Dugesia bifida Stocchino & Sluys

Stocchino, Giacinta Angela, Sluys, Ronald & Manconi, Renata, 2014, A new and aberrant species of Dugesia (Platyhelminthes, Tricladida, Dugesiidae) from Madagascar, ZooKeys 425, pp. 71-88 : 74-79

Taxon classification Animalia Tricladida Dugesiidae

Dugesia bifida Stocchino & Sluys sp. n. Figs 1-7; Tables 1-2

Material examined.

Holotype: ZMA V.Pl. 7189.1, one set of sagittal sections on 8 slides, Central High Plateau, between Antsirabe (19°86'32"S, 47°03'36"E) and Ambositra (20°53'14"S, 47°24'61"E), near the small village of Antsariboti, Madagascar, 16 September 2011, coll. R. Manconi.

Paratypes: CGAS Pla 7.1, ibid., sagittal sections on 7 slides; CGAS Pla 7.2, ibid., sagittal sections on 4 slides; CGAS Pla 7.3, ibid., transverse sections on 20 slides. CGAS Pla 7.4-5, ibid., horizontal sections on 4, 7, slides respectively; ZMA V.Pl. 7189.2, ibid., horizontal sections on 6 slides; ZMA V.Pl. 7189.3, ibid., horizontal sections on 3 slides; ZMA V.Pl. 7189.4, ibid., horizontal sections on 4 slides; ZMA V.Pl. 7189.5, ibid., sagittal sections on 7 slides; ZMA V.Pl. 7189.6, ibid., horizontal sections on 5 slides.

Diagnosis.

Dugesia bifida is characterized by the presence of the following features: body slender; head with smooth, rounded auricles; oviducts that recurve before opening into the bursal canal and provided with a common posterior extension; slightly asymmetrical openings of the oviducts into the bursal canal; absence of ectal reinforcement; large seminal vesicle; asymmetrical openings of the vasa deferentia into the seminal vesicle, the openings situated at halfway along the vesicle; very long spermiducal vesicles; large diaphragm; ventral course of the ejaculatory duct; terminal opening of the ejaculatory duct; unstalked cocoons; chromosomal number 2n = 18.

Etymology.

The specific epithet is derived from the Latin adjective bifidus, split into two parts, and alludes to the fact that the peculiar long common oviduct splits into two branches, each branch subsequently opening into the bursal canal.

Geographical distribution.

Known only from the type locality in the High Tsiribihina hydrographic basin, Madagascar.

Habitat.

Planarians were found in running water in a paddy field area at an altitude ca. 1300 m asl in the Central High Plateau, along Route Nationale 7, between Antsirabe and Ambositra, near the small village of Antsariboti (Fig. 1). The small, unnamed stream is a tributary of the Mania River in the southern branch of the High Tsiribihina hydrographic basin. The animals, scattered and not abundant, were collected from running clear water, under small pebbles on coarse sand at a depth of 3-10 cm. A survey of ca. 50 pebbles, performed at the end of the dry season (September), revealed complete absence of planarian cocoons, as well as other invertebrates, excepting very small larvae of mayflies.

Description.

Body of living specimens slender, ranging from 6 to 7 mm in length and 0.4-0.6 mm in width in fissiparous specimens and from 11-15 mm × 1.5-2 mm in sexualized specimens. Two eyes present in the centre of the head; unpigmented auricular grooves marginally placed just posteriorly to the eyes. Head with smooth, rounded auricles and with five sensory fossae on either side of its anterior margin.

The dorsal surface light grey-brown, with two darker lateral stripes running from the central part of the pharynx to its posterior part, where they form a single median stripe that runs to the tail. In sexualized specimens the pigmentation is darker than fissiparous animals (Fig. 2). The ventral surface is paler than the dorsal body surface.

The pharynx is positioned in the posterior half of the body and measures about 1/9th of the body length. Inner and outer pharyngeal musculature bilayered, i.e. without an extra, third, outer longitudinal muscle layer in the inner sheath of muscles.

The ovaries, localized just behind the brain, are weakly hyperplasic. They occupy half of the dorso-ventral space of the body and are particularly expanded in horizontal direction. The anterior portion of the infranucleated oviducts is expanded into a tuba that may communicate, at a poorly defined position, with the dorsal side of the ovaries or with the center of the ovarian masses, dependent upon the hyperplasic condition of the ovaries (Fig. 6A). The oviducts run ventrally in a caudal direction to beyond the level of the genital pore and, subsequently, recurve anteriad to open at the same level into a long, posterior duct with an histology similar to that of an oviduct. For descriptive purposes we consider this to be a common posterior oviducal extension. The right oviduct opens dorsally into this long common duct while the left oviduct opens ventrally. From this point the common posterior duct divides into two branches, which open separately and asymmetrically through the posterior wall of the bursal canal. The left branch opens slightly dorsally to the right one. The openings of these two branches into the vertically running section of the bursal canal are situated close together (Figs 3A, B, 4). The lumen of the common posterior oviducal extension, and also that of the two branches contains ample sperm. In CGAS Pla 7.1 specimen the most posterior part of the common posterior oviducal extension communicates through a thin ductule with the ventral part of an adjacent vitellarium (Fig. 5C). In the holotype ZMA V.Pl. 7189.1 and in specimens CGAS Pla 7.1 and CGAS Pla 7.2 the lumen of the oviducts has an irregular diameter and is generally quite spacious, thus in some parts forming a kind of vesicle (Fig. 6B).

The numerous mature, fully developed testes are situated dorsally and extend from the level of the ovaries to the posterior end of the body. Spermatogenesis appears to proceed in a regular fashion, in that no anomalies, such as irregularly shaped spermatids and spermatozoa, were observed (Fig. 6C). Vitellaria are located between the testes and the intestinal branches and extend to some distance posteriorly to the copulatory apparatus.

The large copulatory bursa is lined by a columnar, glandular epithelium bearing basal nuclei and it is surrounded by a layer of muscles. In the holotype ZMA V.Pl. 7189.1 a spermatophore full of sperm is present in the lumen of the bursa. From the postero-dorsal wall of the bursa the bursal canal runs in a caudal direction to the left of the copulatory apparatus and, after a narrowing, communicates with the common atrium. The bursal canal is lined with cylindrical, infranucleated, ciliated cells and is surrounded by a thin subepithelial layer of longitudinal muscles, followed by a layer of circular muscles. Ectal reinforcement is absent. The very abundant shell glands open into the vaginal section of the bursal canal, at the level of the oviducal openings (Figs 3A, 5B).

The scarcely developed penis bulb, rich in glands, consists of intermingled longitudinal and circular muscle fibres. Extra-bulbar penial glands (staining yellow with Mallory-Cason) penetrate the penis bulb at its dorsal and ventral side. The penis bulb houses a very large, flask-shaped seminal vesicle, lined with a nucleated epithelium (Figs 3A, 5A). The vasa deferentia penetrate the proximal, anterior section of the penis bulb and open separately and asymmetrically into the seminal vesicle at a position about halfway along the vesicle, at the point where it narrows. The right vas deferens opens dorsally to the left one. The seminal vesicle opens into the ejaculatory duct via a large, valve-like diaphragm. In all specimens examined the sperm ducts form well-developed spermiducal vesicles, packed with sperm. These vesicles are very long and extend over a large distance, viz. from the root of the pharynx to the penis bulb. The diaphragm, located approximately at the base of the penis papilla, receives the openings of penis glands. The stubby, asymmetrical penis papilla is covered by an infranucleated epithelium that is underlain with a subepithelial layer of longitudinal muscles. The ejaculatory duct follows a ventral course and has a terminal opening. A ventrally displaced course of the ejaculatory duct is present in all specimens examined, albeit that this condition is more clearly expressed in some specimens as compared to others, depending on the state of contraction of the penis papilla. For example, in paratype V.Pl. 7189.5 the penis papilla is cone-shaped and shows a distinctly ventrally displaced ejaculatory duct, with a terminal opening. A similar situation is present in paratype CGAS Pla 7.2. In contrast, in the holotype and in paratype CGAS Pla 7.1 the penis papilla is much more stubby, due to contraction, with the result that the ventral course of the ejaculatory duct is much less pronounced. The ejaculatory duct, which in most of the specimens examined contained an empty spermatophore, is lined by a cuboidal, infranucleated epithelium (Figs 3, 5A).

The genital atrium is divided into a common atrium and a male atrium and is lined by an infranucleated epithelium that is underlain by a subepithelial layer of circular muscles, followed by a layer of longitudinal muscle fibres. The common atrium opens ventrally through the gonopore, which receives the openings of the cement glands (Figs 3A, B, 5A, B).