Discocelis persica Maghsoudlou & Rahimian, 2013, sp. nov.

Discocelis persica sp. nov.

(Figures. 2–6)

Material examined and locality. Holotype: One mature specimen with the anterior half mounted on a glass slide and the posterior half containing the copulatory apparatus as series of sagittal sections (ZUTC Platy.1246).

Paratypes: One immature specimen with the anterior half as mounted and the posterior half, contains the copulatory apparatus, as a series of sagittal sections (eight slides) stained with haematoxylin and eosin (ZUTC Platy. 1247); plus one entire immature specimen (ZUTC Platy.1248) preserved in ETOH 70%.

Etymology. The specific name refers to the Persian Gulf, where the specimens of the species were collected.

Type locality. Three specimens were collected, in March 2011, during the low tide, from shallow rocky shores of Baghestan station, Persian Gulf; 27° 09' 02" N, 56° 07' 32.2ʺ E (Fig. 1 station 1).

Description. External morphology. Holotype 2.5 × 1.9 cm in size; average size of paratypes 2.8 × 1.8 cm; body broadly oval, anteriorly rounded, somewhat tapering posteriorly, thick and fleshy; coloration yellowish brown or beige dorsally, becoming darker medially around the pharynx, and translucent toward the margin (Fig. 2 A); ventrally the color is light beige, more or less translucent, revealing the internal structures, e.g. pharynx and vasa deferentia. Tentacles absent. Tentacular eyes (Fig. 3 B) visible on a grayish background, as two converge half-crescent shaped clusters, 1.2 mm apart, each 0.4 mm long, with 23–25 eyespots in each cluster; cerebral eyes (Fig. 3 B) more difficult to observe in living specimens (embedded in epidermis) but readily observable in cleared specimens, immediately behind the cerebral organ and just anterior to the pharynx, in the form of two clusters each subdivided into anterior and posterior groups in relation to the cerebral organ, anterior group with more eyespots (50–52 eyespots in each cluster) than the posterior one, extending to a length equal to the distance between the two tentacular eye clusters, posterior groups with 25–30 eyespots in each cluster. Narrow band (120 µm thick) of small and irregular eyes, in two or three rows (Fig. 3 A) located around the anterior third of the body margin.

Digestive system: Ruffled pharynx occupies the central-anterior part of the body (Figs. 2 B, 3A), with 13 to 15 pairs of folds, somewhat larger posteriorly, in sagittal view the most posterior folds of the pharynx extend ventrally to the first male atrium lobe. Pharynx begins immediately behind the cerebral eye clusters, 5.0 mm from the proximal and 8.3 mm from distal body margins (Fig. 5 B). Pharynx length 7.7 mm, width, at the center, 2.1mm. Mouth located at the posterior end of the pharyngeal cavity, 7.6 mm from the distal body margin (Figs. 2 B, 3A).

Epidermis and body wall: The average thickness of the ventral body wall 42 µm, but it is thinner around the male complex (23µm) and thicker (61 µm) around the female system (Fig. 5 A); the cellular epidermis is without rhabdites. The arrangement of muscle fibers beneath basement membrane is as follows: longitudinal muscle layer (5.8 µm), with well-developed circular fibers (14 µm), the inner most layer composed of longitudinal and diagonal muscular fibers (10 µm). Beneath the muscular layer, in the parenchyma, distinct granular pigmentation are distributed; numerous transverse muscle fibers present (Fig. 4 A).

Dorsal body wall 47 µm thick (Fig. 4 B), with ciliated cellular epidermis, intra-epithelial nuclei and numerous rhabdites; the body wall musculature consists of a: longitudinal layer (7.5 µm), a combination of circular and diagonal muscle fibers (13.5 µm).

Male copulatory complex: The male copulatory apparatus is situated immediately posterior to the pharynx, anterior to the common gonopore. It consist of muscular lobed penis with prostatoid organs in the male atrium; the common gonopore ascends dorsally while curves anteriorly (Figs. 5 A & 6) to enter the male atrium which is bilobed in ventral view (Fig. 3 A). In sagittal view, male atrium is long, without villus-like projections. Muscular lobed penis with three to four lobes, descend from dorsal wall into the long male atrium: one anterior lobe, curves slightly ventro-posteriorly over the mouth and the most posterior fold of pharynx, with prostatoid organs on both sides of the anterior tip; one or two lateral lobes with numerous prostatoid organs; one posterior lobe with prostatoids that are arranged centrally in three rows. Behind the penis the male atrium is subdivided into two branches, one cure posteriorly, parallel and dorsal to vagina, other exit by a slanting route to a common gonopore (Figs. 5 A, 6). The distance between anterior lobe and common gonopore, or the penis length, is about 2.2 mm. Prostatoids cover distally all penis lobes, in the form of crescent cluster, except the posterior lobe which they are arranged as half-crescent cluster. They have two different sizes with faintly eosinophilic content, anterior prostatoides are oval to tear-shaped and slightly larger, 67× 47 µm, than the other prostatoides (Figs. 4 D, 5A, 6), they are filled with fine, darkly staining matrix and with nuclei scattered in the matrix (Fig. 4 D); prostatoid organs are surrounded by muscle fibers (Fig. 4 D), their narrow distal ends pass through the epithelia lining and empty into the atrium wall; numerous extra-vesicular glands are scattered around the prostatoid organs.

The vasa deferentia are visible in ventral views (Figs. 2 B, 3A), posterior to the pharynx they extend about 5.2 mm anteriorly to level of the posterior third of the pharynx, then curve and extend posteriorly while constantly decreasing in diameter, then finally curve anteriorly, about 2.3 mm (Figs. 2 B, 3A), arrive dorsally at the male atrium; in sagittal view they run posteriorly and unite to form a common vas deferens. The distal portion of the vas deferens has dilated, 480 µm×90 µm in size, for storage of sperm, this portion has thin and eosinophilic wall and may be considered as spermiducal vesicle (Figs. 5 A, 6B). The spermiducal vesicle enters the ejaculatory duct which is connected in the middle of lobed penis to the male atrium (Figs. 5 A, 6A). The male atrium opens to a common gonopore about 100 µm in diameter. Testes are arranged ventrally (Fig. 4 C)

Female reproductive apparatus. The female system is located posterior to the male system, with a horseshoeshaped Lang’s vesicle. The female duct narrows before opening in the common genital atrium (Figs, 5A, 6A), from here the female apparatus extends posteriorly in form of the vagina externa which ascends, with a slope parallel to the posterior branch of the male atrium, while becomes wider before it turning into the vagina media, which is surrounded by eosinophilic cement glands, the vagina media ascends in the form of a dorso-anteriorly curve to form vagina interna. The vagina interna after a short distance, with a descending gentle slope receives the common oviduct, it then continues as the duct of Lang’s vesicle which descends ventrally, entering into the Lang's vesicle (Figs. 5 A, 6A). The distance between the gonopore and the entrance of Lang’s vesicle is about 1.2mm; the inner wall of the vagina is lined with elongate ciliated epithelial cells underlain by muscle fibers. Cuboidal epithelia, supported by muscle fibers, cover the duct of Lang’s vesicle. The Lang's vesicle is surrounded by a thin layer of muscle fiber (Fig. 6 A). Ovaries are distributed more or less dorsally.

Remarks. The genus Discocelis Ehrenberg, 1836, with species found worldwide in tropical and temperate waters, is considered the largest genus within the Discocelidae family (Newman & Cannon 2005). Faubel (1983) in his systematic revision of the Polycladida, reduced the number of valid species of Discocelis to four: D. tigrina (Blanchard, 1847) Lang, 1884; D. japonica Yeri and Kaburaki, 1918; D. pusilla Kato, 1938 and D. fulva Kato, 1944. Later, Beveridge (2000) introduced a new species, D. parvimaculata.

D. tigrina is the type of the genus and classically has been described as an endemic species of the Mediterranean (Hofrichter 2005; Novell 2001). However, observations in the North Atlantic Ocean have revealed a broader distribution for this species (Frédéric & Michel 2010). Other Discocelis species separated from D. tigrina only based on a limited number of characters including the number and arrangement of the prostatoid organs, direction of the male complex in relationship to the longitudinal axis of the body and the presence of muscular folds (villus-like projections) in the male atrium.

Differential diagnosis. A list of morphological and histological character variations for all of the aforementioned Discocelis species, along with relevant references, is presented in Table 1. Based on those characteristics, the present species differs from D. tigrina, and D. fulva in terms of cerebral eyes pattern and the position of the mouth. In the present species cerebral eyes form two clusters which are subdivided into two groups: anterior and posterior in relation to the cerebral organ, while in D. tigrina, and D. fulva cerebral eyes are in a single elongate group. In the species described here mouth is located at the posterior end of pharynx, while it’s located in the middle of pharynx in D. fulva and D. tigrina similar to the position of the mouth in D. japonica.

The present species also is distinguished from D. japonica, D. fulva, and D. pusilla, based on the absence of villus-like projections (muscular folds according to Bulnes, 2010) in the male atrium (Figs. 12 B–D) of the Persian Gulf species. In the present species numerous prostatoid organs are distributed in the male atrium which is not the case in D. fulva and D. pusilla. These species based on Figure. 2 of Kato (1944) for D. fulva, and Figure. 3 of Kato (1938) for D. pusilla, show a few scattered prostatoid organs. The structures of the male atrium of these species are also available in redrawn Figs. 14 & 15 of Beveridge (2000) and are reproduced here (Figs.12 C, D).The number of posterior cerebral eyes in our species, with 30 eyespots in each cluster, clearly differs from D. japonica, with 15–16 eyespots and D. pusilla, with one or two eyespots.

The specimens described here differ from D. australis and D. tigrina in the shape of the male atrium. The male atrium is in the shape of a simple semicircle in D. australis and D. tigrina (Figs.12 A, F), while in the present species the male atrium is approximately vertical with four distinct lobes in sagittal sections (Fig. 5 A). D. australis also differs with species described here in having single elongate cerebral eye clusters, few prostatoid organs, equal size of prostatoids and the presence of muscular seminal vesicle (Beveridge, 2000; Hyman, 1959). As will be discussed D. australis is a valid member of the genus Thalamoplana, and therefore combination of T. australis (Hyman, 1959) Faubel, 1983 is correct.

The species presented here lacks a distinct color pattern (Fig, 2A), which separates it from all the other congeners except D. fulva. Other species of Discocelis have a color pattern of a pale reddish, pale brown, vinaceous cinnamon or fawn background with numerous black or dark brown spots (Beveridge 2000; Yeri & Kaburaki 1918).

The present species, therefore, resembles most closely to D. parvimaculata, but with some obvious differences, including the marginal eyes which extend anteriorly in our described species (Fig. 3 A), while they extend to the posterior end of body in D. parvimaculata; the posterior cerebral eyes numbering 5-20 in D. parvimaculata, while in the present species they number 30 in each cluster (Fig. 3 B); the anterior tip of the male atrium in our specimen has prostatoids on either sides (Figs. 5 A, 6A), while in D. parvimaculata the anterior tip of the male atrium has no prostatoids on the ventral side, based on Fig. 6 of Beveridge (2000) for D. parvimaculata, the anterior lobe of the male atrium does not pass the mouth dorsally, while in our species the anterior lobe passes the mouth dorsally (Figs. 5 A, 6). Based on Figs. 12 B & 12D, the anterior lobe of the male atrium does not pass the mouth in D. japonica, D. fulva and D. pussila as well.

The two types of prostatoid in D. parvimacula, one with eosinophilic content and the other with basophilic content, separate this species from our species which has faintly eosinophilic prostatoids (Figs.6, 4 D).

The vagina morphology also varies in the different species of Discocelis (Fig.12). Based on illustrations presented in both plate 13 Figure 1 of Lang (1884) for D. tigrina, Figure 6 of Beveridge (2000) for D. parvimaculata, Figures 14–17 of Beveridge (2000), Figures 12 C–E of Bulnes, (2010), the vagina morphology of our species is similar to that of D. australis and differs from that in all other congeners. In the two aforementioned species vagina have two dorsal and ventral curves before entering the oviduct or Lang's vesicle, while other Discocelis species has one ventral and dorsal curve in their vaginae (Figs. 5 A, 6A).