Metedwardsia akkeshi ( Uchida, 1932 )

Izumi, Takato, Yanagi, Kensuke & Fujita, Toshihiko, 2018, Re-description of Metedwardsia akkeshi (Cnidaria: Anthozoa: Actiniaria: Edwardsiidae), Discovered in Akkeshi, Hokkaido, Almost 80 Years after Original Description, with a Revision of the Diagnosis of Genus Metedwardsia, Species Diversity 23, pp. 135-142 : 138-141

Metedwardsia akkeshi ( Uchida, 1932)

[Japanese name: Hoso-isoginchaku; Uchida 1965] ( Figs 2–4 ; Table 2)

Milne-Edwardsia akkeshi Uchida, 1932: 571 , figs 1–4; Uchida 1940: 266.

Drillactis akkeshi: Uchida 1940: 267 .

Metedwardsia akkeshi: Carlgren 1949: 26 ; Song 2003: 288,

figs 3A–F, 5C, Table 2 (questionable).

Material examined. NSMT-Co 1605, histological sections (3 slides), tissues in paraffin, and prepared nematocysts (4 slides), 30 June 2015, Chikarakotan mud flats ( Fig. 1C ), Lake Akkeshi, Hokkaido (43°01.28′N, 144°52.46′E), intertidal, collected by Takato Izumi and Kensuke Yanagi ; CMNH- ZG09137, two whole specimens, same date and place as NSMT-Co 1605; NSMT-Co 1606, 3 whole specimens, 25 June 2017, Shinryu-hama Beach ( Fig. 1D ), Lake Akkeshi , Hokkaido (43°05.23′N, 144°84.43′E), intertidal, collected by Naoto Jimi; CMNH-ZG 4314 , histological sections (3 slides), tissues in paraffin, and prepared nematocysts (4 slides), 31 July 2006, mud flats near the Onne-to Ohashi Bridge ( Fig. 1E ), Lake Onne-to , Hokkaido (43°26.50′N, 145°49.77′E), intertidal, collected by Kensuke Yanagi and Marymegan Daly; NSMT-Co 1607, 1 whole specimen, 27 June 2015, at same lake as CMNH-ZG 4314 (43°15.56′N, 145°29.56′E), collected by Takato Izumi and Kensuke Yanagi; NSMT-Co 1608, histological sections (3 slides), tissues in paraffin, and prepared nematocysts (4 slides), 25 October 2014, Lake Furen , Hokkaido, collected by Naoto Jimi .

Description. Body elongated and worm-like in shape; little divided into physa, scapus, and scapulus ( Fig. 2A, B ). Surface of column smooth, without nemathybomes. Outside of scapus surrounded thin covering imbued with detritus particles, mucus-membrane secerned by mucus cells. Scapulus and physa mostly concealed in the covering when contracted. Scapulus short and provided with 24 short conical tentacles arranged in three alternating cycles ( Figs 2C , 3 ). Tentacles of inner and middle cycles same length or shorter than those of the outer. Lines indicating the insertion of mesenteries observed from outside in submarginal region but gradually obscure toward proximal end owing to the presence of thick ectoderm and well-developed mesogleal ring muscle. Physa, usually small and barely distinguishable from scapus, well expanded and contractable. Length of body ca. 20–40 mm and breadth of oral disc ca. 3–5 mm. Scapus and physa a little narrower than tentacular cycle. General color of column pale pink, tentacles brownish white, with dark brownish cross band ( Fig. 2C ). Occasional white spot on marginal part of oral disc below base of each tentacle. Epidermis of scapus and scapulus containing glandular cells thicker than mesoglea, furnished with ring muscles ( Fig. 2D, E ). Gastrodermis of scapulus generally similar in structure and thickness to those of both scapus and physa. Epidermis of scapus thick with a large number of glandular cells. Mesoglea thick in scapus, actinopharynx, and physa, but thinner in mesenteries tentacles ( Fig. 2E–H, J ). Ectoderm thicker than mesoglea in physa ( Fig. 2J ). No sphincter muscle in capitulum and no basilar muscle in aboral end ( Fig. 2D ). Actinopharynx folded, ectoderm as thick as its mesoglea, and granular cells present.

Mesenteries 24 in number of which 8 macrocnemes and 16 microcnemes ( Figs 2J , 3 ). Out of 8 macrocnemes, 4 in two directive pairs, dorsal and ventral. Other dorso- or ventro-lateral mesenteries, unpaired with other macrocnemes. Out of 16 microcnemes, 4 between dorsal directives and dorso-lateral mesenteries, 6 between dorso-lateral mesenteries and ventro-lateral ones, and the other 6 between ventro-lateral ones and ventral directives ( Fig. 3 ), comparatively tiny ( Fig. 2E, I ). All mesenteries including microcnemes extend from oral to aboral ends ( Fig. 2E, J ). On 8 macrocnemes, retractor muscle short and circumscribed and arranged in bilateral symmetry. Retractor muscles well developed, generally similar in form from scapulus to aboral end ( Fig. 2E, H ). Muscle fibers in the most developed muscle pennon about 10, each fiber highly branched ( Fig. 2H, I ) into 10 or more; 2–3 in lower part also branched into several fibers. On 16 microcnemes, muscle a little developed or not at all, rarely branched into muscle fiber. Parietal muscle folds few and radially elongated ( Fig. 2G ). Mesenterial filament shape trilobated leaf near upper part, consisting of a median cnidoglandular and two lateral ciliated tracts; in lower part, mesenterial filament shape broadly kidney-like, furnished with glandular cells and nematocysts ( Fig. 2J ). Base of ciliated tract digestive region and arranged by high density of endoderm cells. Reproductive region between muscle pennon and mesenterial filaments. Ovary or testicular saccules in a row imbedded in mesoglea, ovary in histological sections ( Fig. 2I ), dioecious. All microcnemes devoid of filaments and little furnished with parietal muscle similar to that of macrocnemes ( Fig. 2E, H, I ).

Cnidom. Basitrichs (in every tissues), spirocysts (in tentacles), microbasic b -mastigophores (in actinopharynx and filaments), and microbasic amastigophores (in filaments) ( Fig. 4A–H ; Table 2).

Habitat. Inhabits soft mud in the brackish intertidal zone, associated with the sea grass Zostera japonica . Although Shinryu-hama Beach in Akkeshi Bay opens on to the sea, the water is brackish due to input from Lake Akkeshi. This species tends to live in patchy aggregations of several individuals.

Remarks. The examined specimens corresponded well with Uchida’s (1932) description, but some morphological tigophores that differed in size in the filaments. Comparison of the sizes of these microbasic p -mastigophores with those of our specimens suggests that the smallest microbasic p -mastigophores in the Korean specimens would be equivalent to the microbasic amastigophores of the Akkeshi specimens, and the medium sized Korean ones would be equivalent to the microbasic b -mastigophores of our specimens. However, the large Korean microbasic p -mastigophores were not found in our specimens, and we did find any spirocysts in the column or the scapus. Second, Deokjeokdo Island, where the Korean specimen was collected, is over 1500km across the Yellow Sea from Akkeshi; moreover, the distribution of M. akkeshi in Japan appears to be limited to southeast Hokkaido, as the species even has not been found elsewhere in Japan. Finally, Song (2003) reported that the Korean specimens were found in sandy mud flats, whereas M. akkeshi in Hokkaido always inhabits quite muddy seashore environments, which feature substantially different substratum. Thus, we concluded that it is unlikely that the South Korean specimens are M. akkeshi from Hokkaido.

features were found to differ slightly or were not included in the original description. For example, Uchida (1932) noted that M. akkeshi has a capitulum, but we identified the part of the column near the tentacles as scapulus rather than capitulum. The cnidom, which were not included in Uchida (1932), are here described for the first time.

Song (2003) reported M. akkeshi from Deokjeokdo Island, South Korea, which is the sole record of M. akkeshi from localities other than Akkeshi. Based on their description, it is probable that the Korean specimen is at least within the genus Metedwardsia , because it only has 8 macrocnemes and 16 microcnemes extending beyond the actinopharyinx, but we suspect that the Korean specimen is not Metedwardsia akkeshi , for the following reasons. First, several differences between the cnidoms in the specimens of Song (2003) and those of this study are apparent; for example, Song (2003) described three types of microbasic p -mas-

Figures

Fig. 2. Outer and inner views of Metedwardsia akkeshi. A, outer view of Metedwardsia akkeshi (NSMT-Co 1605); B, outer view of live specimen (NSMT-Co 1606); C, oral view (NSMT-Co 1606); D, longitudinal section of oral part of M. akkeshi (NSMT-Co 1605, same hereafter); E, transversal section of actinopharynx and column; F, transversal section of tentacle; G, transversal section of a mesentery and parietal basal muscle; H, transversal section of mesenteries, gonads, and filaments; I, enlarged view of mesenteries. J, transversal section of physa. Abbreviations: Ac, actinopharynx; Fi, filament; Ma, macrocneme; Me, mesoglea; Mi, microcneme; Ov, ovary; Pa, parietal muscle; Ph, physa; Rm, retractor muscle; Scl, scapulus; Scs, scapus; Te, tentacle; Tlm, tentacular longitudinal muscle. Scale: 5mm in A–C; 1mm in D, E, H; 500µm in J; 100µm in F, G, I.

Fig. 3. Tentacular and mesenterial arrangement of Metedwardsia akkeshi. Schematic diagram depicting a cross-section of mesenterial and tentacular arrangement. The 24 tentacles are arranged in three cycles, consisting of an inner six (white), middle six (grey), and outer twelve (black). Abbreviations: Ac, actinopharynx; dd, dorsal directives; dlm, dorso-lateral mesenteries; mi, microcneme; vd, ventral directives; vlm, ventro-lateral directives.

Fig. 4. Cnidae of Metedwardsia akkeshi (NSMT-Co 1605). A, tentacle spirocyst; B, tentacle basitrich; C, actinopharynx basitrich; D, actinopharynx microbasic b-mastigophore; E, column basitrich; F, filament basitrich; G, filament microbasic b-mastigophore; H, filament microbasic amastigophore.

Fig. 1. Location of sites where Metedwardsia akkeshi specimens were collected. A, localities where Metedwardsia akkeshi were collected in Hokkaido, Japan. Sites in Nemuro are indicated by black arrows (Lake Onne-to and Lake Furen); B, enlarged map of Akkeshi. Collection sites in Akkeshi are indicated by black arrows (Akkeshi Bay and Lake Akkeshi); C, the Chikarakotan mud flat of Lake Akkeshi; D, Shinryuhama Beach of Akkeshi Bay (image courtesy of Naoto Jimi); E, mud flat near the Onne-to Ohashi Bridge, Lake Onne-to.