Species of Elasmogorgia and Euplexaura (Cnidaria, Octocorallia) from Japan with a discussion about the genus Filigella

Abstract Octocorals with thread-like colony shape have been re-examined, mainly from Japanese waters. The holotypes of Elasmogorgia filiformis and Filigella boninensis and a syntype of Filigella mitsukurii have been studied. Euplexaura arbuscula is identified and Euplexaura yayoii sp. n. described.


Introduction
The octocoral genera Elasmogorgia, Filigella and Thesea have been underexplored and their taxonomic position remains confusing. One of the Japanese species of these genera, Filigella mitsukurii, is classified with three different genera in WoRMS, as Elasmogorgia mitsukurii (Ofwegen 2016a), Filigella mitsukurii (Ofwegen 2016b), and Thesea mitsukurii (Ofwegen 2016c). In this manuscript, a revision is presented of the genera Elasmogorgia and Filigella and their species in Japan, as well as some Japanese species of Euplexaura.
The genus Filigella Gray, 1868 was established to accommodate F. gracilis from Brazil. Later on Wright and Studer (1899) established the Pacific genus Elasmogorgia with the remark that their new species E. filiformis could be identical to Filigella gracilis. Next, Hickson (1905) described Elasmogorgia flexilis from the Maldives, Kinoshita (1909) described Filigella mitsukurii from Japan, Nutting (1912) described Elasmogorgia ramosa, also from Japan, and finally Aurivillius (1931) described Filigella boninensis from the Ogasawara Islands (Bonin Islands), and Thomson and Dean (1931) described Elasmogorgia filigella from Kalimantan (Indonesia). Both Kinoshita and Aurivillius considered Elasmogorgia and Filigella synonymous and Aurivillius doubted whether Elasmogorgia ramosa of Nutting (1912) belonged to one of these two genera. Kükenthal (1919) first treated them as two separate genera but he synonymized them five years later (Kükenthal 1924). Bayer (1959: 17) was the first to include F. gracilis in the genus Thesea Duchassaing & Michelotti, 1860, although he did not directly synonymize the genus Filigella with Thesea, but much later in his key to the octocoral genera, Bayer (1981: 945). However, he did not re-examine six Pacific species referred to Filigella or Elasmogorgia, and therefore the status of these species has remained doubtful.
In the present study, the type material of Elasmogorgia filiformis, E. filigella, Filigella mitsukurii, and F. boninensis, is examined and their previous identifications are discussed. In addition, two specimens identified as E. filiformis by Nutting (1910) and by Thomson and Dean (1931) were examined. Elasmogorgia filigella Thomson and Dean (1931) from Kalimantan clearly does not belong to Elasmogorgia because it has a red colony and also red sclerites. The type specimen of E. filigella (ZMA 2536) appears to consist of a few branch fragments with disintegrated sclerites. It is considered to represent a species of Astrogorgia in the present study.
Finally, a new thread-like Euplexaura species is decribed from the Pacific side of northern Japan, Euplexaura yayoii sp. n., in addition to E. arbuscula Broch, 1935 from off Chishima Is. (Kuril Is.), which previously was reported from the west coast of Kamchatka, Sea of Okhotsk. These two species are both from northern Japan and northeastern Russia (Figure 1).

Material
Material was collected from depths between 38 and 366 m by dredging, trawling or fishing net onboard RV Tansei-maru, University of Tokyo and Japan Agency for Marine-earth Science and Technology and RV Yayoi, the University of Tokyo, during the years 1975-2010. Type specimens and other historical museum material was examined in collections of the BMNH, NBC, UMUTZ, UUZM, ZIN, and ZMUC. From each specimen a small piece of the distal part of a branch was dissolved in a solution of household bleach (4% hypochlorite) to isolate sclerites. The sclerites were washed with demineralised water, dried on a hot plate, mounted on SEM stubs, and coated with Pd/Au for SEM imaging. For this, either a JEOL JSM6490LV scanning electron microscope was operated at high vacuum at 10 kV, or a JEOL JSM6510LA scanning electron microscope with a Quick Carbon Coater SC-701C, SANYU ELEC-TRON was used. For terminology, see Bayer et al. (1983).
Descriptions of old Japanese material collected by Japanese used "hiro" (Japanese fathom) as the depth unit. One Japanese fathom (hiro) is usually 1.43 m, occasionally 1.51 m, whereas, it is 1.818 m for the length unit on land. The old depth unit fathom is also converted to 1.8288 m. When it was not clear whether the collector used fathom or hiro, the converted depth has wider ranges.

Elasmogorgia filiformis Wright & Studer, 1889
ZMA 2537 of Thomson and Dean (1931) is a thread-like colony fragment containing colourless disintegrated sclerites, which were sufficiently recognizable to identify it as a species of Astrogorgia. In a comparison with Astrogorgia bayeri Ofwegen and Hoeksema, 2001, from Sulawesi, the latter species appears to have shorter spindles, up to 0.5 mm long, whereas Thomson and Dean's (1931) specimen has spindles of over 1 mm long. Because of the disintegrated state of its sclerites, no more differences could be ascertained.
ZMA 2538 of Nutting (1910) was also re-examined; it has characters of the genus Euplexaura. Elasmogorgia filiformis of Nutting (1912) is also unlikely an Elasmogorgia.
Diagnosis. Plexauridae with colonies branched in one plane. Calyces may be present but are mostly absent. Polyps with collaret and points, only point sclerites, or no sclerites at all. The surface of the coenenchyme with robust ovals or spindles with complex tubercles; sometimes with one side that is less tuberculate. The interior with rods or small spindles with simple tubercles. All sclerites colourless. Diagnosis. Branches thread-like. Calyces dome-shaped, arranged all around the branches (Figure 2b). Polyps without sclerites. The surface layer of the coenenchyme has spindles and blunt ellipsoids (Figure 5a), up to 0.15 mm long, with complex tubercles. The interior has small spindles, capstans, and a few crosses, up to 0.15 mm long (Figures 5b-d), all with simple tubercles.

Euplexaura arbuscula
Remarks. The material examined was fragmentary ( Figure 2b) and therefore it resembles a species of Elasmogorgia.
Euplexaura abietina Kukenthal, 1909 resembles E. arbuscula regarding its sclerites, but it differs in having polyp spindles. Since its original description, the species was not found again and its type material could not be retraced, hence some doubts remain about the identification of this species. Broch (1935) (Figure 2c). The polyps have points with flattened spindles, up to 0.15 mm long (Figure 6b), with simple tubercles and spiny distal end. Collaret present, with slightly bent, flattened spindles, up to 0.25 mm long, with simple tubercles (Figure 6c). Tentacles with small scales, up to 0.10 mm long (Figure 6a).
The surface layer of the coenenchyme has spindles (Figure 6d) and blunt ellipsoids (Figure 6e), up to 0.65 mm long, with complex tubercles. Several of them with one side less tuberculate. The interior has small spindles and rods, up to 0.25 mm long (Figure 7), with simple tubercles.
The surface layer of the coenenchyme has spindles (Figure 8d-e), up to 0.35 mm long, with complex tubercles. Some of them with one side that is less tuberculate. The interior has small spindles and rods, up to 0.25 mm long (Figure 9), with simple tubercles.
Remarks. Because the sclerites of this species are spindles with complex tubercles this is actually a species of Euplexaura. Kinoshita (1909) mentioned 13 specimens of Filigella mitsukurii and three of them were complete. He used two specimens for his original description. Nowadays two specimens are present in UMUT and the data fit Kinoshita's, two specimens in his description.
The locality name "Jogaschima, Pagamibai" of this species in Kükenthal (1924 Description. The holotype is 2.5 cm high and 5.5 cm wide (Figure 2e). The colony is branched only once, 1 cm above the base. The two branches are very slender, only 1 mm thick; the calyces are low, dome-shaped, arranged spirally around the branches.
The polyps have points with slightly bent, flattened spindles, up to 0.30 mm long, with a few tubercles and a slightly spiny distal end (Figure 10b). The collaret has bent, flattened spindles, up to 0.30 mm long, with simple tubercles, the largest tubercles present in the middle (Figure 10c). The tentacles have flattened rods, up to 0.15 mm long, with hardly any tubercles (Figure 10a). The surface layer of the branches has spindles and blunt ellipsoids, up to 0.15 mm long, with complex tubercles (Figure 11). The deeper layer has short spindles, up to 0.10 mm long, and a few crosses (Figure 10d-e); all with simple tubercles.
Etymology. Named after the research vessel that was used to collect the specimens. Remarks. The live colony has blue-coloured polyps. E. yayoii differs from the two other Japanese Euplexaura species with thread-like branches, E. boninensis and E. mitsukurii, by its very small sclerites.

Discussion
Originally, there were four species of Elasmogorgia: E. filiformis Wright & Studer, 1889, E. filigella Thomson and Dean 1931, E. flexilis Hickson, 1905, and E. ramosa Nutting, 1912. Based on the present re-examination, it is obvious that Elasmogorgia filiformis,  with spindles covered by simple tubercles, is not a species of Thesea. Corals of this genus have coarse rugose plates, sometimes tuberculate spindles and double heads (Bayer 1981). Therefore the genus Elasmogorgia is reinstated here. The only two species from Japan previously recognized as Filigella, i.e., F. mitsukurii and F. boninensis, were reexamined and both proved to belong to the genus Euplexaura. Elasmogorgia filigella from Kalimantan is a species of Astrogorgia. This leaves E. ramosa and E. flexilis unexamined. E. ramosa was collected by the Steamer Albatross at Satamisaki Light, south of Kyushu I., Kagoshima prefecture, Japan, 103 fms (188 m), and E. flexilis from the Maldives. From the descriptions of these two species it is obvious that E. ramosa, with a heavily branched colony, is not a Thesea or Elasmogorgia. Elasmogorgia flexilis, with spindles with complex tubercles probably is a species of Euplexaura, and therefore the genus Elasmogorgia is considered here monotypic with E. filiformis as its only member. E. filiformis mostly resembles a species of Astrogorgia. Following Bayer (1981) we also consider Filigella a synonym of Thesea. All Japanese species previously included in Filigella are assigned to Euplexaura in this study. All Japanese thread-like plexaurid material South of Kashima Sea was previously identified as F. mitsukurii and it clearly is the most common thread-like plexaurid species of Japan.