Oregoniplana geniculata, 2019

OREGONIPLANA GENICULATA LI & WANG , SP. NOV.

http://zoobank.org/ urn:lsid:zoobank.org:act:88E6DE08-6C98-4B4D-892D-FC2F5D5CECEA

Material examined

Holotype: China, Guangdong Province, eastern Shenzhen City, Xichong beach, 22°28′N, 114°31′E, temperature of the water was 26–27 °C and its salinity 19‰ –20 ‰, 29 April 2017, coll. Wei-Xuan Li, PLA-Or001, sagittal sections on three slides. GoogleMaps

Paratypes: China, Guangdong Province, eastern Shenzhen City, Xichong beach, 22°28′N, 114°31′E, 29 April 2017, water temperature: 26–27°C, salinity: 19‰–20‰, coll. Wei-Xuan Li, PLA-Or002, horizontal sections on two slides; PLA-Or003, horizontal sections on one slide; PLA-Or004, whole mount on one slide; PLA-Or005, whole mount on one slide; PLA-Or006, whole mount on one slide; PLA-Or007, whole mount on one slide; PLA-Or008~PLA-Or012, whole mounts on five slides; PLA-Or013, sagittal sections on three slides; PLA-Or014, sagittal sections on three slides. All material is deposited in the Institute of Zoology, Chinese Academy of Sciences ( IZCAS) GoogleMaps .

Etymology

The specific epithet is derived from the Latin adjective geniculatus, like the bent knee, alluding to the highly curved penis papilla.

Diagnosis: Oregoniplana geniculata is characterized by the following features: brown pigmentation on the entire dorsal body surface; a pair of large, prepharyngeal testes; absence of a stromatic sac attached to the ovaries; a long and slender penis papilla that is attached to the anteroventral wall of the male atrium, from where the papilla turns towards the anterior end of the body and then abruptly recurves, exhibiting a sharp U-turn in posterior direction; a horizontal female genital duct, communicating with a ventral gonopore and a secondary gonopore. Molecular diagnosis: this species comprises all individuals that are identified as a single evolutionary unit together with individuals from this study (18S rDNA GenBank Accession numbers MK140757 View Materials – MK140759 View Materials ; 28S rDNA GenBank Accession numbers MK140760 View Materials – MK140762 View Materials ), when using a coalescence-based molecular method for species delimitation.

Description

The body shape varies from elongate to ovatelanceolate, with a rounded front end and hind end ( Figs 3A–C, 4A). In elongated state, their length ranges between 1600 and 2500 μm and their width between 290 and 510 μm (N = 5). Brownish pigmentation over a cream-colour ground is present on the whole dorsal body surface, while accumulations of pigment occur along the body margin and around the testes and ovaries ( Fig. 3A–C).

A pair of black, semilunar eyes is located at a considerable distance from the anterior body margin, i.e. at about 1/5 of the total body length as determined from the anterior margin. The eyes are placed rather close to the body margin, the mean distance to the margin being 87 ± 9 μm (N = 5), while the distance between the eye cups ranges from 67 to 171 μm (N = 5). The eyes are located more or less dorsally to the brain and contain about 5 – 6, retinal cells ( Fig. 5A).

The cylindrical pharynx lies fully in the posterior half of the body ( Figs 3C, 4A) and measures 1/6–1/5 of the total body length; it measures about 365 ± 34 μm in length and 84 ± 8 μm in width (N = 5).

The outer epithelium of the pharynx is underlain by a thin subepithelial layer of circular muscles, followed by a thick layer of longitudinal muscle fibres. The inner pharynx epithelium, which is densely ciliated, is underlain by a thin subepithelial layer of longitudinal muscle fibres, followed by a thick layer of circular muscles. The mouth opening is located at a distance of about 1/4 of the body length as measured from the posterior body margin; the mouth is situated at the posterior end of the pharyngeal cavity.

The anterior branch of the intestine extends anteriorly to the eyes and gives off a pair of pre-ocellar diverticula. The posterior branches extend backwards into the tail region of the body, each of the gut trunks giving rise to 13 – 15 short, lateral diverticula; the posterior trunks do not communicate in the hind end of the body.

A pair of large, ellipsoidal testes is situated immediately in front of the pharynx and directly posterior to the ovaries ( Figs 3A–D, 4A). The testes’ follicles occupy the entire dorsoventral space and measure about 258 ± 66 μm in anteroposterior direction and about 83 ± 39 μm in width (N = 5). Narrow vasa deferentia arise from the posteroventral side of the testes and enlarge to form spermiducal vesicles at about the middle of the pharyngeal pocket, with a diameter that is about 33 ± 3 μm (N = 5). These spermiducal vesicles then extend posteriorly along either side of the pharyngeal pocket and fuse to form a common vas deferens immediately behind the pharyngeal pouch ( Figs 5E, 6).

The common vas deferens opens into an irregular egg-shaped seminal vesicle that is situated at the base of the penis papilla and, subsequently, communicates with the ejaculatory duct ( Fig. 3E). The latter is lined with a nucleated epithelium and surrounded by a well-developed layer of circular muscles, the duct gradually becoming very narrow before opening at the tip of the penis papilla.

The penis papilla is a long and slender cone with a unique orientation:its root is attached to the anteroventral wall of the male atrium, from where the papilla turns towards the anterior end of the body and then abruptly recurves, exhibiting a sharp U-turn in posterior direction above its own root; hereafter the distal portion of the penis papilla shows an oblique, ventrocaudal orientation, its tip more or less pointing into the ventral gonopore ( Figs 4C, 5E, 6). A thin flap of tissue originates from the ventral atrial wall immediately posterior to the root of the penis papilla. This flap projects into the male atrium, thereby following the same course as the basal portion of the penis papilla ( Figs 5E, 6).

At the level of the ventral gonopore, which is located at a distance of 1/19 of the body length as determined from the posterior margin, the male atrium communicates also with a horizontally oriented female genital duct that runs to the posterior body end, where it opens to the exterior via a secondary, caudal gonopore ( Figs 4D, 6).

A pair of sickle-shaped, as well as pear-shaped, ovaries is situated at some distance behind the brain, i.e. at 1/3 to 1/2 of the distance between the brain and the root of the pharynx, the mean distance being about 262 ± 72 μm (N = 6); their length (in anterior– posterior direction) ranging between 77 and 135 μm, and their width between 46 and 58 μm (N = 5; Fig. 4B). No stromatic, sac-shaped structure was found in at least ten specimens examined. The oviducts arise from the anterolateral surface of the ovaries and continue their course on either side of the pharyngeal pocket.

The oviducts turn medially at the level immediately behind the ventral gonopore to open separately into the narrow, duct-like female atrium ( Figs 5B, 6). The latter receives the openings of shell glands and the separate oviducal openings, after which it communicates with the female genital duct that leads to the caudal gonopore ( Figs 5C, E; 6).

Feeding and reproduction: When feeding, the thin pharynx of O. geniculata extends through the mouth opening and sucks up the soft tissues of the prey item. The worms were not able to consume either untreated pork or liver, or even to prey on live, untreated Dugesia specimens; O. geniculata was only able to consume these prey items when the tissues were artificially softened, which suggests that the pharynx is rather weak and that in nature the species depends on the availability of appropriate prey.

When not starved, copulations may occur, after which one cocoon may be laid after 1 – 2 days. The flat side of the hemispherical, brown-red cocoon is attached tightly to the substrate; it is devoid of a pedicel and measures about 238 ± 4 μm in diameter (N = 5). Only one young worm hatches from each cocoon ( Fig. 3F–G).

Habitat, ecology and distribution: The animals were collected at the outflow of a freshwater river or spring onto a shallow beach and thus the habitat was brackish water. From the same locality we obtained also the marine triclads Nerpa fistulata and Paucumara falcata Li & Wang, 2019 , and also species from the suborder Kalyptorhynchia Graff, 1905 (see: Zhong et al., 2018), as well as many other unidentified flatworm species. Vegetation type and rainfall at this locality varies between the seasons. Alternations between the rainy season and dry season underly great variation in salinity, ranging between 15‰ to more than 30‰; the specimens of Oregoniplana geniculata are resistant to these great variations in salinity. Two plants are dominant during the various seasons, viz. Ulva lactuca and Saccharina japonica (Areschoug) Lane, Mayes, Druehl & Saunders. It is noteworthy that the marine triclad species composition is related to the dominant type of plant. In the season suitable for the growth of U. lactuca , great numbers of Paucumara falcata , an as yet unverified species of Pacifides Holmquist & Karling, 1972 and Oregoniplana geniculata are present. But when the season changes and becomes suitable for S. japonica , the only marine triclad species to be found is Nerpa fistulata .