Paraplehnia seisuiae, Oya, Yuki, Kimura, Taeko & Kajihara, Hiroshi, 2019

Oya, Yuki, Kimura, Taeko & Kajihara, Hiroshi, 2019, Description of a new species of Paraplehnia (Polycladida, Stylochoidea) from Japan, with inference on the phylogenetic position of Plehniidae, ZooKeys 864, pp. 1-13 : 1

Paraplehnia seisuiae sp. nov. Figures 1, 2

Etymology.

The specific name is a noun in the genitive case and taken from the TRV Seisui-maru.

Material examined.

One specimen: holotype, ICHUM 5345, 44 slides (14 slides for the anterior part and 30 slides for the posterior part of the body), dredged from 298-310 m depths, the Sea of Kumano, between 34°08.0'N, 136°37.8'E to 34°07.8'N, 136°37.9'E, Japan.

Description.

Live specimen 26 mm in length, 11 mm in width. Body thick, elongate, oval, narrow toward posterior end (Fig. 1A, B). Anterior and posterior ends pointed. Body ground color translucent to whitish opaque. General appearance of body light brown. Dorsal body without any pattern. Body margin translucent. Tentacles lacking. Pharynx, ruffled in shape, 7.4 mm in length, located at center of body. Mouth opening at center of pharyngeal cavity (Fig. 1B). Intestine highly branched and not anastomosing, spreading throughout body except margin. Pair of sperm ducts and oviducts whitish, visible through ventral surface. Male and female gonopores separate; male gonopore opening at 9 mm from posterior end; female gonopore situated 2.5 mm posterior to male gonopore.

Marginal and cerebral eyespots small and embedded in parenchyma (Fig. 1C, D). At least 47 and 28 eyespots arranged in anterior body margin and from just behind brain to anterior to brain, respectively, but detailed distribution of eyespots could not be observed.

Male copulatory apparatus located posteriorly to pharynx, consisting of pair of spermiducal bulbs, prostatic vesicle, and penis papilla (Fig. 2 A–E). Distal end of each sperm duct forming oval spermiducal bulb, latter having thick muscular wall (Fig. 2A). Distal end of each spermiducal bulb slender and separately connecting to neck of prostatic vesicle (Fig. 2 B–D). Prostatic vesicle pear-shaped, having strong muscular wall occupying its proximal one-third, distally coated with connective tissue and enclosed by muscular bulb (Fig. 2F). Canals of extra-vesicular gland penetrating prostatic-vesicle wall. Glandular epithelium with numerous teardrop-shaped cells folded in prostatic vesicle. Ejaculatory duct lacking; distal end of prostatic vesicle directly forming a part of penis papilla. Penis papilla large, conical, and projecting posteriorly into male atrium. Male atrium lined with thin, non-ciliated epithelium.

Pair of oviducts forming common oviduct, which run postero-dorsally to enter vagina (Fig. 2E). From this point, elongated duct of Lang’s vesicle, lined with ciliated epithelium, running posteriorly. Lang’s vesicle sac-shaped, lined with squamous cells, positioned posterior to female gonopore. Vagina lined with smooth ciliated epithelium, running antero-dorsally, curving postero-ventrally as it becomes slenderer, turning postero-dorsally as it becomes wider, eventually leading ventrally to exit at female atrium (or vagina externa). Medial part of vagina surrounded by numerous cement glands (Fig. 2E). Female atrium large, folded, with thick basement membrane opening at female gonopore.

Genital pit with smooth epithelium and basement membrane similar to those in vagina (Fig. 2G), located between male and female gonopores (Fig. 2F, H).

Habitat.

Judging from the nature of the dredged material, the sediment type of the species’ habitat is likely to be sandy mud.

Molecular phylogeny.

The resulting BI and ML trees were almost identical to each other in topology. Paraplehnia seisuiae sp. nov. was nested in a clade composed of stylochoids except Koinostylochus elongatus and Pseudostylochus obscurus (Fig. 3); the latter two appeared to be more closely related to leptoplanoids than to stylochoids, as indicated by Tsunashima et al. (2017). The majority of stylochoids except Callioplana marginata , Koinostylochus elongatus , and Pseudostylochus obscurus formed a clade which also included Stylochus sp. of Bahia et al. (2017) and was supported by 0.99 BI posterior probability and 73% ML bootstrap (Fig. 3). Given Bahia et al.'s (2017) generic identification of Stylochus sp., this clade can be regarded as representing the “true” Stylochoidea, because Stylochus is the type genus for this family-group name. While Paraplehnia seisuiae sp. nov. appeared as sister to Hoploplana spp., its supporting values were low (0.64 BI posterior probability; 27% ML bootstrap). The inter-family relation of Plehniidae among Stylochoidea was thus not fully resolved in the present study.

Remarks.

In this paper, we follow the classification system by Prudhoe (1985), in which Plehniidae consists of five genera ( Discocelides , Myoramyxa , Nephtheaplana , Paraplehnia , and Plehnia ); for Faubel’s (1983) system, see Discussion below. Hyman (1953) characterized Paraplehnia as possessing a prostatic vesicle that has i) a strong muscular wall in its proximal end and ii) a reduced glandular part. Paraplehnia seisuiae sp. nov. is characteristic of the genus by possessing these characteristics. By these characters, our new species cannot be placed in Plehnia sensu Prudhoe (1985), because the latter is diagnosed to have a prostatic vesicle whose proximal end is not particularly thick-walled. Paraplehnia seisuiae sp. nov. differs from Discocelides and Myoramyxa in that it does not have a vaginal duct (ductus vaginalis) and from Nephtheaplana in that our specimen has a pair of spermiducal bulbs.

Paraplehnia has contained two species, P. japonica (Bock, 1923) and P. pacifica (Kato, 1939), and both were originally described from the sublittoral zone in Japan. Paraplehnia seisuiae sp. nov. can be distinguished from the two congeners by the thickness of the muscular wall of the prostatic vesicle (about one-third of the prostatic vesicle in P. seisuiae sp. nov.; about one-half in P. japonica and P. pacifica ), the presence/absence of a common duct between spermiducal bulbs and prostatic vesicle (absent in P. seisuiae sp. nov.; present in P. japonica and P. pacifica ), and the presence/absence of a genital pit between the male and the female gonopores (present in P. seisuiae sp. nov.; absent in P. japonica and P. pacifica ) (Table 2). In addition, P. seisuiae sp. nov. differs from P. japonica by the length of the Lang’s duct (elongated in P. seisuiae sp. nov.; short in P. japonica ) and from P. pacifica by the range of developed connective tissues in the female copulatory apparatus (from the female atrium to around the female gonopore and the genital pit in P. seisuiae sp. nov.; only around the female atrium in P. pacifica ).

The eyespots were invisible in the living specimen (Fig. 1A), probably because of the thickness and opaqueness of the body, as well as the small size of each eyespot. We noticed the presence of eyespots only after sectioning (Fig. 1C, D). Bock (1923: 3) also remarked for Paraplehnia japonica that eyespots were undetectable in the living specimens and became apparent only after histological sectioning. Because we failed to observe the arrangement of eyespots from dorsal view in intact body, we had to categorize each eyespot into marginal ones or cerebral ones according to the relative position from the body margin and the brain.

It is for the first time that a genital pit (or genital sucker) was found in a species of plehniid. Among Acotylea, genital pits have been known in Itannia ornata Marcus, 1947 ( Hoploplanidae Stummer-Traunfels, 1933), three species of Leptoplana ( Leptoplanidae Stimpson, 1857) ( Gammoudi et al. 2012), and Persica qeshmensis Maghsoudlou, Bulnes, & Rahimian, 2015 ( Pleioplanidae Faubel, 1983). Genital pits in I. ornata are present in a pair, situated on both sides of the female gonopore ( Marcus 1952). On the other hand, a single genital pit is present between the male and female gonopores in three Leptoplana species and Persica qeshmensis , as well as in Paraplehnia seisuiae sp. nov. (Fig. 2H, I).