Composetia tokashikiensis sp. nov.

[New Japanese name: Tokashiki-nagare-gokai]

(Figs 4B, 6–8)

Composetia sp. A: Sato and Sakaguchi 2016: 85.

Composetia sp. 1: Sato 2017: 483.

Type material. Holotype (NSMT-Pol H-774), female, the upper reaches of a small estuary in the Tokashiki-gawa river on Tokashiki-jima island, Okinawa Prefecture (26°11′46.81″N, 127°21′46.82″E) in the central Ryukyu Islands, southern Japan, 20 November 1991, coll. M. Sato, fixed in 80% ethanol. 18 paratypes: 9 individuals (NSMT- Pol P-775–783), data as for holotype; 9 individuals (NSMT- Pol P-784), locality same as holotype, 27 May 2012, coll. M. Sato, fixed in 80% ethanol.

Non-type materials examined. Two individuals, data as for holotype. No longer preserved since whole body used for a DNA analysis (Sato et al. 2020) after morphological examination.

Diagnosis. Notoacicula present in first 2 chaetigers. Notopodial prechaetal lobe absent throughout body. Neuropodial postchaetal lobe present only in anterior body. Upper neurochaetae comprising of homogomph spinigers and heterogomph falcigers except for around first 20 chaetigers, where most or all of heterogomph falcigers replaced by heterogomph spinigers. Lower neurochaetae comprising of heterogomph spinigers and heterogomph falcigers except for around first 20 chaetigers, where most or all of heterogomph falcigers replaced by heterogomph spinigers. Oral ring greatly enlarged in full-everted proboscis.

Description. Holotype (Figs 6A, 7 D–G, 8A–C), complete female, 19 mm BL, 1.5 mm BW, with 59 chaetigers (Fig. 6A). Paratypes 15–21 mm BL, 1.0–1.6 mm BW, with 57–63 chaetigers.

Body stout almost throughout, tapering around pygidium. Dorsum convex, venter relatively flat with longitudinal midventral groove. Colour in live specimens brownish. Colour in preserved specimens whitish cream with brownish pigmentation on anterior dorsum.

Prostomium pear-shaped or triangular. Antennae short, tapered, separated from each other (Figs 6B, C, 7A). Palps with massive palpophores and short subconical palpostyles. Both pairs of eyes arranged trapezoidally, anterior pair reniform, more separated and as large as (or larger than) posterior pair; posterior pair round. Mid-longitudinal white cleft present on dorsal anterior surface of prostomium, bordered by dark pigmentation.

Apodous segment slightly longer than subsequent chaetigers, with four pairs of tentacular cirri of unequal length; posterior dorsal tentacular cirri longest, reaching back to chaetiger 8 in holotype (chaetigers 6–12 in paratypes, usually chaetigers 6–10) (Fig. 7A).

Proboscis with pair of amber jaws, each with 8 marked teeth in holotype (7–9 teeth in paratypes). Brown paragnaths usually with sharply pointed tip present only on maxillary ring (Figs 6C, D, 7B, C). Paragnath numbers in holotype (range for all materials in parentheses): area I: 0 (0–0, n =21); area II: 26 on left and 24 on right in two or three arched rows, total 50 (35–58, n =21); area III: 23 (14–25, n =21) in ovoid patch along base of maxillary ring; area IV: 22 on left and 21 on right in triangular patch, total 43 (16– 47, n =20). Oral ring greatly enlarged into trapezoidal shape in full-everted proboscis, 1.7 times longer and 1.8 times wider than maxillary ring in holotype, without any paragnaths or papillae (Figs 6 A–D, 7C).

Sub-biramous parapodia of first 2 chaetigers with thin notoacicula (Fig. 7D). Notopodial dorsal ligule conical with tapering tip throughout. Notopodial prechaetal lobe absent throughout. Notoacicular process present in few parapodia in chaetigers 5–10 in holotype and some large paratypes more than 1.5 mm BW (Fig. 7F). Notopodial ventral ligule conical with tapering tip throughout, shorter than notopodial dorsal ligule in anterior parapodia and subequal to that in posterior parapodia. Dorsal cirri slender, tapering, as long as or shorter than notopodial dorsal ligule throughout, except for posteriormost few parapodia where dorsal cirri longer than notopodial dorsal ligule. Three whitish glandular patches present on dorsal edge of notopodia; distalmost glandular patch larger than others, covering whole conical projection of notopodial dorsal ligule throughout (Fig. 7 E–H).

Neuropodial postchaetal lobe with tapering tip present in first 18 chaetigers in holotype (12–25 chaetigers in paratypes) (Fig. 7 D–F), absent in following chaetigers (Fig. 7G, H). Superior lobe in acicular ligule absent throughout. Inferior lobe conical in anterior parapodia, diminishing in middle parapodia, and absent in posterior parapodia (Fig. 7 D–H). Ventral ligule conical with tapering tip throughout, diminishing from middle parapodia, shorter than neuracicular ligule. Ventral cirrus slender with tapering tip, shorter than ventral ligule throughout.

Notochaetae all homogomph spinigers, having long blades with finely serrated edge (Fig. 8A); in holotype, 3, 8 and 6 spinigers present in chaetiger 3, 5 and 32, respectively; up to 11 spinigers in paratypes.

Upper neurochaetae consisting of homogomph spinigers and heterogomph falcigers except for anterior chaetigers (around first 20 chaetigers), where most or all of heterogomph falcigers replaced by heterogomph spinigers (Fig. 4B). Heterogomph falcigers with short finely-serrated blades located at superior/anterior position; in holotype, no falcigers present in chaetigers 3 and 5; up to 3 falcigers in paratypes; in holotype, 3 falcigers present in chaetiger 32; up to 3 falcigers in paratypes. Heterogomph spinigers with short finely-serrated blades (Fig. 8B) present only in anterior chaetigers (most abundant around chaetiger 5); in holotype, 4 and 6 spinigers present in chaetigers 3 and 5, respectively; up to 9 spinigers in paratypes. Homogomph spinigers with long finely-serrated blades located at posterior position; in holotype, 6, 8 and 5 spinigers present in chaetigers 3, 5 and 32, respectively; up to 10 spinigers in paratypes.

Lower neurochaetae consisting of heterogomph spinigers and heterogomph falcigers except for anterior chaetigers (around first 20 chaetigers), where most or all of heterogomph falcigers replaced by heterogomph spinigers (Fig. 4B). Heterogomph falcigers with short serrated blades (Fig. 8E) located at inferior/anterior position; in holotype, no falcigers present in chaetigers 3 and 5; up to 2 falcigers in paratypes; in holotype, 2 falcigers present in chaetiger 32; up to 6 falcigers in paratypes. Heterogomph spinigers with finelyserrated blades present throughout (most abundant around chaetiger 5); in holotype, 12, 18 and 7 spinigers present in chaetigers 3, 5 and 32, respectively; up to 21 and 12 spinigers in anterior and middle chaetigers, respectively, in paratypes; spinigers with long blades (Fig. 8C) located at posterior position; spinigers with short blades (Fig. 8D) located at inferior/anterior position.

Pygidium with anus on dorsal side, with slender anal cirri.

Variations. In our subsequent extensive surveys, additional specimens of this species were collected from 20 additional sites on six islands in the Ryukyu Islands, and also from a site in Thailand. The variations of morphological characteristics among the geographically separated populations will be shown in a subsequent paper (Sato et al. 2020).

Reproduction. The coelom of a paratype specimen (NSMT-Pol P-775) collected in November in 1991 was filled with large oocytes (about 250 µm in maximum diameter). None of the specimens show epitokous metamorphosis.

Habitat. Intertidal sandy bottom in the upper reaches of a small estuary (Fig. 6E). Salinity of interstitial water that drained into the remaining holes after taking the sediment samples was 0.3 psu at a low tide around 16:00 on 27 May 2012.

Etymology. The species name is an adjective derived from the island name of the type locality, Tokashiki-jima.

Remarks. Composetia tokashikiensis sp. nov. is distinguishable from C. kumensis sp. nov. by the arrangement of neurochaetae in anterior chaetigers around chaetiger 5, where heterogomph falcigers are mostly or completely replaced by heterogomph spinigers with short blades in both upper and lower fascicles of neurochaetae.