Ikedosoma abemama, Tanaka, 2019

Ikedosoma abemama View in CoL sp. nov. ( Figs 1 View Fig , 2A, B View Fig , 3 View Fig )

Material examined. Holotype: NSMT-Ec 187, male, intertidal sandy bottom, lagoon side of Abatiku , Abemama

Atoll , the Gilbert Islands, Kiribati (approximate geolocation: 0°23′12.1″N, 173°47′15.4″E), collected by Teruaki Nishikawa, 26 July 1984 GoogleMaps .

Diagnosis. A species of Ikedosoma without proboscis lower lip; more than two pairs of gonoducts present anterior to the ventral setae.

Description. Coloration in life unknown. Trunk and proboscis pale yellow in preservative, probably due to discoloration by Bouin’s fluid ( Fig. 1A View Fig ). TL ca. 140 mm; PL ca. 70 mm. Proboscis elongate, laterally curled inward, truncated at anterior extremity; dorsal surface covered with minute papillae; lower lip absent, lateral margins not united at base; margin of mouth opening weakly undulated due to shrinkage ( Figs 1A View Fig , 2A, B View Fig ). Trunk wall thin, uniformly covered with numerous papillae, particularly prominent (up to ca. 400 µm in height) posteriorly ( Figs 1A View Fig , 3 View Fig ). Trunk musculature comprising outermost circular, middle longitudinal, and innermost continuous oblique muscle layers. LMB nine, rather inconspicuous dorsally ( Fig. 1B View Fig ). Paired ventral setae hook-shaped; no interbasal muscle between setal sacs ( Fig. 3A View Fig ). Gonoducts filled with sperm masses, 11 in total, arranged in approximate pairs along ventral nerve cord; in front of ventral setae, two and three gonoducts present on left and right side, respectively; behind ventral setae, three further pairs of gonoducts ( Fig. 3A View Fig ). All pairs of gonoducts occupying ca. anterior one-fifth of trunk, with largely consistent longitudinal separation between adjacent pairs ( Figs 1B View Fig , 3A View Fig ). Gonostome proximal, lips elongated and spirally coiled ( Fig. 3A View Fig ).

Alimentary canal long, convoluted, some parts filled with fine white sand not molded into fecal pellets; foregut, intestine, and rectum present ( Figs 1B View Fig , 3 View Fig ). Foregut, fastened to ventral trunk wall by strong sheet-like mesentery, almost straight along ventral nerve cord, and divided into pharynx, esophagus, gizzard, and crop ( Fig. 3A View Fig ). Intestine fastened to trunk wall through numerous thread-like mesenteries, and divided into presiphonal, siphonal, and postsiphonal sections; ciliated groove undetectable in presiphonal section but present in postsiphonal section ( Fig. 3 View Fig ). Rectum fastened to trunk wall by several robust mesenteries; rectal caecum absent ( Fig. 3B View Fig ). Paired simple anal vesicles extending laterally from rectum; ca. one-third of TL, basally attached to rectum by several robust mesenteries, entirely covered with numerous microscopic ciliated funnels ( Fig. 3B View Fig ).

Vascular system consisting of dorsal, neurointestinal, ventral, and ring vessels ( Fig. 3 View Fig ). Dorsal vessel branched, one branch attached to posterior end of gizzard, the other connected to ring vessel; branches connected by additional narrow vessel between distal end of former and middle part of latter, forming a loop ( Fig. 3A View Fig ). Ring vessel incompletely encircling posterior end of crop ( Fig. 3A View Fig ). Ventral vessel running along almost entire length of ventral nerve cord, terminating at posterior end of postsiphonal intestine with mesenteries ( Fig. 3 View Fig ). Neurointestinal vessel linked to ventral vessel at level of first pair of gonoducts, immediately bifurcating into a large loop, and terminating on each side of ring vessel ( Fig. 3A View Fig ).

Etymology. The specific name is after the type locality of the new species, Abemama Atoll, and is used as a noun in apposition.

Remarks. The new species has the following diagnostic characteristics of Ikedosoma , as redefined by Tanaka et al. (2014): (1) regularly thickened longitudinal muscle and continuous oblique muscle layer in the trunk, (2) elongated, spirally coiled gonostomal lips, and (3) absence of a rectal caecum.

The diagnostic characteristics of the species of Ikedosoma are summarized in Table 1. Ikedosoma abemama sp. nov. differs from I. elegans ( Fig. 2C View Fig ) and I. gogoshimense ( Fig. 2D View Fig ) in the absence of a proboscis lower lip, in contrast to the presence of such in the latter two species [see also Ikeda (1904: fig. 19) for I. gogoshimense , and Ikeda (1907: fig. 4) for I. elegans ]. In addition, the former is unique in the genus in having at least two pairs of gonoducts in front of the ventral setae, although the precise number of anterior pairs (two or three) could not be determined because it was unclear whether or not the proximate second and third gonoducts on the right side represented a single cluster corresponding to the second gonoduct on the left side.

The vascular system of I. abemama sp. nov. is unique among Thalassematidae in having a branched dorsal vessel [usually simple in other members of the family ( Stephen and Edmonds 1972; Amor 1973)]. Because some intraspecific variations in blood vessel architecture have been reported (e.g., Jameson 1899; Tanaka and Nishikawa 2013; Tanaka et al. 2014), more material is needed to determine whether or not the branched dorsal vessel is typical in I. abemama sp. nov.

Previously only two species of Echiura have been recorded from the Gilbert Islands: Achaetobonellia maculata Fisher, 1953 (Bonelliidae) from Onotoa Atoll and Ochetostoma sp. (Thalassematidae) from Aranuka Atoll ( Bock 1942; Fisher 1953). Thus, Ikedosoma abemama sp. nov. represents the third echiuran species known from that region.

Distribution. Known only from Abemama Atoll, the Gilbert Islands, Kiribati.