Polycelis eudendrocoeloides ( Zabusova, 1929 )

Sun, Xin-Xin, Wang, Lei, Li, Ning, Sluys, Ronald, Liu, De-Zeng, Dong, Zi-Mei & Chen, Guang-Wen, 2024, Two species of Polycelis (Platyhelminthes, Tricladida, Planariidae) newly recorded for the Qinling Mountains and the Loess Plateau in China, with a comparative discussion on their karyotypes, Zootaxa 5536 (4), pp. 581-599 : 590-596

Polycelis eudendrocoeloides ( Zabusova, 1929)

Material examined. ZMHNU-JX8 , Wang Jiabian Town (38°14’50”N 110°14’24’’E; alt. 988 m a.s.l.), Jiaxian County, Shaanxi Province, China, 2 August 2017, coll. G-W. Chen, Z-M. Dong, Y-Q. Yu-wen, and co-workers, sagittal sections on 28 slides ; ZMHNU-JX2–7 , ibid., sagittal sections on 13, 21, 9, 35, 9 and 9 slides, respectively ; ZMHNU-JX9 , ibid., horizontal sections on 19 slides ; ZMHNU-JX10 , ibid., transverse sections on 34 slides ; RMNH. VER.21596.1 , ibid., sagittal sections on 12 slides ; RMNH.VER.21596.2 , ibid., sagittal sections on 22 slides .

Habitat and reproduction. All animals were collected from a stream on the Loess Plateau ( Figs 1 , 7A, B ) in northern China at 988 m a.s.l. (air temperature 5 ℃, water temperature 15 ℃). Many animals were sexually mature at collection; however, no cocoons were collected. After 90 days under laboratory conditions, the sexual animals laid cocoons. Newly laid cocoons were yellow, but turned dark brown after 2 to 3 days. Cocoons were spherical in shape (<1 mm in diameter) and without a stalk. Since no young worms hatched from any of the cocoons, these were likely infertile. This population failed to adapt to the laboratory environment, and eventually there were no surviving individuals.

Karyology. Each of the five randomly selected specimens exhibited diploid chromosome complements. From a total of 109 metaphase plates examined, 90 chromosome complements were diploid with 2n = 2x = 36 ( Fig. 8 ), while the chromosome complements of the remaining 19 plates failed to be determined, due to either lack of well-dispersed chromosomes or over-dispersed sets of chromosomes. All chromosomes were metacentric. Karyotype parameters, including relative length, arm ratio, and centromeric index, are shown in Table 2 .

Description. In living sexual specimens, the body measured 8–12 mm in length and 4.3–4.7 mm in width. The frontal margin of the body is slightly convex, and provided with a pair of bluntly pointed, laterally projecting auricles. Dorsal surface brownish, and the ventral surface is paler than the dorsal one, being slightly light brown ( Fig. 7C ). About 49–80 small eyes are arranged into two bands that run from the frontal margin of the body along the lateral margins to a short distance posterior to the auricles ( Fig. 7C ).

The pharynx, situated in the posterior half of the body, measures about 1/3–1/4 th of the body length; the mouth opening is located at the posterior end of the pharyngeal pocket ( Fig. 7C ). Outer pharyngeal musculature is composed of a subepithelial layer of longitudinal muscles, followed by a layer of circular muscles. Inner pharyngeal musculature is composed of a thick subepithelial layer of circular muscles, followed by a thin layer of longitudinal muscles ( Fig. 9A ).

The ventral, oval-shaped ovaries are located at 1/3–1/4 of the distance between the brain and the root of the pharynx, occupying about 1/3 of the dorso-ventral space ( Fig. 9B ). The oviducts arise from the posterior wall of the ovaries and follow a ventral course in caudal direction. At the level of the male atrium, the two oviducts curve towards the dorsal body surface and dorsally to the atrium they unite to form a common oviduct, which opens into the dorsal portion of a very small and/or indistinct common atrium ( Figs. 9C, D , 10C , 11 ).

The large, sac-shaped copulatory bursa occupies the entire dorso-ventral space and is lined by a vacuolated epithelium with basal nuclei ( Figs. 9E , 10E ). From the postero-dorsal wall of the copulatory bursa, the fairly long bursal canal runs in a caudal direction to the level of the gonopore and then curves ventrally to communicate with the common atrium and the gonopore ( Figs. 10C, E , 11 ). The bursal canal is lined with cylindrical, ciliated cells with basal nuclei and is surrounded by a subepithelial layer of circular muscles, followed by a thin layer of longitudinal muscles ( Fig. 9C ).

The tall, ventrally located testes occupy almost the entire dorso-ventral space ( Fig. 9F ). In fully mature individuals, there are about 20–30 testicular follicles extending from the level of the ovaries to the root of the pharynx. Mature spermatozoa are present in the testes and the vasa deferentia. The latter form spermiducal vesicles at the posterior level of the pharyngeal pocket in most specimens (RMNH.VER.21596.1, ZMHNU-JX2, 3, 4, 6, 7, 10, RMNH.VER.21596.2), except specimens ZMHNU-JX5, 8, and 9 ( Fig. 10A ). At the level of the penis bulb, the vasa deferentia bend towards the dorsal body surface and then penetrate the antero-lateral or lateral wall of the penis bulb to open separately and symmetrically into the mid-lateral portion of the seminal vesicle. ( Figs. 10B , 11 ).

The large and well-developed penis bulb generally occupies most of the dorso-ventral space and is composed of intermingled muscle fibres. The intrabulbar seminal vesicle is very large in several specimens, except specimen ZMHNU-JX8, and then extends from near the ventral body surface to occupy most of both the penis bulb and the dorso-ventral space of the body ( Figs. 10C–E , 11 ). The seminal vesicle is lined by a flat, nucleated epithelium and is surrounded by a layer of irregularly crosswise arranged muscle fibres. Broad bands of erythrophil, granular secretion anterior to the penis bulb have their glandular ducts opening into the seminal vesicle. The secretion is present in the seminal vesicle in all specimens.

The penis papilla is a very short and blunt cone that projects into the seminal vesicle by means of an equally stubby kind of flagellum ( Figs. 10C–E , 11 ). The papilla is covered with an infranucleated epithelium, which is underlain with a subepithelial layer of circular muscles, followed by a layer of longitudinal muscle fibers.

The elongated male atrium is lined with a tall, papillose epithelium. It is surrounded by a thick subepithelial layer of circular muscles, followed by a thin layer of longitudinal muscles. The male atrium communicates with a very small common atrium, which is not always well discernible ( Figs. 10C, E, F , 11 ).

Discussion. The anatomy of the copulatory apparatus of our specimens from the Loess Plateau closely resembles that described by Zabusova (1929) for Polycelis eudendrocoeloides from Kamchatka. It is immediately obvious that both the type specimens and the Chinese animals exhibit a feature that is unique among species of Polycelis , viz., a highly reduced and blunt penis that is provided with a kind of flagellum that projects into the seminal vesicle. Although Zabusova (1929) also used the term flagellum and pointed to the resemblance with species of Dendrocoelum , we did not find any indication in our specimens that this structure could be evaginated, as is the case in species of the last-mentioned genus.

Another noteworthy feature comparable between the type material of P. eudendrocoeloides and our specimens concerns the musculature of the male atrium. Zabusova (1929) mentioned that in the specimens from Kamchatka particularly the posterior portion of the male atrium was surrounded by a well-developed coat of circular and longitudinal muscles. Our Chinese specimens exhibited a relatively thick coat of circular and longitudinal muscles around the entire male atrium. Although this coat of muscles is well-developed, it does not yet reach the dimensions of the muscular coat surrounding the male atrium in species of Seidlia . Therefore, we agree with the classical taxonomic view that P. eudendrocoeloides belongs to the genus Polycelis s.s.

The dorsal surface of the animals from Kamchatka was described as pale yellow-brown. In the Chinese specimens, the dorsal surface is brownish, while the ventral surface is paler than the dorsal one, slightly light brown.

The number of eyes in our Chinese animals ranged between 49–80 on either side of the head. Zabusova (1929) mentioned a considerably lower number for the type specimens, viz. 9–12 eyes on either side of the head. However, as already mentioned above, eye number in Polycelis is variable and generally increases during growth of an animal. The preserved specimens examined by Zabusova (1929) were about half the size of the live Chinese animals that we had at our disposal. This may, perhaps, signal that Zabusova’s animals were at a less advanced stage in their growth.

Our new finds of P. eudendrocoeloides are taxonomically and nomenclaturally interesting for several reasons. First of all, it falsifies Kenk’s (1974) conclusion that P. eudendrocoeloides would be a species inquirenda. The reasons for such a conclusion are difficult to apprehend, in view of the rather good description provided by Zabusova (1929), in contrast to Kenk’s (1953) evaluation of it, while our new findings clearly support the species status of P. eudendrocoeloides .

Second, although Kenk (1974) considered P. eudendrocoeloides to be a species inquirenda, he did also designate it as the type species of the genus Sorocelides Zabusova, 1929 , albeit he placed the latter under Polycelis s.l. This taxonomic placement is supported by our examination of the new, Chinese specimens of P. eudendrocoeloides .

Third, according to Dyganova & Porfirjeva (1990) and Kawakatsu & Mitchell (1998), P. eudendrocoeloides would be a junior synonym of Seidlia elongata ( Zabusova, 1929) . Although the specific epithet eudendrocoeloides has page precedence (which does not imply priority), the act of Dyganova & Porfirjeva (1990) may be interpreted as being in accordance with the First Revisor principle ( ICZN 1999, art. 24.2), so that the name elongata has priority over eudendrocoeloides . However, it is doubtful that the names are synonyms, as there are clear differences between the two species.

It is true that both species possess a very blunt penis papilla, which is provided with a kind of flagellum that projects into the seminal vesicle, a similarity that was already pointed out by Zabusova-Zhdanova (1956). The latter doubted that this flagellum would be an eversible structure. However, Dyganova & Porfirjeva (1990) found that in S. elongata this flagellum-like structure that usually projects into the seminal vesicle, on occasions sits at the tip of the penis papilla as, what they called, a pseudo-flagellum (see Dyganova & Porfirjeva 1990, fig. 35). Thus, apparently, it concerns an eversible flagellum.

However, S. elongata is different from P. eudendrocoeloides in that the former is an unpigmented species that also exhibits some notably different features in its copulatory apparatus. In particular, in S. elongata the posterior wall of the common atrium expands to form a distinct bulge towards the surrounding parenchyma (cf. Zabusova 1929, fig. 9; Dyganova & Porfirjeva 1990, fig. 35), which is absent in P. eudendrocoeloides . Furthermore, S. elongata is characterized by a very thick coat of muscles around the male atrium, as well as the bursal canal (cf. Zabusova-Zhdanova 1956, fig. 4; Sluys et al., 2009b). The excessive musculature around the male atrium signals that S. elongata is indeed a member of the genus Seidlia and not of Polycelis s.s. Although in P. eudendrocoeloides the musculature around the bursal canal is well developed, it does not reach the dimension of that in S. elongata , in which it comprises ten layers of circular muscles, bounded by two layers of longitudinal muscles ( Zabusova-Zhdanova 1956).

According to Kawakatsu & Mitchell (1998), the following species would also be synonymous with S. elongata : Seidlia eurantron , S. relicta and Polycelis polyopsis . That may well be true for P. eurantron as it also has a very stubby penis papilla and also shows the posterior extension at the base of the bursal canal, near its communication with the atrium. The latter feature was not specifically mentioned in the description but is apparent from the figures ( Zabusova 1936, figs 7, 8). On the other hand, P. eurantron does not exhibit the flagellum-like structure pointing into the seminal vesicle that is characteristic for S. elongata . Furthermore, S. eurantron is a yellowish-brown species, whereas S. elongata is unpigmented or at least pale, while in S. eurantron the gonopore is displaced towards the posterior end of the body, in contrast to the standard position of the gonopore in S. elongata ( Zabusova 1936) .

Although Zabusova (1936) considered S. eurantron to be very similar to S. relicta , the latter has a much more developed penis papilla, provided with a wide ejaculatory duct.

Finally, P. polyopsis is a member of the genus Polycelis s.s. as Zabusova (1936) specifically described the musculature around the male atrium as being rather poorly developed, so that it cannot be synonymous with S. elongata . Furthermore, in P. polyopsis the penis papilla is conical, while its tip is curved towards the ventral body surface, in contrast to the condition in S. elongata .

Figures

FIGURE 1. Location of collection sites of Polycelis in Shaanxi Province, China.

FIGURE 7. Habitat and external appearance of Polycelis eudendrocoeloides. A & B sampling site and habitat. C sexually mature, live individual. D Head with eyes, Scale bar: 500 μm.

FIGURE 8. Polycelis eudendrocoeloides. A metaphase plate of diploid complement. B karyogram of diploid complement. C idiogram. Scale bar: 5 μm.

FIGURE 9. Polycelis eudendrocoeloides. Photomicrographs. A sagittal section of ZMHNU-JX8, showing pharyngeal musculature. B transverse section of ZMHNU-JX10, showing ovaries. C sagittal section of ZMHNU-JX8, showing common oviduct and longitudinal muscles and circular muscles around bursal canal. D transverse section of ZMHNU-JX10, showing longitudinal muscles and circular muscles around bursal canal. E sagittal section of ZMHNU-JX8, showing large copulatory bursa. F sagittal section of ZMHNU-JX8, showing testes. Scale bars: 100 μm.

FIGURE 10. Polycelis eudendrocoeloides. A sagittal section of ZMHNU-JX2, showing spermiducal vesicles. B horizontal section of ZMHNU-JX9, showing seminal vesicle, oviduct and vas deferens. C sagittal section of ZMHNU-JX8, showing seminal vesicle, bursal canal, male atrium and common oviduct. D sagittal section of ZMHNU-JX2, showing seminal vesicle, bursal canal and male atrium. E sagittal section of ZMHNU-JX4, showing copulatory bursa, bursal canal, male atrium and seminal vesicle. F sagittal section of ZMHNU-JX8, showing the thick layer of circular muscles and male atrium. Scale bars: 100 μm.

FIGURE 11. Polycelis eudendrocoeloides. Sagittal reconstruction of the copulatory apparatus of ZMHNU-JX8. Scale bar: 100 μm.