Scolanthus isei, Izumi, Takato & Fujita, Toshihiko, 2018

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 isei sp. n. Figs 4, 6C, 7 J–M

Material examined.

Holotype. NSMT-Co 1611. One specimen cut into several parts, histological sections (5 slides) and prepared nematocysts (5 slides), on August 1, 2014, Sugashima Island (Fig. 1 B– 2), Mie, Japan (34°29'4"N, 136°52'31"E), cavity of a rock at a depth around 50 cm at low tide, collected by snorkeling by hand, by Yuji Ise. Paratype. NSMT-Co 1612. Histological sections (5 slides), damaged slightly when collected, on August 4, 2014, Sugashima, Mie, Japan (34°28'51"N, 136°52'46"E), cavity of a rock at a depth around 30 cm at low tide, collected by hand, by Yuji Ise.

Description.

External anatomy. Column rough, rugged and uneven, ca. 30 mm in whole length in fixed holotype, and 10-12 mm in width, truncated cone-like form both in living and fixed (Fig. 4A) specimen, comparatively tubby form for edwardsiids. Paratype a little small, ca. 18 mm in length and ca. 9 mm in width. Upper part narrower than lower part. No apparent capitulum, all parts of column uniformly scapus. Periderm brownish or whitish, no pattern in color, with trichome-like structure (Fig. 4A), and easily stripped. Column with highly densely scattered nemathybome but no papillae, and the surface on the mesenteries slightly sunken (Fig. 4A). Aboral end rounded, not differentiated from scapus, with nemathybome. No pedal disk, but no physa or physa-like structure (Fig. 4A, G). Tentacles slender, no acrosphere, completely transparent and white patches or stripes on each surface. Tentacles 20 in number, in two cycles; eight in inner and twelve in outer cycle (Figs 4B, 6C), 5.0-7.0 mm in length, longer than oral disk diameter and the inner tentacles shorter than outer ones. Oral disk ca. 4.0 mm in diameter. Mouth not swollen.

Internal anatomy. Eight perfect mesenteries, all macrocnemes. Four dorsal and ventral directives, and four lateral mesenteries not paired with other macrocnemes (Fig. 6C). 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. 6C). Twelve tiny microcnemes, without muscles, confined only in distal-most part. Four microcnemes between dorsal directives and dorso-lateral mesenteries, four between dorso- and ventro-lateral mesenteries, and four between ventro-lateral mesenteries and ventral directives, in unusual arrangement for Edwardsiidae . One tentacle each between exocoels and endocoels (Fig. 6C). Retractor muscles pennon-like, diffused throughout the whole body, smaller next to the actinopharynx (Fig. 4F), but largely developed and almost integrated to gonads in lower part (Fig. 4G). Each muscle pennons consisting of approximately 20-30 single or slightly branched muscular processes (Fig. 4F, G). Parietal muscles with approximately 15-20 muscular processes (Fig. 4G). Actinopharynx short, no distinct siphonoglyphs (Fig. 4F). Both tentacular circular muscle and longitudinal muscle too weakly developed to observe (Fig. 4C, D). Mesoglea thickest in the body wall and aboral end, approximately 200-300 and in some parts over 500 µm thick. However, mesoglea far thinner in actinopharynx and mesenteries (Fig. 4E, F, G), and thinnest in tentacles (Fig. 4C, D). Nemathybomes, around 150 µm in diameter, protruded from body wall (Fig. 4I) in the column but a little buried into the mesoglea in the aboral end (Fig. 4G, J). Marginal sphincter muscle and basilar muscle absent (Fig. 4E, G). Gonads next to retractor muscle, short, and wide (Fig. 4G). Ovary between retractor muscle and filament, and oocytes in gonads of holotype.

Cnidome. Spirocysts (in tentacles), basitrichs (in all tissues), microbasic b-mastigophores (in actinopharynx) and microbasic p-mastigophores (in filament) (Table 2, Fig. 7 J–M). Basitrichs in tentacles, nemathybomes and filaments are distinguished into two types by their size. No nematocysts in body wall.

Distribution.

Southwest coast of Sugashima Island, Mie. Known only from the type locality.

Etymology.

The species name was named after Yuji Ise, the collector of both holotype and paratype specimens. Origin of Japanese name: New Japanese name: sugashima-gareba-ashinashi-mushimodoki; “gareba” means rocky seashore, the habitat where this species inhabits.

Remarks.

In terms of having 20 tentacles, Scolanthus isei sp. n. resembles S. ignotus and S. ena sp. n., while all other edwardsiid species have 16 tentacles ( England 1987, Daly and Ljubenkov 2008, and this study). This species is different from S. ignotus in having two types of basitrichs of different sizes in nemathybomes ( Carlgren 1920), and the larger type is far bigger than the basitrichs of S. ignotus . The differences between S. isei sp. n. and S. ena sp. n. are principally regarding the tentacular arrangement and body size (see Remarks of S. ena sp. n.).

Exceptionally for edwardsiids, this species inhabits in the cavities of the underside of boulders, adhering by their aboral end, on rocky seashores. Edwardsiids usually inhabit in sand or mud, and only two species had been reported from other different environments: Edwardsiella andrillae Daly, 2013 living on ice ( Daly et al. 2013), and Tempuractis rinkai Izumi, Ise & Yanagi, 2018 living in sponges ( Izumi et al. 2018).