Prosthiostomum hibana, Tsuyuki & Kohtsuka & Kajihara, 2021

Tsuyuki, Aoi, Kohtsuka, Hisanori & Kajihara, Hiroshi, 2021, Fig. 4. A in Saphonecrus globosus Schweger and Tang 2015, Zoological Studies (Zool. Stud.) 60 (29), pp. 1-20 : 11-14

Prosthiostomum cf. ostreae Kato, 1937b ( Fig. 5 )

? Prosthiostomum ostreae Kato, 1937b: 365–366 , pl. 22, figs. 4–5, text-figs. 25–27; Kato 1944: 308; Faubel 1984: 232; Prudhoe 1985: 192.

Material examined: Three specimens, all collected by T. Miura, K. Oguchi, and H. Kohtsuka in Arai-hama (35.1609°N, 139.6105°E), Misaki, Kanagawa, Japan. ICHUM 6151, 4 slides, February 19, 2019; ICHUM 6152, 4 slides, March 25, 2019; ICHUM 6153, 8 slides, March 25, 2019.

Description: Body elongated, tapered posteriorly, 9.0–13.0 mm long and 2.4–3.3 mm wide at its widest point when slightly contracted while alive (n = 3); anterior margin rounded ( Fig. 5A–D ). Tentacles absent. Dorsal surface smooth, light brown, covered with numerous brown maculae ( Fig. 5A ). Brown pigments aggregating mid-dorsally to form posteriorly fading band, which runs from behind brain; non-pigmented specks forming line along midline on brown band ( Fig. 5A ). Body margin translucent. Ventral surface translucent, without color pattern ( Fig. 5B ). Pair of cerebral-eyespot clusters, each consisting of 20–22 eyespots (n = 3); cerebral-eyespot clusters medially approaching each other at three points; anterior end of clusters located at distance of 1.20 mm posterior to anterior margin of body ( Fig. 5C, D ). About 70 marginal eyespots irregularly scattered along anterior margin, extending to half position of brain ( Fig. 5D ). One pair of ventral eyespots present near front end of brain ( Fig. 5D ). Anterior branch of main intestine extending to position 0.52 mm posterior from anterior margin of brain. Plicated pharynx tubular in shape, 4.7 mm in length (about one-third of body), located in anterior half of body ( Fig. 5B ). Mouth situated at anterior end of pharynx, located at 1.53 mm posterior from anterior margin of body. Male gonopore, female gonopore, and sucker closely set on body center ( Fig. 5B ); distance between male and female gonopores being 0.34 mm; distance between female gonopore and sucker being 0.31 mm. Male copulatory apparatus consisting of large seminal vesicle, pair of prostatic vesicles, and armed penis papilla, located immediately posterior to pharyngeal pocket. Pair of spermiducal vesicles, running on each side of midline and curving posteriorly behind penis to separately enter into anteroventral end of seminal vesicle ( Fig. 5E ). Ejaculatory duct with thin muscular layer, entering penis papilla. Prostatic ducts with thin muscular layer, connected to ejaculatory duct separately at proximal end of penis papilla. Pair of spherical prostatic vesicles coated with 0.04-mm-thick, non-nucleated muscular wall, located on both sides of ejaculatory duct ( Fig. 5E, F ). Seminal vesicle oval, coated with 0.04-mm-thick muscular wall ( Fig. 5E, F ). Seminal vesicle (long axis 0.26 mm, short axis 0.17 mm) more than twice as large as prostatic vesicle (0.12 mm in diameter) ( Fig. 5E ) (n = 1). Penis papilla armed with rather thick, pointed tubular stylet (0.13 mm in length; n = 1), enclosed in penis pouch, protruding into male atrium ( Fig. 5E, F ). Male atrium elongated anteriorly from gonopore to penis pouch (0.27 mm in length; n = 1); inner wall slightly ruffled, lined with ciliated and muscularized epithelium ( Fig. 5F ). Female copulatory apparatus posterior to male reproductive system ( Fig. 5G, H ). Cement glands concentrated around vagina and releasing their contents in cement pouch when developed ( Fig. 5H ). Oviducts not observed. Sucker large (0.26 mm in diameter; n = 1), situated immediately behind female reproductive system ( Fig. 5G ), at 7.4 mm anterior from posterior margin of body (n = 1).

Distribution: Arai-hama, Misaki, Kanagawa, Japan.

Habitat: Among branching coralline algae Corallinales spp. and holdfasts of the kelp Eisenia bicyclis subtidally in Misaki.

Sequences: Parital 28S rRNA (1012 bp) and COI gene sequences from two individuals. LC625889 (28S rRNA) and LC625896 ( COI, 585 bp) from ICHUM 6151; LC625890 (28S rRNA) and LC625897 ( COI, 553 bp) from ICHUM 6152.

Remarks: We tentatively identified our specimens as P. cf. ostreae . Kato (1937b) originally described P. ostreae based on three specimens found on cultivated oyster shells from Moroiso, Misaki, Kanagawa, Japan. Unfortunately, the type series of P. ostreae is not extant ( Kawakatsu 2004), and therefore we could not compare our samples to the type series. The dorsal body color pattern, the arrangements of cerebral and marginal eyespots, and the form of male copulatory apparatus in our specimens are largely consistent with those given by Kato (1937b). However, our specimens differ from the original description of P. ostreae by i) the body-margin coloration (translucent in our specimens; partly lemon-yellow in the original description) and ii) the frontal eyespots (absent in our specimens; four eyespots present in the original description). In addition, the pair of the ventral eyespots were confirmed in our materials although they were not mentioned in the original description. Our materials were collected from coralline algae and kelp holdfasts whereas the original specimens were found on oyster shells. Additional data are needed to test whether the two (possibly three) morphological differences between our specimens and the original description represent interspecific or intraspecific ones stemming from the habitat difference.

Figures

Fig. 5. Prosthiostomum cf. ostreae Kato, 1937b, ICHUM 6153 (A, B, H), ICHUM 6151 (C, E–G), ICHUM 6152 (D); photographs taken in life (A– C) and after being cleared in xylene (D), schematic diagram (E), and photomicrographs of sagittal sections (F–H) (anterior to the left); A, entire body, dorsal view; B, entire body, ventral view; C, magnification of head, dorsal view; D, magnification of head, dorsal view, showing ventral eyespots (arrowheads); E, copulatory organs and sucker; F, male copulatory apparatus; G, female gonopore and sucker; H, cement glands. Abbreviations: ce, cerebral eyespots; cg, cement glands; cp, cement pouch; fg, female gonopore; it, intestine; ma, male atrium; me, marginal eyespots; mg, male gonopore; ph, pharynx; pv, prostatic vesicle; spv, spermiducal vesicle; st, stylet; su, sucker; sv, seminal vesicle. Scale bars: A, B = 5 mm; C, D = 1 mm; E–G = 300 µm; H = 100 µm.