Polycladids are a diverse group of marine Flatworms (Bilateran Worms which lack a body cavity) found from litoral (tidal) environments to deep sea hydrothermal vents, though they are most numerous and diverse around Coral Reefs. They typically range from about 3-20 mm in length, with a flattened oval bodyshape, and often have paired tentacles at their front ends. Members of the Family Prosthiostomidae are characterised by (i) an elongated body with a ventral sucker after the female gonopore, (ii) a plicate tubular pharynx, and (iii) paired prostatic ducts, each of which extends from a spherical prostatic vesicle and enters the penis or the ejaculatory duct independently, instead of uniting to each other before the entrance. The Prosthiostomidae is composed of five genera: Enchiridium, Enterogonimus, Euprosthiostomum, Lurymare, and Prosthiostomum. The genus Enchiridium is distinguished from other Prosthiostomids by having a muscle sheath (or bulb) that encloses just the two prostatic vesicles among other male reproductive organs; i.e., the seminal vesicle and the male atrium are not enclosed by the muscle sheath.
In a paper published in the journal ZooKeys on 12 March 2020, Aoi Tsuyuki of the Graduate School of Science at Hokkaido University and Hiroshi Kajihara of the Faculty of Science at Hokkaido University, describe a new species of Enchiridium from Kagoshima and Okinawa.
Three Polyclad specimens were collected subtidally from under rocks on the coast of Bonomisaki in Kagoshima Prefecture and the coast of Nago on Okinawa Island, southwestern Japan. Worms were anaesthetised in seawater containing menthol before fixation. The relaxed worms were photographed with a Nikon D5600 digital camera with external strobe lighting provided by a pair of Morris Hikaru Komachi Di flash units. For DNA extraction, a posterior piece of the body was removed and stored in 99.5% ethanol. The rest of the body was fixed in Bouin’s solution for 24 hours and preserved in 70% ethanol for long-term storage.
Map showing distribution of Enchiridium daidai: point (A) off the coast of Bonomisaki, Kagoshima (type locality); point (B) Nago, Okinawa Island. Tsuyuki & Kajohara (2020).
The new species is named Enchiridium daidai, where 'daidai' means 'orange', in reference to a thin marginal orange line surrounding the entire dorsal fringe. The species is described from three specimens, all collected by Aoi Tsuyuki. One was collected at 13–14 m depth off the coast of Bonomisaki in Kagoshima Prefecture, and the other two were both collected at 5 m depth at Nago on Okinawa Island.
Enchiridium daidai, photograph taken in life and eyespots observed in fixed state after being cleared in xylene. Entire animal, dorsal view (left) and ventral view (right). Abbreviations: fg, female gonopore; mg, male gonopore; op, oral pore; ph, pharynx; su, sucker. Scale bar is 10 mm. Tsuyuki & Kajohara (2020).
The body of Enchiridium daidai is elongate, tapered posteriorly, 28–77 mm long (77 mm in the holotype) and 4.6–14 mm maximum width (14 mm in the holotype) in the living state; the anterior margin is rounded; the mid-point of the posterior margin is acute. Tentacles are absent. The dorsal surface is smooth, translucent, and fringed with a thin marginal orange line. The ventral surface is translucent, without colour pattern. A pair of cerebral-eyespot clusters is present, each consisting of 20–52 eyespots (left 20 and right 23 in holotype); each cluster is of an antero-posteriorly elongated spindle shape. Marginal-eyespot clusters form a single marginal band, extending to position of mouth (about anterior one-eighth of the body length) along margins on both sides; marginal eyespots are abundant along the anterior margin, diminishing posteriorly. Ventral eyespots are absent. The intestine is highly branched, spreading all over body. A plicated pharynx is tubular in shape, about one-fifth of the body length, and located in the anterior one-third of the body. The oral pore is situated at the anterior end of the pharynx, behind the brain. The Male gonopore and female gonopore are closely set, both situated behind the posterior end of pharynx. The male copulatory apparatus consists of a large seminal vesicle, a pair of prostatic vesicles, and an armed penis papilla. The antero-posterior length of the seminal vesicle is more than twice as long as the diameter of each prostatic vesicle. Spermiducal vesicles form a single row on each side of the midline, separately entering into seminal vesicle. An ejaculatory duct with a thick muscular layer, enters the penis papilla. Prostatic ducts with muscular layer are connected to the ejaculatory duct separately at the proximal end of penis papilla. A pair of spherical prostatic vesicles is coated within thin non-nucleated muscular wall, arranged anterodorsally to the ejaculatory duct. A common muscular sheath encloses the two prostatic vesicles. The seminal vesicle is oval, coated with a thick muscular wall, narrowing anteriorly and forming the ejaculatory duct; the latter almost immediately penetrating the common muscular sheath. The penis papilla is armed with a pointed tubular stylet, enclosed in a penis pouch, and protrudes into the male atrium. The male atrium is elongated anteriorly, and lined with a ciliated, muscularised epithelium. The female reproductive system is immediately posterior to the male reproductive system. Cement glands are numerous, concentrated around the vagina and release their contents into a cement pouch. The vagina curves anteriorly, leading to two narrow lateral branches of uteri. Each branch of uteri turns laterally and then runs backwards. The Lang’s vesicle is absent. A sucker is set on the body centre.
The specimens from Kagoshima and Okinawa differed in body size. The holotype from Kagoshima was 77 mm long and 15 mm wide, whereas the paratype specimens from Okinawa were 28–37 mm long and 4.6–7.4 mm wide. In spite of the noticeable difference in body size, specimens from Kagoshima and Okinawa, all having reached sexual maturity, were identified as conspecific. They shared the following morphological characteristics: (i) a body dorsally fringed with a thin orange line, (ii) a marginal-eyespot band extending to the position of the mouth (about anterior one-eighth of the body), (iii) two prostatic vesicles covered by a common muscle sheath, and (iv) common muscle sheath penetrated by ejaculatory duct. In addition the proportion of nucleotide sites at which two sequences being compared were different was very low, indicative of being the same species.
Difference in mature body size among Enchiridium daidai. (A) ICHUM 5993 (holotype), from Kagoshima, (B) ICHUM 5995 (paratype), from Okinawa, (C) ICHUM 5994 (paratype), from Okinawa. Scale bar 10 mm. Tsuyuki & Kajohara (2020).
Reaching 77 mm in body length, Enchiridium daidai is the largest species in the genus, superseding Enchiridium punctatum (about 40 mm in body length). Indeed, Enchiridium daidai is the second largest species in the Prosthiostomidae after Prosthiostomum cyclops, which reaches 90 mm. Among about 80 species of Prosthiostomids, only Enchiridium daidai and Prosthiostomum cyclops are known to exceed 70 mm in body length, while most of the other species are less than 30 mm long. Therefore, Tsuyuki & Kajohira's new species is considered to be unusually big in body size for a Prosthiostomid.
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