Choeradoplana albonigra (Riester, 1938)

Lago-Barcia, Domingo, Silva, Marcos Santos & Carbayo, Fernando, 2021, Revision and description of six species of Choeradoplana (Platyhelminthes, Tricladida), with an emendation to the genus, ZooKeys 1016, pp. 1-48 : 1

Choeradoplana albonigra (Riester, 1938) Figures 6 , 7 , 8 , 9

Geoplana albonigra Riester, 1938: 7-9, figs 4, 5, 86, 87, taf. 1, fig. 2.

Notogynaphallia albonigra : Ogren & Kawakatsu, 1990: 140.

Choeradoplana albonigra : Carbayo et al., 2013: 514, 517.

Material examined.

Holotype SMF N° 702, sexually mature: Teresópolis, Rio de Janeiro, Brazil, coll. E. Bresslau, April 9th, 1914; transverse sections of a body portion seemingly being close to the anterior tip of the body on 1 slide; sagittal sections of the pharynx and copulatory apparatus on 1 slide; anterior and posterior tips in Canada balsam on 1 slide.

Reserva Biológica Augusto Ruschi, municipality of Santa Teresa, State of Espírito Santo, Brazil. May 26-27th, 2008. MZUSP PL 1109 (field code, F2313), only anterior half of the body collected: transverse sections of anterior extremity on 10 slides; sagittal sections of a portion behind anterior extremity on 5 slides; horizontal sections containing testes on 32 slides. MZUSP PL 1113 (field code, F2391), sexually mature: transverse sections of anterior extremity on 9 slides; sagittal sections of ovaries and testes on 6 slides; sagittal sections of pharynx and copulatory apparatus on 11 slides.

Parque Estadual do Desengano, municipality of Santa Maria Madalena, State of Rio de Janeiro, Brazil. August 10-13th, 2009. MZUSP PL 2273 (field code, F3991), juvenile: horizontal sections of anterior extremity on 3 slides. MZUSP PL 2153 (field code, F4031), sexually mature: transverse sections of anterior extremity on 12 slides; horizontal sections of ovaries on 22 slides; transverse sections of pre-pharyngeal region on 4 slides; sagittal sections of pharynx and copulatory apparatus on 8 slides; horizontal sections of posterior extremity on 4 slides. MZUSP PL 2152 (field code, F4024), sexually mature: horizontal sections of anterior extremity on 7 slides; sagittal sections of ovaries on 19 slides; horizontal sections of testes on 21 slides; transverse sections of pre-pharyngeal region on 9 slides; sagittal sections of pre-pharyngeal region on 12 slides; sagittal sections of copulatory apparatus on 12 slides. MZUSP PL 1083 (field code, F4081), sexually mature: transverse sections of anterior extremity on 55 slides; sagittal sections of the ovaries on 25 slides; horizontal sections of testes on 9 slides; transverse sections of pre-pharyngeal region on 6 slides; sagittal sections of pharynx and copulatory apparatus on 38 slides; horizontal sections of posterior extremity on 24 slides.

Distribution.

Reserva Biológica Augusto Ruschi, Santa Teresa, State of Espírito Santo; Parque Estadual do Desengano, Santa Maria Madalena, State of Rio de Janeiro; Municipality of Teresópolis, State of Rio de Janeiro, Brazil.

Diagnosis.

Choeradoplana species with a white dorsum, covered by a wide median black band, darker at its margins; an additional thin black median stripe may be present. The anterior third of the body is progressively thinner towards the pointed tip; its extremity has no lateral dilations or “neck” differentiating the head from the body. The ventral side of the cephalic region is concave and without glandular cushions. The proximal third of the prostatic vesicle is extrabulbar. The copulatory apparatus is relatively long; penis papilla is absent, and the female atrium is approximately funnel-shaped.

Description.

Living specimens range between 50-63 mm in length and 2-3 mm in width (n = 2). Preserved specimens range between 45-73 mm in length and 2-4 mm in width (n = 4). The body is slender and subcylindrical, with the anterior third becoming progressively thinner to the anterior tip. The anterior extremity is very thin and coiled up so that the ventral surface is facing out (Fig. 6A, B ). The ventral side of the cephalic region is slightly concave with indistinct glandular cushions (Fig. 6B-D ). The posterior extremity is pointed. The creeping sole is 80-86% of width in the pre-pharyngeal region (n = 3) (Fig. 7A ). Its mouth is at a distance from the anterior extremity ranging between 50.9-64.4% of body length, gonopore at 63.2-75.3% (n = 3).

The background color of the body is traffic white (RAL 9016). It is dorsally covered by a wide graphite black (RAL 9011) band, darker at its margins, as wide as three-quarters of the body width, in the middle of which runs a thin jet black (RAL 9005) stripe which is not apparent in some individuals. Anterior and posterior extremities of the body are slightly orangish. The color has faded out in preserved specimens. The dorsal graphite black band in the body tips of the holotype split into two brownish stripes.

The eyes are of one-pigment cup type, 25-45 µm in diameter; with no clear halos. They are marginally distributed in an irregular row of 2-6 eyes, from the anterior tip of the body (Figs 6B, C , 7A ), backwards to the posterior end. Anterior extremity devoid of eyes.

The sensory pits are 22-27 μm deep and ventro-laterally are distributed in a single row along approximately the anterior one-seventh of the body. The pits are absent at the very anterior tip of the body (300 µm).

Numerous rhabditogen cells open onto the dorsal surface of the body (Fig. 7B ) in the pre-pharyngeal region and its margins; these margins are also pierced by scarce gland cells producing granular, erythrophil secretion. The ventral epithelium is pierced by scarce gland cells producing granular, xanthophil secretion, and abundant gland cells producing strong erythrophil secretion (Fig. 7C ). There is no glandular margin.

The cutaneous musculature consists of a subepithelial circular muscle, followed by a diagonal layer with decussate fibers, and a strong longitudinal muscle organized in bundles (Fig. 7A-C ). This longitudinal muscle is 81.2-175 μm thick dorsally; it is ventrally divided into a 37.5-50.0 μm-thick muscle, organized in bundles with 20-35 fibers each, and an equally thick muscle sunken into the parenchyma, and constituted by scattered bundles with 7-30 fibers each (Fig. 7B ). A few dorsal longitudinal fibers are medially intermingled with those of the parenchymatic dorsal layer of diagonal decussate fibers. The thickness of the cutaneous muscle coat is 22-25% (n = 3) of the body height.

The pre-pharyngeal region, namely the dorsal decussate muscle (40-55 μm thick, n = 2), transverse supra-intestinal muscle (74-100 μm); and transverse subintestinal muscle (45-65 μm) (n = 2) (Fig. 7A-C ).

The retractor muscle of the head is delta-shaped in a cross-section along 0.3 mm (or 0.5% of body length) from behind, 0.1 mm (or 0.15%) of the anterior extremity of the body (Fig. 7C ), and its thickness equals 36% of the height of the cephalic region. The Muskelgeflecht is 18-25 μm thick (24% of body height). The subneural parenchymal muscle consists of a few scarce transverse fibers. The glandular cushions are composed of a relatively small quantity of rhabditogen cells (Fig. 7D-F ).

The mouth is located at a distance from the anterior section of the pharyngeal pouch ranging between 55.1-78.9% (n = 4) (Fig. 8A ). The pharynx is bell-shaped with dorsal insertion near the mouth level. An esophagus is absent. The outer epithelium of the pharynx is cuboidal, ciliated and underlain by a thin longitudinal muscle (3-5 μm); followed by a circular muscle (15-20 μm) with interspersed longitudinal fibers. The inner pharyngeal epithelium is ciliated, underlain by a circular muscle (50 µm thick; n = 3) with longitudinal fibers interspersed. There is abundant granular secretion of three types, cyanophil, erythrophil, and xanthophil, respectively, and pierce the distal pharyngeal epithelium.

The testes are dorsal, located under the supra-intestinal transverse muscle layer, partially placed between the intestinal diverticula. They extend from the level of the ovaries (a distance from the anterior extremity of the body equal to 27% of the body length) to nearly the root of pharynx (53% of the body length). Sperm ducts run immediately above the subintestinal muscle layer, dorsally and slightly laterally to the ovovitelline ducts. Distal portions of sperm ducts contain sperm, and are surrounded by a circular muscle. Sperm ducts communicate with the two roughly horizontal branches of the extrabulbar portion of the prostatic vesicle. These branches open laterally into an irregular, pear-shaped cavity, which is located more or less under the anterior section of the penis bulb (Figs 8B, C , 9A-C ). The prostatic vesicle continues as an almost vertical, tubular portion inside the very dense penis bulb to bend posteriorly towards a loose small ring-shaped horizontal fold (or 'small penis-shaped fold’, after Riester 1938) which may be narrowed and elongated as a finger (Fig. 8B , arrow). This fold gives passage to the male atrium. The penis bulb is very thick and consists of very numerous muscle fibers which are continuous with those underlying the epithelium of the male atrium. There is no ejaculatory duct as a differentiated portion. The prostatic vesicle is lined with a columnar, ciliated epithelium. The very abundant secretions discharging into it are zoned along the prostatic vesicle: paired branches receive very fine granular erythrophil and pink-reddish staining secretion; the dilated portion takes gross granular erythrophil intensely reddish staining secretion; a proximal intrabulbar portion receives finely erythrophil secretion; the distal section takes both finely cyanophil granular secretion and xanthophil variously sized secretion granules (Fig. 9B ). The extrabulbar portion is surrounded by interwoven muscle fibers, and the intrabulbar portion by a muscular layer of circular fibers interspersed with longitudinal ones, both portions are 20-35 μm thick. The loose small ring-shaped horizontal fold is lined with a 3-6 μm high non-ciliated epithelium and is surrounded by a few seemingly circular muscle fibers.

The proximal half of the male atrium is slightly folded and narrow. The distal half is ample and is narrowed distally by a large, dorsal fold extending through both the male and female atria. The stroma of this fold is strongly muscularized with longitudinal and oblique fibers. Additional lateral folds may be present in the distal half (Figs 8B, C , 9B ).

The proximal half of the male atrium is lined with a 5 μm high non-ciliated, infra-nucleated epithelium which is pierced by scarce gland cells producing fine erythrophil granules, and by gland cells producing variously sized xanthophil granules. This epithelium is surrounded by a 20-60 μm-thick dense muscle of very thin muscle fibers (2 μm); followed by a muscle (<150 μm thick) of 4 μm-thick fibers. The distal half of the male atrium is lined with an epithelium which is pierced by gland cells producing erythrophil granules. An intensely reddish erythrophil is found beneath this epithelium secretion. The lining epithelium of the distal half of the male atrium is surrounded by a circular muscle (18-45 μm); followed by a longitudinal muscle (80-150 μm) (n = 3).

A spermatophore is in the stroma of the large dorsal fold leveled with the gonopore in three specimens (F2391, F4024, F4081) (Fig. 8B, C ). The spermatophore is ovoid in shape, with approximately 150 μm in diameter, and seems to be constituted of a central mass of sperm partially surrounded by irregular strands of erythrophil granules. The biological meaning of the position of the spermatophore will be discussed elsewhere.

The ovaries are mature, very elongated, and club-shaped due to the dilated proximal extremity. The thin portion can be divided into smaller segments. The size of the ovaries ranges between 700-1200 µm in length and 120-170 µm in width (n = 4). They are located above the ventral nerve plate, and at a distance from the anterior extremity equal to 27.1-31.3% of the body length (n = 2). Ovovitelline ducts emerge from the dorso-lateral aspect of the ovaries and run above the nerve plate. The proximal segment of the oviducts is dilated and contains sperm (n = 4). They ascend posteriorly and medially laterally to the female atrium, then unite dorsally to the common glandular ovovitelline duct (Fig. 9A-C ). The distal portion of the oviducts is pierced by shell gland cells. The common glandular ovovitelline duct is downwards directed; it is continuous with a posteriorly and upward directed diverticulum of the female atrium, i.e., the female genital canal. The female atrium is roughly funnel-shaped, narrowed by a large dorsal fold continued with that of the male atrium. The female genital duct is lined with epithelial cells, with the apical portion containing fine erythrophil granules, and is surrounded by a thin layer of circular and longitudinal muscle fibers.

The female atrium is lined with a columnar, non-ciliated epithelium, and is pierced by two types of cells producing erythrophil and cyanophil granular secretions, respectively. The female atrium is as long as half the male atrium. The common muscular coat of the copulatory apparatus is composed of a weak layer of intermingled fibers; it is 20-40 μm thick in the anterior section, and thinner in the posterior section. The length:height ratio of the copulatory apparatus enveloped by the common muscle coat ranges between 3.0-3.5:1 (n = 5; mean 3.2:1).

Behavioral note.

Creeping on the Petri dish, the animals sometimes rise up to three-quarters of the body from the substrate and swing to the sides as they would be searching for ground. When touched, they can react by tumbling. A similar behavior, named 'escape reaction’, was observed for Ch. marthae by Froehlich (1959).

Remarks.

This species was originally described as Geoplana albonigra Riester, 1938, from Teresópolis, State of Rio de Janeiro, Brazil. The external aspect and the internal morphology of the pharynx and copulatory apparatus were given in the original description. The species was later transferred to the recently proposed new genus Notogynaphallia Ogren & Kawakatsu, 1990, erected for species of Geoplaninae without penis papilla and dorsally located female genital canal. Based on morphological and molecular information, Carbayo et al. (2013) transferred the species to Choeradoplana , but they did not provide morphological evidence supporting that taxonomic decision.

On the identity of our specimens. Our specimens and the holotype of Ch. albonigra are much alike, with the exception of the size of the male and female atria. Whereas the male and female atria are relatively narrow in the holotype, these atria are higher in two out of four mature individuals. Differences of the same nature have been observed in other land planarians (e.g., Pasipha pasipha (Marcus, 1951); Obama josefi (Carbayo & Leal-Zanchet, 2001)), and this might be caused by different states of maturation of each specimen, albeit all being mature, or by the physiological state of the individuals such as a recent copulation, as seems to be the case of the specimens bearing a spermatophore.

The systematic position of Ch. albonigra . Riester described the species from a single individual and assigned the species to Geoplana . In Riester’s description, there is no reference to the inner structures of the cephalic region, nor to the cutaneous muscle coat. The cephalic region of the holotype is embedded in Canada balsam on a histological glass slide. Our study revealed a set of additional aspects, including the delta-shaped retractor muscle of the head and the in-sunk ventral longitudinal cutaneous muscle which only agree with Choeradoplana . Our molecular data also support this conclusion. However, Ch. albonigra deviates from the genus in the body shape, since its anterior third narrows progressively towards the anterior extremity, with the latter also lacking the typical glandular cushions of Choeradoplana. Accordingly, the diagnosis of the genus is revised below.

Figures

Figure 6. Choeradoplana albonigra (Riester, 1938) A habitus of living specimen F 4024 in dorsal view B detail of anterior extremity of living specimen F 2313 in dorsal view. Debris is glued on the body C detail of anterior extremity of living specimen F 4024 in lateral view D detail of anterior extremity of preserved specimen F 4031 in ventral view. Abbreviations: cm common muscle coat, co common glandular ovovitelline duct, dd decussate dorsal cutaneous muscles, dm diagonal decussate muscles, e eye, ej ejaculatory duct, ep esophagus, er erythrophil secretion, fa female atrium, fd female genital duct, g gonopore, i intestine, lc longitudinal cutaneous muscles, ma male genital atrium, mk Muskelgeflecht (Graff, 1899), mo mouth, o ovary, ov ovovitelline duct, ph pharyngeal pouch, pp penis papilla, pv prostatic vesicle, px pharynx, rg rhabditogen glands, r retractor muscle, sb subintestinal transverse muscles, sd sperm duct, sg shell glands, sk sunken longitudinal cutaneous muscles, sm spermatophore, sn subneural transverse muscles, sp supra-intestinal transverse muscles, t testis, vi vitellaria, vn ventral nerve plate.

Figure 7. Choeradoplana albonigra (Riester, 1938) A diagrammatic transverse section of a portion, seemingly near the anterior extremity, of holotype. Photomicrographs of transverse sections (B-F) B dorsal epidermis in pre-pharyngeal region of specimen F 4024 C ventral epidermis in pre-pharyngeal region of specimen F 4024 D-F cephalic region of specimen F 4081 at 1.2, 0.4, and 0.15 millimeters from anterior extremity of the body, respectively. Scale bar: 200 µm. Abbreviations: cm common muscle coat, co common glandular ovovitelline duct, dd decussate dorsal cutaneous muscles, dm diagonal decussate muscles, e eye, ej ejaculatory duct, ep esophagus, er erythrophil secretion, fa female atrium, fd female genital duct, g gonopore, i intestine, lc longitudinal cutaneous muscles, ma male genital atrium, mk Muskelgeflecht (Graff, 1899), mo mouth, o ovary, ov ovovitelline duct, ph pharyngeal pouch, pp penis papilla, pv prostatic vesicle, px pharynx, rg rhabditogen glands, r retractor muscle, sb subintestinal transverse muscles, sd sperm duct, sg shell glands, sk sunken longitudinal cutaneous muscles, sm spermatophore, sn subneural transverse muscles, sp supra-intestinal transverse muscles, t testis, vi vitellaria, vn ventral nerve plate.

Figure 8. Choeradoplana albonigra (Riester, 1938) A diagrammatic representation of the pharynx from sagittal sections of holotype B sagittal section of the copulatory apparatus of specimen F 2391; note the small finger-shaped fold of the male atrium (arrow) C photomicrograph of a sagittal section of the copulatory apparatus of specimen F 4024. Abbreviations: cm common muscle coat, co common glandular ovovitelline duct, dd decussate dorsal cutaneous muscles, dm diagonal decussate muscles, e eye, ej ejaculatory duct, ep esophagus, er erythrophil secretion, fa female atrium, fd female genital duct, g gonopore, i intestine, lc longitudinal cutaneous muscles, ma male genital atrium, mk Muskelgeflecht (Graff, 1899), mo mouth, o ovary, ov ovovitelline duct, ph pharyngeal pouch, pp penis papilla, pv prostatic vesicle, px pharynx, rg rhabditogen glands, r retractor muscle, sb subintestinal transverse muscles, sd sperm duct, sg shell glands, sk sunken longitudinal cutaneous muscles, sm spermatophore, sn subneural transverse muscles, sp supra-intestinal transverse muscles, t testis, vi vitellaria, vn ventral nerve plate.

Figure 9. Choeradoplana albonigra (Riester, 1938) A diagrammatic reconstruction of copulatory apparatus of holotype from sagittal sections B photomicrograph of a sagittal section of the unpaired portion of the prostatic vesicle and proximal section of male atrium of specimen F 4024. Arrow points the small ring-shaped horizontal fold communicating prostatic vesicle with male atrium C diagrammatic reconstruction of copulatory apparatus of specimen F 4081 from sagittal sections D photomicrograph of a sagittal section of the female atrium of specimen F 4081. Abbreviations: cm common muscle coat, co common glandular ovovitelline duct, dd decussate dorsal cutaneous muscles, dm diagonal decussate muscles, e eye, ej ejaculatory duct, ep esophagus, er erythrophil secretion, fa female atrium, fd female genital duct, g gonopore, i intestine, lc longitudinal cutaneous muscles, ma male genital atrium, mk Muskelgeflecht (Graff, 1899), mo mouth, o ovary, ov ovovitelline duct, ph pharyngeal pouch, pp penis papilla, pv prostatic vesicle, px pharynx, rg rhabditogen glands, r retractor muscle, sb subintestinal transverse muscles, sd sperm duct, sg shell glands, sk sunken longitudinal cutaneous muscles, sm spermatophore, sn subneural transverse muscles, sp supra-intestinal transverse muscles, t testis, vi vitellaria, vn ventral nerve plate.