Choeradoplana tristriata (Schultze & Mueller , 1857)

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 tristriata (Schultze & Mueller, 1857) Figures 1 , 2 , 3 , 4 , 5

Geoplana tristriata Schultze & Müller, 1857: 23.

Geoplana tristriata Not Geoplana tristriata in Graff 1899: 327-328, 331, taf. V, figs 25, 26.

Pseudogeoplana tristriata : Ogren & Kawakatsu, 1990: 161.

Material examined.

MZUSP 2271 (field code, F3226), sexually mature: Parque Estadual da Serra de Tabuleiro, State of Santa Catarina, Brazil (-27.94, -48.79). coll. F. Carbayo and co-workers, 11 January 2009; transverse sections of the cephalic region on 6 slides; horizontal sections of the portion behind the cephalic extremity on 4 slides; transverse sections of the pre-pharyngeal region on 6 slides; sagittal sections of the pharynx and copulatory apparatus on 17 slides.

Distribution.

Municipality of Blumenau and Parque Estadual da Serra de Tabuleiro, State of Santa Catarina, Brazil.

Diagnosis.

Choeradoplana species with yellow green background color, with three thin discontinuous longitudinal black lines; ventral side is zinc-yellow. Copulatory apparatus compact, without penis papilla; female atrium funnel-shaped.

Description.

The preserved specimen measures 22 mm long and 2.5 mm wide (Fig. 2A ). The body is slender and subcylindrical. The cephalic region is differentiated from the remaining body by means of a ‘neck’, laterally dilated and rolled up so that the ventral surface is provided with glandular cushions, and is facing out (Fig. 2B ); the posterior extremity is pointed. The creeping sole is 90% of body width at the pre-pharyngeal region. The mouth is positioned at a distance from the anterior extremity equal to 60% of the body length, and the gonopore is at 74%.

The dorsal coloration consists of a yellow green (RAL 1018) background color, with three thin discontinuous longitudinal lines of small black spots. These spots are less concentrated in the median line (Fig. 2A ). The ventral side is zinc yellow (RAL 1018), except for a silver grey (RAL 7001) spot on the glandular cushions (Fig. 2A ).

The eyes are formed by one pigmented cup of 80 μm in diameter (Fig. 2C ). There are no clear halos around them. Eyes are absent in the very anterior extremity of the body; at 1.5 mm behind the anterior tip the eyes are marginally distributed in a row of two or three eyes; 3.8 mm posterior to the anterior tip, the eyes are placed in a single marginal row which runs along the whole body until posterior extremity, with each eye at a distance of ~ 0.3 mm from each other (Fig. 2B ).

Sensory pits are 25 μm deep (Fig. 2C , arrowhead), and are distributed ventro-laterally in a uniserial row between ~ 0.4 mm behind the anterior extremity to at least the ovarian level.

Numerous rhabditogen cells discharge through the glandular cushions of the cephalic region. These cells are scarce in the dorsal epidermis. Abundant erythrophil gland cells pierce the dorsal and marginal epithelium in the pre-pharyngeal region. The entire epithelium is additionally pierced by scarce gland cells of two types, producing cyanophil and xanthophil granules, respectively. No glandular margin.

The cutaneous musculature of the pre-pharyngeal region comprises a subepithelial circular muscle, followed by a diagonal layer with decussate fibers, and a longitudinal muscle organized in tight bundles (Figs 2D , 3A ). This longitudinal muscle is 70 μm thick dorsally; it is ventrally divided into a 35 μm-thick muscle, organized in bundles with 4-8 fibers each, and a 50 μm-thick muscle sunken into the parenchyma consisting of scattered bundles with 5-12 fibers each (Fig. 3B ). The thickness of the cutaneous muscle coat is 17% of the body height.

There are three parenchymal muscle layers in the pre-pharyngeal region (Fig. 3A, B ): a well-developed dorsal layer of diagonal, decussate fibers (12-18 μm thick); a transverse supra-intestinal muscle (22-25 μm); and a transverse subintestinal muscle (30-35 μm).

The ventral longitudinal cutaneous muscle is modified into the retractor muscle in the cephalic region. This retractor muscle is delta-shaped in cross-section along 0.7 mm (or 3% of body length), starting 0.2 mm (or 1%) from the anterior extremity of the body (Fig. 3C ), and its thickness is 24% of the height of the cephalic region. The dorsal decussate and subintestinal parenchymal muscles in this region are weak, whereas the supra-intestinal is strongly developed and mixed with dorso-ventral muscle fibers giving rise to the Muskelgeflecht or interwoven muscle, and is 75 μm in thickness. A fourth subneural parenchymal muscle is present in the cephalic region, and is located beneath the central nerve system and above the retractor muscle. The paired glandular cushions are pierced by numerous rhabditogen cells. The arrangement of cutaneous and parenchymal muscles in the cephalic region and the glandular component of the cephalic cushions match those of the type species of the genus, Choeradoplana iheringi Graff, 1899.

The mouth is located in the middle of the pharyngeal pouch (Fig. 4A ). It is a bell-shaped pharynx, with the dorsal insertion posterior to the ventral insertion ~ 45% of the pharyngeal length. An esophagus is not present. The pharyngeal pouch is lined with a non-ciliated, cuboidal-to-flat epithelium, underlain by a one-fiber thick layer of longitudinal muscle, followed by a 10 μm-thick layer of circular muscle. The outer pharyngeal epithelium is cuboidal, ciliated, underlain by a longitudinal muscle (7.5 μm thick), followed by a circular muscle (45 μm thick) with interspersed longitudinal fibers; the inner epithelium is flat, ciliated, underlain by a circular muscle (62 μm thick) with interspersed longitudinal fibers. Numerous erythrophil and xanthophil gland cells open and discharge their contents through the distal portion of the pharynx.

The testes are dorsal, located between the intestinal diverticula, with some of them reaching the parenchymal supra-intestinal transverse muscle (Figs 2D , 3A ). The testes are arranged in two paramedian rows. They extend from 1 mm behind the level of the ovaries (30% of body length) to 1.2 mm from the root of the pharynx. Sperm ducts run straight and immediately above the subintestinal parenchymatic muscle. They distally penetrate the anterior region of the common muscle coat (Fig. 4B, C ) to open into the proximal portion of the paired branches of the prostatic vesicle. The prostatic vesicle consists of a proximal half of these paired branches and a distal unpaired half. This vesicle runs postero-dorsally to open into the dorso-anterior section of the male atrium. An ejaculatory duct and penis papilla are absent (Figs 4C , 5A ). The penis bulb is thick and consists of numerous muscle fibers continuous with those underlying the epithelium of the male atrium. The prostatic vesicle is lined with a columnar-to-cuboidal epithelium, underlain by a thin longitudinal muscle (5 μm thick), followed by a 30 μm-thick circular muscle interspersed with longitudinal fibers. The paired portion of the prostatic vesicle receives abundant erythrophil and xanthophil granules from the respective gland cells, while the unpaired portion receives abundant cyanophil and xanthophil granules.

The male atrium is long and narrow with folded walls. The proximal third of the atrium runs postero-dorsally; distal two-thirds runs ventrally almost vertically above the gonopore canal. The atrium is lined by a cuboidal-non-ciliated epithelium, and underlain by an 80 μm-thick circular muscle with interspersed longitudinal fibers. The male atrium is 1.2 × longer than the female atrium.

The ovaries are mature, ovoid, and 200 μm in length. They are located above the ventral nerve plate at a distance from the anterior body tip equal to 25% of its body length (5.5 mm from anterior tip). Ovovitelline ducts emerge from the dorso-lateral aspect of the ovaries, where-after they run backwards above the ventral nerve plate. The ducts bend medially, posteriorly to the female atrium, and subsequently ascend vertically and medially to communicate with each other above the postero-dorsal section of the female atrium (Fig. 5B ). The ovovitelline ducts open directly into a very short female genital canal lined by a cuboidal ciliated epithelium. A small number of small shell gland cells can be spotted around the junction of the two ovovitelline ducts.

The female atrium is funnel-shaped, not folded, and lined by a ciliated columnar-to-cuboidal epithelium, which is surrounded by circular muscle fibers with interspersed longitudinal fibers. This muscle is continuous with the common muscle coat. Most of the abundant gland cells discharging into the female atrium have a fine granular xanthophil and erythrophil secretion. The length:height ratio of the copulatory apparatus enveloped by the common muscle coat is 1.4:1.

Remarks.

On the identity of our specimen. Schultze and Müller (1857) described Geoplana tristriata from near Blumenau, in the State of Santa Catarina. The original description reads: " Geoplana tristriata , pale yellowish-green, with three narrow, dark, longitudinal lines on the back; belly paler. Greatest breadth at approx. the second third part of the length, where the mouth is situated. It likes to bend the head upwards. At the point of curvature on each side there is a closed packed group of eye-spots, which are continued in an irregular series to the posterior extremity. The anterior margin of the head appears to be destitute of eyes. Length 1 1/2 inch [38.1 mm]; breadth 1 1/2 line [3.81 mm]. Abundant."

There is no record of the deposition of the type series. It was probably not preserved. Our specimen was collected 115 km south from the type locality, and matches Schultze and Müller’s species in all characteristics. The conspecificity of our specimen could be questioned since the copulatory apparatus is the most important organ assuring identification in triclads. However, this species combines a set of unusual features among Geoplanins: the color pattern of the body, the cephalic region bent to the dorsal side (actually only found in Choeradoplana and Cephaloflexa among Geoplaninae ), and the absence of eyes in the very anterior tip of the body. Schultze and Müller’s and our specimen sharing these uncommon attributes supports their conspecificity.

The species redescribed herein also matches all diagnostic characteristics of Choeradoplana and should therefore be transferred to this genus. The species is unique in the external aspect in that there are no other species of Choeradoplana with dorsal green color with three longitudinal dark stripes. Internally, Choeradoplana bilix Marcus, 1951; Ch. crassiphalla Negrete & Brusa, 2012; Ch. langi (Dendy, 1894); and Ch. marthae Froehlich, 1955 are similar to Ch. tristriata in the compact aspect of the copulatory apparatus, having a length:height ratio equivalent to 1.8:1 or less, as calculated in drawings ( du Bois-Reymond Marcus 1951; Marcus 1951; Froehlich 1959; Negrete and Brusa 2012). However, Ch. bilix , Ch. crassiphalla and Ch. marthae possess a penis papilla (vs. absent in Ch. tristriata ), and the female atrium of Ch. langi is a narrow canal (vs. a funnel-shaped cavity).

The identity of Graff’s specimen and subsequent taxonomic actions. Herman von Ihering collected one specimen in Taquara, State of Rio Grande do Sul, Brazil, which he identified as a member of this species. Ihering sent it to Graff (1899), who endorsed his identification, but only provided a description of the external aspect. Froehlich (1959) disagreed with Ihering’s identification because of the body shape, the relative position of the mouth, and the different width of the paramedian dorsal stripes. Froehlich concluded that both Schultze and Müller’s and Graff’s species remain obscure ( Froehlich 1959), but did not propose taxonomic changes. Ogren and Kawakatsu (1990) considered Schultze and Müller’s and Graff’s specimens conspecific, and transferred the species to the Pseudogeoplana collective genus, which houses species lacking information about the internal organs, especially the copulatory apparatus. We agree with Froehlich’s opinion that Graff’s species is different from Schultze and Müller’s species. Accordingly, we propose the name Pseudogeoplana aevipandemiae Lago-Barcia & Carbayo, sp. nov. for Graff’s species. The specific epithet means 'from the times of the pandemic’. The epithet alludes to the COVID-19 pandemic and is intended to keep the memory of the negative effects caused by the long months of closure of the laboratories for the conclusion of this paper.

We did not find Graff’s specimen in the museums where part of this collection was disseminated (Naturhistorisches Museum, Basel; Museum of Natural History, Vienna; Senckenberg Museum, Frankfurt; Zoological Museum, Hamburg; Natural History Museum, London). Therefore, we consider Graff’s specimen lost.

Figures

Figure 1. Phylogenetic tree of Choeradoplana obtained from the COI gene by Bayesian inference. Black numbers at the nodes correspond to the posterior probability (only values above 0.99 shown) and blue numbers to Bootstrap support in the tree by maximum likelihood (only values above 95 % shown). See text for details.

Figure 2. Choeradoplana tristriata (Schultze & Mueller, 1857), specimen F 3226 A dorsal view of the live specimen B lateral view of the anterior region of the live specimen C photomicrograph of a transverse section of the cephalic region, showing an eye and a sensory pit (arrowhead) D photomicrograph of a transverse section of the pre-pharyngeal region. 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 3. Choeradoplana tristriata (Schultze & Mueller, 1857), specimen F 3226. Photomicrographs of transverse sections A dorsal portion of the pre-pharyngeal region B ventral region of the pre-pharyngeal region C cephalic region. 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 4. Choeradoplana tristriata (Schultze & Mueller, 1857), specimen F 3226 A photomicrograph of a sagittal section of the pharynx B photomicrograph of a sagittal section of the copulatory apparatus C diagrammatic representation of the copulatory apparatus. 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 5. Choeradoplana tristriata (Schultze & Mueller, 1857), specimen F 3226. Photomicrographs of sagittal sections A prostatic vesicle and the male atrium B female atrium. 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.