wall of the seminal vesicle, with the left sperm duct opening dorsally to the right one.

The characteristic nozzle at the tip of the penis papilla in D. circumcisa is paralleled in D. bakurianica Porfirjeva, 1958, D. bijuga , and perhaps also in D. sinensis . From that perspective, it is interesting that D. bijuga also possesses two diaphragms (see above). However, other parts of the male copulatory apparatus of D. circumcisa are different from that of D. bijuga (e.g., glands opening at a major portion of the blunt penis papilla, as well as atrial folds in the latter species). Furthermore, in the phylogenetric tree, D. circumcisa is far removed from D. bijuga (Fig. 2 View Figure 2 ). Dugesia circumcisa differs from D. sinensis in the presence of atrial folds in the latter, while it differs from D. bakurianica in that this species only has a single diaphragm.

Another characteristic feature of D. circumcisa is the rather wide and long duct connecting the male atrium with the common atrium. Generally, in species of Dugesia the male atrium opens more or less directly into the common atrium, without interpolation of a well-defined duct. Apart from D. circumcisa , other exceptions to this ground-plan condition can be found in D. bactriana de Beauchamp, 1959, D. bengalensis Kawakatsu, 1983, D. bifida , D. capensis Sluys, 2007, and D. colapha Dahm, 1967. However, other characters prevent synonymisation of D. circumcisa with any of these species, which in the case of D. bifida is supported also by completely different positions in the phylogenetic tree (Fig. 2 View Figure 2 ).