Paucumara trigonocephala (Ijima & Kaburaki, 1916)

Yang, Hee-Min, Sluys, Ronald, Kawakatsu, Masaharu & Min, Gi-Sik, 2018, New molecular sequences for two genera of marine planarians facilitate determination of their position in the phylogenetic tree, with new records for two species (Platyhelminthes, Tricladida, Maricola), ZooKeys 781, pp. 1-17 : 1

Paucumara trigonocephala (Ijima & Kaburaki, 1916)

Material examined.

NIBRIV0000821277, Sacheon-si, Gyeongsangnam-do, Republic of Korea (35°05'05"N 128°03'14"E), 7 June 2017, coll. H-M. Yang, sagittal sections on 2 slide; ZMA V.Pl. 7279.1, ibid., sagittal sections on 3 slides; V.Pl. 7279.2, ibid., horizontal sections on 1 slide; V.Pl. 7279.3, ibid., transverse sections on 6 slides.

ZMA V.Pl. 6807, shore of Lake Hi-numa, near the Park, Ibaraki-machi, Higashi-Ibaraki-gun, Ibaraki Prefecture, Kantô Region, Honshû, Japan, 13 August 2007, coll. S. Chinone, preserved specimens. ZMA V.Pl. 6810, ibid., 31 August 2007, coll. S. Chinone, preserved specimens.

Comparative description and discussion.

The external features and anatomy of the specimens from South Korea correspond in all essential details to the descriptions of this species published earlier (see Sluys 1989, Sluys and Ball 1990, Sluys and Kawakatsu 2000). Preserved specimens measured up to approx. 3 mm in length and 1 mm in width. In particular, the shape of the body and the position of the eyes in living specimens (Figure 2) are very similar to the situation in Australian specimens, as documented in Sluys (1989) and Sluys and Kawakatsu (2000). The two eyes are set close to the mid-line of the body and positioned at a considerable distance posterior to the anterior body margin.

The shape of the front end of the body is very characteristic: anterior to the eyes the body first narrows to give rise to a kind of “neck” and then widens to form a triangular, obtusely pointed head with broadly rounded auricles (Figure 2). At the level of the auricles there is a broad, creamy-white patch that extends across the body. A similar kind of patch is located immediately behind the eyes, albeit that it does not extend from one lateral body margin to the other but is confined to mid-dorsum. Additional creamy-white spots may be located directly in front of and behind the pharyngeal pocket and at the very tip of the tail. In point of fact, each of the pharyngeal patches may actually consist of two spots situated close together. These pharyngeal spots as well as the one at the tip of the tail were not present in every specimen examined and neither were they reported earlier in the available literature.

In the specimens from South Korea the entire dorsal surface is provided with a brownish pigmentation. The pigment granules are more or less evenly distributed, but accumulations occur in front of the eyes, where there is a broad, transverse band, and in the form of a brown stripe on either side of the pharyngeal pocket and a band of brown pigment running between the eyes. A brownish colouration, on both dorsal and ventral body surface, was described also for specimens from northern Australia ( Sluys and Kawakatsu 2000).

With respect to their anatomy, the South Korean animals exhibit a distinct lens in each of their eyes (Figure 3) and a copulatory apparatus (Figs 4, 5) similar to that documented for animals from other parts of the range of the species (see Sluys and Ball 1990, Sluys and Kawakatsu 2000, and references therein). The penis papilla is a stubby cone. Immediately after having penetrated the penis bulb, the vasa deferentia unite to form a common vas deferens, which communicates with a much broader ejaculatory duct. At its distal, ventral section the latter receives the openings of erythrophilic penis glands. The sac-shaped copulatory bursa fills the entire dorso-ventral space; it is connected with the common atrium by means of a bursal canal that is lined with an infranucleated epithelium. The major portion of the bursal canal is rather wide and irregularly shaped, but the part near its opening into the common atrium is narrow. This lower, proximal portion of the bursal canal receives the separate openings of the oviducts. Unfortunately, in specimens NIBRIV0000821277, ZMA V.Pl. 7279.1, and ZMA V.Pl. 7279.2 we were unable to trace the oviducts and only in the transversally sectioned specimen ZMA V.Pl. 7279.3 did we observe the oviducts separately opening into the bursal canal. The entire bursal canal is covered with a well-developed, subepithelial layer of circular muscle, followed by a thin layer of longitudinal muscle.

According to Sluys (1989), Sluys and Ball (1990), and Sluys and Kawakatsu (2000), the bursal canal receives along its entire length numerous openings of erythrophilic glands, the cell bodies of which are located ectally to the surrounding coat of muscle. These glands are different from the shell glands, which open into the bursal canal ectally to the oviducal openings. In the present material we observed that indeed at places a granular, erythrophilic secretion is discharged into the lumen of the bursal canal. However, this secretion is not easy to detect and apparently in our specimens this situation is much less developed than as described in the literature for other specimens of Paucumara trigonocephala . We were unable to detect shell glands in our animals from South Korea.

Previous records of Paucumara trigonocephala from Japan, Australia, the Bismarck Archipelago and probably Hong Kong were summarized in Sluys and Ball (1988), Sluys (1989), and Sluys and Kawakatsu (2000). We do here document a new locality for Japan, viz., Lake Hi-numa; this is a lake that is connected with the sea and, thus, has a low level of salinity. Histological sections have not been prepared of the preserved specimens available from this locality (see Material examined) but a picture of a live specimen (Figure 6) leaves little doubt about its specific identity. It is noteworthy that this animal also exhibits the whitish patches in the pharyngeal region and at the tip of the tail (see above).

Although the species was probably observed in Hongkong as early as 1857 (see Sluys 1989) our present animals from South Korea represent the first substantiated record of P. trigonocephala from continental Asia. Here, the animals were collected from brackish water with a low salinity level, the bottom consisting of mud with small stones and being devoid of any aquatic vegetation. This habitat is in agreement with the fact that all over its range the animals show the same characteristic ecology, in that they live in low-salinity biotopes, which during ebb tide even may be entirely fresh ( Kaburaki 1922, Sluys and Kawakatsu 2000).

In the molecular phylogenetic trees generated by Souza et al. (2018) their new genus and species Sluysia triapertura Leal-Zanchet & Souza, 2018 is the sister-group of the ectocommensal species Ectoplana limuli (Ijima & Kaburaki, 1916), albeit that this relationship only has low support in their trees. In our tree (Figure 1) the topology has changed slightly in that P. trigonocephala has become the sister-species of E. limuli , while S. triapertura forms part of a group of five species that constitutes the sister-taxon of E. limuli plus P. trigonocephala . That E. limuli and P. trigonocephala share a sister-group relationship conforms with the current taxonomy of the Maricola , in which both species are classified amongst members of the subfamily Ectoplaninae Bresslau, 1933 ( Sluys 1989).