Scolanthus armatus (Carlgren, 1931)

Izumi, Takato & Fujita, Toshihiko, 2018, Description of three new species of Scolanthus (Cnidaria, Anthozoa, Actiniaria, Edwardsiidae): first records of the genus from Japan, ZooKeys 794, pp. 1-21 : 1

Scolanthus armatus (Carlgren, 1931) Figs 2, 6A, 7 A–E

Edwardsia armata Carlgren, 1931: 2, figs 1-2; Carlgren 1949: 24

Scolanthus armatus : England 1987: 229, figs 13-14.

Material examined.

NSMT-Co 1609, histological sections (7 slides), dissected tissues, tissues embedded in paraffin, and prepared nematocysts (6 slides), 27 June 2014, Seihyo Coast (Fig. 1 A– 2), Chichijima Island, Ogasawara Islands, Tokyo, Japan (27°09'47"N, 142°20'26"E), coral sand, 3 m depth, collected by scuba diving with a shovel and a sieve, by Takato Izumi.

Description.

External anatomy. Column rough, ca. 40 mm in whole length, and ca. 5 mm in width, worm-like form, and the proximal part narrower to some extent. The column consists of capitulum, scapulus and scapus. The most proximal part of column capitulum, distinct, extremely short, whitish and semitransparent, but scapulus and scapus indistinct. The periderm-like cuticle, brownish orange with no pattern in color, covering the whole column except capitulum and tentacle but easily stripped off from epidermis (Fig. 2A). The scapus beneath periderm semitransparent, with scattered small prominent nemathybomes but no papillae (Fig. 2A). Aboral end of the column tapered or flattened, not differentiated from scapus, with scattered nemathybomes (Fig. 2, A and 2I). No pedal disk, and no physa or physa-like structure. Tentacles in two cycles, 16 in number, eight in inner and eight in outer cycle (Fig. 6A). All tentacles long and slender, 1.5-2.0 mm in length, the inner tentacles as long as outer ones, transparent or semi-transparent, and capitated on their tip (Fig. 2B). Oral disk ca. 1.5 mm in diameter.

Internal anatomy. Eight perfect mesenteries, all macrocnemes. Four dorsal and ventral directives, and four lateral mesenteries not-paired with other macrocnemes, arranged in normal Edwardsia pattern (Fig. 6A). All macrocnemes present along whole length of the body from oral to aboral end, and bearing distinct retractor and parietal muscles. The retractor muscle of lateral mesenteries all ventrally facing (Fig. 6A). Eight tiny microcnemes, without muscles, confined only in distal-most part. Four microcnemes between dorsal directives and dorso-lateral mesenteries, two between dorso- and ventro-lateral mesenteries, and two between ventro-lateral mesenteries and ventral directives. Each tentacle between either exo- or endocoelic. (Fig. 6A). Retractor muscles strongly developed and diffused (Fig. 2F-H), pennon-like, configured with 15-30 muscular processes, each slightly branched (Fig. 2F). Parietal muscles with approximately 10 muscular processes (Fig. 2F). Actinopharynx short, ca. 2.5 mm in length, no distinct siphonoglyph (Fig. 2C). Tentacular circular muscle indistinct (Fig. 2D) and longitudinal muscle ectodermal and distinct (Fig. 2E). Mesoglea thin in the whole body, less than 100 µm even at the thickest part of body wall (Fig. 2 C–I). Nemathybomes, ca. 100 µm in diameter, protrude from mesoglea on the column including the aboral end (Fig. 2I). Marginal sphincter muscle and basilar muscle absent (Fig. 2C, H). Gonads adjacent to the retractor muscle, relatively long (Fig. 2H). Testes between filament and retractor muscle, but no mature gametes in this specimen.

Cnidome. Basitrichs (in all tissues), spirocysts (in tentacles), microbasic b-mastigophores (in filaments) (Table 2, Fig. 7 A–E). Basitrichs in filaments are distinguished into two types by their size.

Distribution.

Fiji ( Carlgren 1931: type locality), Australia, Singapore ( England 1987), and Japan (this study).

Origin of Japanese name.

New Japanese name: Ogasawara-ashinashi-mushimodoki. “Ogasawara” is the locality name. In Japanese waters, this species has been collected only in the Ogasawara Islands. “Ashinashi-mushimodoki” is short for the Japanese name of this genus (see the etymology of genus).

Remarks.

Scolanthus armatus was originally described by Carlgren (1931) as Edwardsia armata when the genus Scolanthus was a junior synonym of Edwardsia . After Scolanthus was revived by Manuel (1981), this species was transferred to Scolanthus by England (1987). This species had no physa-like structure but many nemathybomes in the aboral end. This is the most characteristic feature of Scolanthus , and this feature is not found in Edwardsia . The specimen from the Ogasawara (Bonin) Islands almost completely agrees with the description of Scolanthus armatus in England (1987); e.g. 16 tentacles which are capitated on the tip; rounded or flattened aboral end; scattered nemathybomes extending to the proximal end; strong and diffused retractor muscle (slightly branched muscular processes also correspond to England, 1987; fig. 13). The proportion and size of body is also similar to England’s description. There are, however, a few differences in the cnidome; England (1987) mentioned nothing about b-mastigophores in the description; stating that basitrichs and microbasic b-mastigophores could be distinguished by both the large diameter of the capsule and the broadened shaft shape ( England 1991). However, both cnidomes are still easily confused and they are difficult to delineate. Thus, microbasic b-mastigophores with their broadened shape were judged by England (1987) to probably be basitrichs. This probability is reinforced by England (1987: table 6): two types of “basitrichs” in filament of S. armatus , but one type of basitrich with apparently broader capsules than the other type while the lengths are not different. In addition, there are very long basitrichs in our specimen, but they are few in number. If the numbers of these long basitrichs are very low, it is possible that England (1987) did not observe them. Moreover, the other cnidae size ranges resemble those of specimens of England (1987).

In conclusion, we identified this specimen as S. armatus because of its similarity in almost all morphological features to the original description of the species. The slight difference observed in the cnidome may be simply individual variation.