Othelosoma lineaenigrum Sluys & Neumann

Sluys, Ronald, Neumann, Matthias, De Lima, Ricardo F. & Drewes, Robert C., 2017, Land flatworms (Platyhelminthes, Geoplanidae) of São Tomé: a first account on their diversity, with the description of five new species, Zootaxa 4221 (3), pp. 291-322 : 309-313

Othelosoma lineaenigrum Sluys & Neumann , sp. nov.

Material examined. Holotype: ZMA V.Pl. 7254.1, São Tomé Island, Lagoa Amélia (N 0 1652.6 E63527.7), April 2014, coll. Miko Nadel, sagittal sections of the anterior part of the specimen on 29 slides; transverse sections of the prepharyngeal part on 8 slides; sagittal sections of the part with the pharynx on 16 slides; sagittal sections of the copulatory apparatus on 34 slides; sagittal sections of the tail end on 20 slides. The specimen was collected from moss on a tree trunk, bordering Lagoa Amélia, approximately 1.5 – 2 meters above the ground, at an altitude of 1425m.

Etymology. The specific epithet is derived from the plural lineae of the Latin noun linea, line, and the Latin adjective niger, black; it alludes to the two black stripes running on the dorsal surface of this species.

Diagnosis. Pale yellow Othelosoma species with two black, widely separated dorsal stripes. With a well developed penis papilla, an antero-ventrally curved seminal vesicle without constrictions, and a horizontally orientated vaginal duct and bursal canal. Opening of vaginal duct into the very large copulatory bursa located midventrally. Vaginal duct receives opening of bursal canal immediately ventrally to the bursa. Sphincter and retractor muscle absent.

Description. Dorsal surface pale yellow and provided with a pair of black bands running from the most anterior tip to the posterior end of the body ( Fig. 33 ). The bands are widely separated from each other and located near the lateral sides of the body. Ventral surface of a slightly lighter yellow hue. A pair of eyes is situated at the very anterior end of the body.

The preserved specimen measured 38mm in length and had a body width of 5mm. Clearly, the live animal was much more slender in appearance ( Fig. 33 ). Narrow creeping sole occupying less than one-fourth of the body width in the prepharyngeal region of the specimen.

Subepidermal musculature consisting of a thin layer of circular muscle, followed by a thin layer of longitudinal muscle. Parenchymal longitudinal muscles well developed, particularly on the ventral side, where longitudinal muscles are present also dorsally to the ventral nerve cords. No retractor muscle in the anterior end of the animal.

The cylindrical pharynx almost completely fills the pharyngeal pouch. The root of the pharynx is placed at 43% of the distance between the anterior tip and the posterior end of the specimen. The pharynx measures almost 1/10th of the total body length in the preserved specimen. Mouth located halfway between the anterior and the posterior tip of the specimen and at two-thirds of the distance between the root of the pharynx and the posterior wall of the pharyngeal cavity. Pharyngeal pouch musculature composed of few layers of subepithelial circular muscles, followed by a weak and simple layer of longitudinal muscles. Outer and inner pharynx epithelium underlain by a simple layer of longitudinal muscles, followed by few layers of circular muscle; after that followed by stronger layers of intermingled muscles with both, circular and longitudinal fibres. Inner pharynx musculature thicker than outer one, in some areas two times as thick ( Fig. 34 ).

The first testes are located 0.7mm behind the ovaries and 1.4mm behind the centre of the brain. There are at least 144 testes, extending posteriorly to the anterior end of the spermiducal vesicles, which are located at about 1.56mm from the anterior end of the seminal vesicle in the penis. In other words, the testes extend posteriorly to well behind the mouth and to about 1200µm anterior to the muscular seminal vesicle. The testes are located ventrally on either side of the body between the intestinal branches and the sub-intestinal transverse parenchymal muscle layer. The shape of the testes follicles is oblong, the follicles occupying between one-fourth and one-fifth of the dorsoventral diameter in the prepharyngeal part of the body ( Fig. 35 ).

The sperm ducts run amongst the fibers of the layer of parenchymal muscle located dorsally to the ventral nerve cords. The distribution of muscle fibers around the duct is irregular, being more abundant on the dorsal than on the ventral side. The sperm ducts expand to form approximately 700µm long spermiducal vesicles, full of spermatozoa. The spermiducal vesicles run posteriorly to a level at about halfway the muscular seminal vesicle ( Fig. 36 ). Hereafter the ducts recurve and suddenly sharply decrease in diameter before penetrating the muscle layer surrounding the postero-ventral portion of the seminal vesicle. Thus, the ducts separately open into the proximal section of the seminal vesicle, which is located near the ventral body surface. From thereon, the seminal vesicle makes an antero-dorsally oriented U-turn and grades into the much narrower, but still broad, ejaculatory duct. The latter opens at the tip of the penis papilla, which is a slender cone, covered with a ciliated, thin, and nucleated epithelium. The penis papilla proper, i.e. from its point of insertion to the tip, measures 610µm, while the complete U-shaped seminal vesicle measures 1750µm in length ( Fig. 36 ).

The seminal vesicle is surrounded by a very strong, subepithelial layer of circular muscles that gradually diminishes in diameter around the ejaculatory duct. Furthermore, longitudinal muscles lie interspersed in the layer of circular muscles of the seminal vesicle. Ectally to this thick zone of circular muscle lies a zone that is traversed by sparse longitudinal muscle fibres, forming a kind of penis bulb or muscle tunic. Part of these longitudinal fibres also extend over the ejaculatory duct ( Fig. 36 ).

Penis glands are mainly distributed ventrally to the root of the penis papilla. Furthermore, the penis papilla receives secretion from glands around the anterior third of the seminal vesicle, or prostatic vesicle, while the latter receives secretion from penis glands located mainly at the anterior and dorsal side of the spermiducal vesicles. Genital atrium lined with a thin nucleated epithelium, underlain with a subepithelial circular muscle layer, followed by longitudinal muscles.

Gonopore located at two-thirds of the distance between the anterior and posterior end of the specimen. Distance between mouth and gonopore 6.2mm, i.e. 1/5th of the total body length in the preserved specimen. The gonoduct is surrounded by a weak layer of circular muscles, followed by a layer of longitudinal muscles.

Ovaries situated above the ventral nerve cords, occupying less than 1/5th of the dorsoventral diameter of the anterior part of the body. They are positioned at approximately 1/10th of the distance between the brain and the root of the pharynx. The oviducts arise from the ventral side of the ovaries. Oviducts run in posterior direction between the fibers of the sub-intestinal transverse parenchymal muscle layer. The dispersion of muscle fibers around the duct is irregular, more fibres being present on the dorsal than on the ventral side. Posterior to the gonopore each oviduct rises slightly and then bends medially for about 800µm. Hereafter, the oviducts fuse to form a short common oviduct, which opens into the bursal canal ( Fig. 37 ). The oviducts receive the openings of shell glands before they have fused into a common oviduct, while the latter also receives the openings of shell glands. From the point of the opening of the common oviduct the bursal canal (1) runs obliquely, antero-ventrally to open into the atrium (this section of the bursal canal corresponds to the so-called canalis anonymus), about 180µm dorsally to the ventral surface, (2) turns dorsally for a short distance, after which it makes a sharp, posteriorly directed bend, following its course more or less parallel to the body surface (this section of the duct correponds to the so-called Beauchamp's canal) ( Fig. 38 ) to open into the vaginal duct, immediately ventral to the copulatory bursa. The vaginal duct opens through the mid-ventral surface of the copulatory bursa ( Fig. 37 ). From this point, the vaginal duct follows a very shallow S-shaped course towards the atrium, opening into the latter at about 200µm dorsally to the ventral surface ( Fig. 39 ).

Vaginal duct and bursal canal are lined with a nucleated, ciliated, pseudostratified epithelium. The musculature around both bursal canal and vaginal duct consists of a thin layer of circular muscles, followed by a layer of diagonally, criss-cross arranged muscle fibres, the latter being slightly more developed on the vaginal duct than on the bursal canal ( Fig. 37 ).

The large, sack-shaped bursa measures about 1100µm in antero-posterior direction and about 700µm in dorsoventral direction, thus occupying about 60% of the dorsoventral space of the body in this part of the specimen ( Fig. 39 ). The bursa is lined with a well developed, vacuolated epithelium. The bursal musculature is composed of a weak layer of decussate fibres. The bursa contains a large amount of partly degraded sperm ( Fig. 39 ). Sperm is present also in Beauchamp's canal.

Distribution. The species is known only from the type locality ( Fig. 9 ).

Discussion. Othelosoma lineaenigrum can be characterised by the presence of a well developed penis papilla, a curved seminal vesicle, a bursal canal that fuses with the vaginal duct shortly before the latter enters into the bursa, and by the absence of a sphincter and a retractor muscle in the anterior end of the body. This combination of anatomical characters invites comparison with O. angolense ( De Beauchamp, 1951) and O. speciosum (Von Graff, 1896) (examination of new, as yet undescribed, material of O. angolense from Nigeria revealed presence of a small retractor, which was not mentioned by De Beauchamp (1951) since he had sectioned parts of three specimens that included only the pharynx and the copulatory apparatus). Othelosoma angolense and O. speciosum further share the following anatomical features with O. lineaenigrum : (a) vertically orientated penis papilla, (b) seminal vesicle with broad lumen that only gradually grades into a still broad ejaculatory duct, (c) sperm ducts separately entering the seminal vesicle, (d) presence of a large copulatory bursa that receives the combined opening of the vaginal duct and bursal canal immediately after their unision.

However, O. speciosum can be distinguished from O. lineaenigrum by its external appearance. While the living specimen of O. lineaenigrum is pale yellow with two dark dorsal stripes and a thin anterior part, O. speciosum , in contrast, has a rusty dark yellow and non-striped body of which the blunt anterior half forms the broadest part ( Von Graff 1899). Furthermore, in O. speciosum two yellowish suckers are present on either side of the ventral anterior third of the body, whereas in O. lineaenigrum suckers are absent. Anatomical differences between O. lineaenigrum and O. speciosum concern the female reproductive system. The copulatory bursa of O. speciosum has an elongated shape and receives the opening of the vaginal duct from the antero-ventral side. In contrast to this, the bursa of O. lineaenigrum is not so elongated, while the vaginal duct penetrates the mid-central wall of the bursa.

Othelosoma angolense differs externally from O. lineaenigrum in the fact that it has dark brown bands and a grey-brown ventral surface ( De Beauchamp 1951), contrasting with the black dorsal stripes and light yellow ventral surface of O. lineaenigrum .

Anatomical differences between the two species can be found in the male and the female reproductive system. The penis papilla of O. angolense is very broad and has a blunt tip, whereas the penis papilla of O. lineaenigrum narrows gradually from a broad base to a pointed tip. While the bursal canal of O. angolense receives the separate openings of the oviducts shortly before the bursal canal communicates with the vaginal duct (near the bursa), the short common oviduct of O. lineaenigrum enters into the bursal canal near the atrial opening of the latter. Furthermore, a connection between the bursa and the gut is present in O. angolense , whereas such an intestinal connection has not been found in O. lineaenigrum .

Apart from another point of communication between bursal canal, vaginal duct and oviducts in O. nigrescens ( Mell, 1904) , this species is in some other features comparable with O. lineaenigrum . Similar to O. lineaenigrum , the long vertically orientated penis papilla of O. nigrescens has a pointed tip and a broad ejaculatory duct. Furthermore, the seminal vesicle is curved and the two sperm ducts form spermiducal vesicles ( Mell 1904). Even the size of the pharynx is similar, occupying in both species around 1/10th of the total body length of the preserved specimen. However, O. nigrescens differs from O. lineaenigrum in the shape of the seminal vesicle, which is curved anteriorly and posteriorly in O. lineaenigrum , whereas in O. nigrescens it is only posteriorly curved. Further differences between these two species are as follows: (a) the antero-lateral opening of the bursal canal into bursa of O. nigrescens stands in contrast to the mid-central combined opening in O. lineaenigrum , (b) the lower part of the vaginal duct of O. nigrescens , which communicates with the atrium, forms various diverticula, whereas the vaginal duct of O. lineaenigrum communicates with the atrium without conspicuous folds of the epithelium, and (c) the mouth of O. nigrescens is located much more anteriorly than in O. lineaenigrum .

Figures

FIGURES 33 – 35. Othelosoma lineaenigrum. Fig. 33. Holotype, living specimen. Scale bar not available. Fig. 34. Holotype, sagittal reconstruction of the pharynx; anterior to the left. Fig. 35. Holotype, sagittal section through pre-pharyngeal part of the body, showing the location of the testes.

FIGURES 36 – 37. Othelosoma lineaenigrum. Fig. 36. Holotype, sagittal reconstruction of the copulatory apparatus; anterior to the right. Fig. 37. Holotype, sagittal reconstruction of the vaginal duct and the bursal canal; anterior to the right.

FIGURES 38 – 39. Othelosoma lineaenigrum. Fig. 38. Holotype, sagittal section of the bursal canal and oviduct. Fig. 39. Holotype, sagittal section of the copulatory apparatus; anterior to the right.

FIGURES 9 – 12. Fig. 9. Distributional records of terrestrial planarians used in this study. Red filled circle: Othelosoma lineaenigrum; blue filled triangle: Othelosoma duplamaculosum; purple filled square: Othelosoma impensum; brown filled hexagon: Othelosoma laticlavium; green filled asterisk: Othelosoma simile; black cross: Bipalium kewense. Fig. 10. Othelosoma impensum, holotype, drawing of anterior end of specimen in ethanol. Fig. 11. Othelosoma impensum, holotype, dorsal side of living specimen; scale bar not available. Fig. 12. Othelosoma impensum, holotype, ventral side of living specimen. Scale bar not available.