Othelosoma simile 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 : 313-318

Othelosoma simile Sluys & Neumann , sp. nov.

Material examined. Holotype: ZMA V.Pl. 7255.1, São Tomé Island, Bom Sucesso , 30 January 2015, coll. Leonel Viegas, sagittal sections of the anterior part on 7 slides; sagittal sections of the pharyngeal portion on 12 slides; sagittal sections of the copulatory apparatus on 15 slides.

Etymology. The specific epithet is derived from the Latin similis, like, resembling, and alludes to the fact that externally this species is very similar to one form of Othelosoma impensum .

Diagnosis. Blackish Othelosoma species with two light grey dorsal bands, which are separated by a median black line. Anterior end orange. Dorsal retractor extending from the anterior margin to one-fifth of the total body length of the preserved specimen. Ventral testes extending posteriorly to two-thirds of the distance between the root of the pharynx and the anterior end of the seminal vesicle. Short penis papilla with a dorsally shifted ejaculatory duct. Vaginal duct and bursal canal with separate openings into copulatory bursa. Sphincter muscle around the vaginal duct absent.

Description. Dorsal surface blackish with two light greyish bands, separated by a narrow blackish stripe. The anterior tip is pale orange, in the live animal being devoid of bands or stripe; however, bands and stripe do extend to the tail end ( Fig. 40 ). Ventral surface dirty-white ( Fig. 41 ). Anterior tip with a pair of well developed eyes at the very front. The preserved specimen measured 23mm in length and had a body width of 2.3mm. Narrow creeping sole comprising between one-third and one-fourth of the total body width of the preserved specimen ( Fig. 41 ).

Subepidermal musculature consisting of a thin layer of circular muscles, followed by a thin layer of longitudinal muscles. Parenchymal longitudinal muscles well developed, particularly on the ventral side, where longitudinal muscles are present also dorsally to the ventral nerve cords. Strong retractor muscle developed in the anterior end of the body, extending throughout the anterior fifth of the body in the preserved specimen ( Fig. 42 ).

The root of the cylindrical pharynx is placed at 1/3rd of the distance between the anterior tip and the posterior end of the specimen. Pharynx length is approximately 1/16th of the total body length in the preserved specimen. Mouth located at 1/3rd of the distance between the anterior and the posterior tip of the specimen. The mouth is located at 4/5th 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 intermingled muscles with both circular and longitudinal muscles.

Inner and outer pharynx epithelium underlain by layers of intermingled muscles with both circular and longitudinal fibres.

The first testes are located 2.1mm behind the ovaries and 3.9mm behind the centre of the brain. The testes extend posteriorly to almost 1.5mm anterior to the muscular seminal vesicle. In other words, the testes extend posteriorly to well behind the mouth, to two-thirds of the distance between the root of the pharynx and the anterior end of the seminal vesicle.

The testes are located ventrally on either side of the body between the intestinal branches and the parenchymal muscle band that is located dorsally to the nerve cords. The shape of the testes follicles is oblong, the follicles occupying approximately 1/6th of the dorso-ventral diameter in the prepharyngeal part of the body ( Fig. 43 ).

The sperm ducts run amongst the fibers of the layer of longitudinal parenchymal muscle located dorsally to the ventral nerve cords. Upon approaching the proximal, ventrally orientated portion of the seminal vesicle, one of the sperm ducts immediately opens into the very proximal section of the vesicle. The other sperm duct initially proceeds a short distance (about 100µm) beyond the proximal section of the seminal vesicle, after which it recurves to open also into the vesicle. From this point, the elongated seminal vesicle gently curves posteriad, then running more or less parallel to the body surface. Its lumen is rather broad and gradually narrows, thus giving rise to the ejaculatory duct ( Fig. 44 ). The latter runs a decentral, dorsally displaced course through the penis and opens at the tip of the papilla ( Fig. 45 ). The lining epithelium of the ejaculatory duct is nucleated and stratified, while the epithelium of the seminal vesicle is lined by a nucleated, pseudostratified epithelium. The entire seminal vesicle is surrounded by a well developed sheath of circular muscles, interspersed with longitudinal muscle fibres.

The short, cone-shaped penis papilla is covered with a nucleated epithelium. The penis papilla proper, i.e. from its point of insertion to the tip, measures 380µm in length, while the seminal vesicle measures 1250µm in length.

Penis glands are mainly distributed dorsally and ventrally of the root of the penis papilla and dorsally and ventrally of the anterior sixth of the seminal vesicle. Further there are many glands anterior to the seminal vesicle ( Fig. 44 ). These glands discharge their erythrophil secretion into the ejaculatory duct and the seminal vesicle, or prostatic vesicle.

Genital atrium lined with a thin, nucleated epithelium, underlain with a subepithelial circular muscle layer, followed by longitudinal muscles.

Gonopore located halfway of the distance between the anterior and posterior end of the specimen. Distance between mouth and gonopore approximately 6.5mm. i.e. between 1/4th and 1/3rd of the total body length in the preserved specimen. The gonoduct is surrounded by a thin, subepidermal layer of circular muscles, followed by a thin layer of longitudinal muscles.

The slightly dorso-ventrally elongated ovaries are located above the ventral nerve cords, occupying 1/10th of dorso-ventral diameter of the anterior part of the body. They are positioned at approximately 1/4th of the distance between the brain and the root of the pharynx.

The oviducts arise from the latero-ventral sides of the ovaries. Oviducts run in posterior direction between the fibers of the parenchymal muscle band, dorsally to the nerve cords. Approximately 300µm posterior to the gonopore, the oviducts curve dorso-medially and, subsequently, unite to form a short common oviduct ( Fig. 44 ). The latter opens into the bursal canal.

From the point where it receives the oviducal opening, the bursal canal runs for about 300µm into anterior direction to open into the atrium, about 290µm dorsally to the ventral body surface (this section of the bursal canal corresponds to the so-called canalis anonymus). After one-third of the way between the oviducal opening and the atrial opening of the canalis anonymus, a dorsal branch of the bursal canal (corresponding to Beauchamp's canal) slightly rises antero-dorsally for about 50µm, thereafter curving postero-dorsally to enter the copulatory bursa after running strait into dorsal direction for about 150µm. The bursal canal opens through the ventral wall of the copulatory bursa. The lining epithelium of the bursal canal extends for a short distance into the bursa.

The separate opening of the vaginal duct into the bursa is located 40µm anteriorly from the opening of the bursal canal. The lining epithelium of the bursal canal extends also for a short distance into the bursa. The vaginal duct is slightly curved into anterior direction and runs from the bursa about 420µm into ventral direction to open into the atrium, 210µm dorsally to the ventral body surface.

Vaginal duct and bursal canal are lined with a nucleated, ciliated and pseudostratified epithelium. The musculature around the vaginal duct consists of well developed layers of intermingled circular and longitudinal muscles, especially strong around the middle part of the duct ( Fig. 46 ). The musculature around the bursal canal is composed of a thin layer of circular muscles.

The small egg-shaped bursa measures about 350µm in length and about 200µm in height, thus occupying around 1/4th of the dorso-ventral diameter of the body in this part of the specimen ( Fig. 47 ). The bursa is lined with a well developed, pseudostratified, nucleated epithelium. No distinct muscles surround the copulatory bursa.

Distribution. The species is known from the type locality, near Monte Café in the northern part of São Tomé Island, and from Ribeira Peixe, about 25km south of the type locality ( Fig. 9 ).

Discussion. The external appearance of Othelosoma simile is very similar to the holotype specimen of O. impensum in that both have a dark ground colour, a dorsal narrow dark line with on either side a greyish band, and an orange tip. Although the colour pattern is the same, in O. simile the colour is made up of a black pigment, whereas in the holotype specimen of O. impensum it is dark green.

Two aspects concerning the colouration of these two species are confusing. First, the preserved specimen ZMA V.Pl. 7251.1 of O. impensum had a pale yellow dorsal surface, in contrast to the striped holotype specimen ZMA V.Pl. 7250.1. Although photos of live animals are not available, upon collection the sample V.Pl. 7251.1 was described as "all white." Photos of the holotype specimen of O. impensum clearly indicate that the ground colour of the body is dark green and that the dorsal stripe is also dark green ( Fig. 11 ), but we noted that in the preserved animal the ground colour and the stripe actually appear dull black. Despite these confusing and partly contradictory data on external appearances, anatomical information unequivocally showed that specimens V.Pl. 7250.1 and V.Pl. 7251.1 belong to one and the same species, viz. O. impensum (see above), which is specifically different from O. simile .

However, there are also some anatomical similarities between O. simile and O. impensum . Both species have (a) a well developed antero-dorsal retractor, extending from the anterior margin to the anterior fifth of the specimen, (b) a dorsally shifted ejaculatory duct running through a small penis papilla and (c) a long male atrium. However, O. impensum differs from O. simile in the presence of a long, wide and muscular seminal vesicle that runs parallel to the body surface. Further, the slightly curved seminal vesicle of O. simile is not so muscular as that of O. impensum and is also much smaller, in comparison to the dorsoventral diameter of the animal in that part of the body.

Other morphological differences in the male reproductive system of these two species are that the testes of O. impensum extend much farther into posterior direction, reaching the proximal section of the seminal vesicle, whereas in O. simile the testes extend posteriorly to almost 1.5mm anterior to the muscular seminal vesicle. Also the course of the sperm ducts is different. In O. impensum the sperm ducts run posteriorly to about three-quarters of the length of the seminal vesicle before they recurve. But in O. simile the sperm ducts recurve immediately after reaching the anterior end of the seminal vesicle.

Differences in the female reproductive systems of the two species O. impensum and O. simile are numerous. The copulatory bursa of O. impensum is much bigger than in O. simile , in comparison to the dorso-ventral diameter of the animals in that part of the body. In O. simile the bursa has two distinct separate openings, receiving the separate entries of the bursal canal and the vaginal duct, whereas in O. impensum bursal canal and vaginal duct first join to form a short common section that, subsequently, opens into the bursa.

The course of the bursal canal in O. simile and O. impensum is very similar but only in O. simile it receives the opening of a short common oviduct, whereas in O. impensum the oviducts open separately into the bursal canal. The vaginal ducts follow different courses, while the vaginal duct of O. impensum is surrounded by a mighty sphincter, which is absent in O. simile , in which the duct is surrounded only by a very thick coat of intermingled muscles.

The presence of a well developed antero-dorsal retractor, two bursal openings and absence of a sphincter surrounding the vaginal duct or the bursal canal appears to be a relatively rare combination of features among species of Othelosoma . Othelosoma simile shares this combination of characters with O. cylindricum ( De Beauchamp 1913) , O. gnaum , and O. marcusi (cf. Marcus 1955a; De Beauchamp 1956). Othelosoma huntum also has a well developed dorsal retractor and no sphincter surrounding the vaginal duct. However, O. gnaum and O. huntum are characterised by a sinuous course of the seminal vesicle, which can be divided into various parts, due to the presence of several constrictions ( Marcus 1955a; De Beauchamp 1956). In contrast to Othelosoma simile , O. gnaum has a huge bursa, which occupies about 80% of the dorso-ventral diameter of the animal. Further it is provided with a long penis papilla, traversed by a central ejaculatory duct, contrasting with the decentral course of the duct in O. simile . In both O. simile and O. marcusi a small bursa and a short common oviduct are present, but in the latter ejaculatory duct and seminal vesicle follow a highly contorted course, while the penis papilla is much longer.

Othelosoma cylindricum shares another anatomical feature with O. simile , viz. the presence of two bursal entries, one for the vaginal duct, the other one for the bursal canal. But in contrast to the two, close to each other and ventrally located bursal openings of O. simile , in O. cylindricum one bursal entry is located on the ventral side and the other on the dorsal side of the bursa ( De Beauchamp 1913). The male reproductive system of O. cylindricum differs from that of O. simile in various ways. The long penis papilla of O. cylindricum is much longer and has a centrally orientated ejaculatory duct, in contrast to the penis papilla of O. simile , while the ejaculatory duct of O. simile is shifted dorsad and has a bigger lumen. Futhermore, the seminal vesicle of O. cylindricum is Cshaped and very muscular, whereas the seminal vesicle of O. simile is just slightly curved and only has a relatively thin muscle coat.

There are several other species of Othelosoma with a dorsal pattern resembling that of O. simile , such as O. gnaum and O. huntum . In addition to the anatomical differences discussed above, the ground colour of O. gnaum and O.huntum is light brown and brownish yellow, contrasting with the black ground colour of O. simile . However, in such animals it is debatable what is the ground colour and what constitutes the stripes and bands. Nevertheless, the regions directly adjacent to the dorsal band are light brown or brownish in O. gnaum and O. huntum , whereas they are greyish in O. simile . Other Othelosoma species that also have three dorsal stripes are O. africanum ( Von Graff, 1899) , O. marlieri De Beauchamp, 1956 , O. rudebecki Marcus, 1955 and O. symondsi Gray, 1869 . The lastmentioned species shares with O. simile the characteristic of a paler anterior end. However, anatomical differences between O. simile and other more or less three-striped Othelosoma species are numerous. None of these threestriped species has the combination of the following characters: presence of a dorsal retractor, two bursal openings, absence of a sphincter around the vaginal duct and a dorsally shifted ejaculatory duct.

Further anatomical differences between these four species and O. simile are as follows: (1) O. africanum has an irregularly shaped penis papilla that points in dorsal direction and a seminal vesicle that has several constrictions; (2) O. marlieri lacks a bursa and has a penis papilla that is thicker on its tip; (3) O. rudebecki has an extra-bulbar seminal vesicle and a more vertically orientated penis papilla with a central ejaculatory duct; (4) O. symondsi has a vertically orientated penis papilla with a central ejaculatory duct, while the sperm ducts, which expand to form spermiducal vesicles, extend posteriorly to well behind the anterior third of the seminal vesicle ( Pantin 1953). These anatomical characteristics of O. africanum , O. marlieri , O. rudebecki and O. symondsi stand in contrast to those of O. simile , which shows a small penis papilla with a dorsally shifted ejaculatory duct, presence of an intrabulbar seminal vesicle, and also a copulatory bursa.

It is noteworthy that three of the species discussed above ( O. simile , O. gnaum and O. marcusi ) show the following combination of anatomical features: presence of an antero-dorsal retractor, two distinct bursal openings, absence of a sphincter. This combination applies also to Othelosoma polecatum , O. fuscum (Moseley, 1877) , and O. voleum . However, in contrast to the small bursa of O. simile , the two last-mentioned species show an extremely elongated bursa, while O. polecatum has a penis papilla which is as long as the seminal vesicle, contrasting with the much shorter papilla in O. simile and the extremely short papilla in O. voleum . Although O. polecatum , O. fuscum and O. voleum have a retractor muscle, their extensions are different. In O. fuscum the retractor starts at the level of the ovaries and in O. voleum even behind the ovaries ( Freisling 1935). The retractor muscle of O. polecatum starts at the very anterior end, as in O. simile but extends posteriorly to about 1/9th of the total body length of the preserved specimen, whereas in O. simile the retractor measures only 1/5th of the total body length.

Figures

FIGURE 44. Othelosoma simile. Holotype, sagittal reconstruction of the copulatory system.

FIGURES 40 – 43. Othelosoma simile. Fig. 40. Living specimen (not collected) from Ribeira Peixe. Scale bar not available. Fig. 41. Holotype, in ethanol. Fig. 42. Holotype, sagittal section through the anterior end showing eye, retractor and ventral nerve cord. Fig. 43. Holotype, sagittal section through the pharyngeal part of the body; anterior to the left.

FIGURES 45 – 47. Othelosoma simile. Fig. 45. Holotype, sagittal section of the penis papilla and the seminal vesicle. Fig. 46. Holotype, sagittal section through copulatory bursa, vaginal duct and penis papilla; anterior to the right. Fig. 47. Holotype, sagittal section through the copulatory system; 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.