Ikedosoma sp.

Tanaka, Masaatsu, 2019, Ikedosoma (Annelida: Echiura: Thalassematidae) from the Tropical Pacific, with Description of a New Species, Species Diversity 24, pp. 267-273 : 271-272

Ikedosoma sp.

( Fig. 4 )

Listriolobus sp. : Marin 2014: 564, fig. 5c.

Ikedosoma sp. : Anker et al. 2015: 447, fig. 8c.

Material examined. CBM-ZW 1100, one specimen, sex unknown, mud-sand flat below mangroves, southeast of Tre Island , Dam Bay, Nha Trang, Vietnam [original geolocation: 12°12′19.53″N, 109°18′11.12″E (sic)], collected by yabby suction pump, I . Marin and S. Sinelnikov, 06 April 2014.

Description. Trunk pale pink, proboscis pale yellow base color in life, covered with dark green spots; LMB indiscernible externally due to shrinkage ( Marin 2014: fig. 5c; Anker et al. 2015: fig. 8c). Coloration faded to cream in preservative ( Fig. 4 ). TL 53 mm; PL 22 mm. Proboscis elongate, laterally carved inwards, anterior extremity truncated; dorsal surface covered with minute papillae; lower lip absent, lateral margins not united at base ( Marin 2014: fig. 5c; Anker et al. 2015: fig. 8c). Trunk wall thin, uniformly covered with numerous papillae, particularly prominent (up to ca. 600 µm in height) posteriorly. Trunk musculature comprising outermost circular, middle longitudinal, and innermost continuous oblique muscle layers ( Fig. 4 ). Due to poor fixation, circular layer wrinkled ventrally; although precise number of LMB unclear, at least eight present, somewhat inconspicuous dorsally ( Fig. 4 ). Paired ventral setae hookshaped; internally no interbasal muscle between setal sacs. All gonoducts damaged, proximal parts only remaining; 12 in total, comprising six pairs along ventral nerve cord, three pairs anterior to and remainder posterior to ventral setae. All pairs of gonoducts occupying ca. anterior one-fifth of trunk, with consistent longitudinal separation between adjacent pairs ( Fig. 4A ). Gonostome proximal, lips elongated and spirally coiled.

Most of alimentary canal fragmented; examination restricted to foregut remnant, posterior part of postsiphonal intestine, and rectum; contents included a few pebbles but no fecal pellets ( Fig. 4A ). Remnant of foregut fastened to ventral trunk wall by sheet-like mesentery, almost straight along ventral nerve cord. Ciliated groove present in remaining postsiphonal intestine. Rectum fastened to trunk wall by several strong mesenteries; rectal caecum absent. Fragments of paired simple anal vesicles remaining.

Most of vascular system missing, although ventral blood vessels apparent along ventral nerve cord.

Biological notes. The burrows of Ikedosoma sp. typical- ly accommodate two commensal shrimps, Eupontonia nudirostris Marin, 2014 (Palaemonidae) and Alpheus echiurophilus Anker, Komai, and Marin, 2015 (Alpheidae) , although the burrow shape is unknown ( Marin 2014; Anker et al. 2015). According to the specimen label, the species inhabits bottom muddy sand sediments at a depth of ca. 50 cm.

Remarks. Although the internal anatomy was in poor condition, the generic identification of this specimen by Anker et al. (2015) was confirmed by the presence of the diagnostic characteristics of Ikedosoma , as mentioned above. Furthermore, the specimen was similar to I. abemama sp. nov., rather than I. elegans , previously recorded from Vietnam by Dawydoff (1959) (see above), in the following characteristics: (1) absence of the proboscis lower lip, and (2) presence of three pairs of gonoducts in front of the ventral setae. However, whether the specimen belonged to I. abemama sp. nov. or not was debatable, because (1) only a single poor condition specimen was available, and (2) the habitat of the specimen was a muddy sand flat in an inner bay associated with mangroves, whereas that of the holotype of I. abemama sp. nov. was a fine sandy lagoon bottom in an atoll. Additional specimens from the same or adjacent localities should clarify the specific identity of this specimen.

Figures

Fig. 4. Ikedosoma sp. collected from Dam Bay, Nha Trang, Vietnam (CBM-ZW 1100). A, dorsal view of dissected trunk showing musculature (with aid of light box); B, magnified trunk wall showing two longitudinal muscle bands (marked by asterisks) and a continuous oblique muscle layer between them.Scales: A, 1cm; B, 1mm.