Composetia kumensis Sato 2020, sp. nov

Composetia kumensis sp. nov

[Japanese name: Kumejima-nagare-gokai]

(Figs 1–3, 4A, 5)

Ceratonereis (Composetia) sp.: Sato 2012: 223.

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

Composetia sp. 2: Sato 2017: 483.

Type material. Holotype (NSMT-Pol H-766), female, Gushicha Gusuku on Kume-jima island, Okinawa Prefecture (26°22′52.6″N, 126°45′15.2″E) in the central Ryukyu Islands, southern Japan, 22 November 2013, coll. M. Sato, fixed in 80% ethanol. 17 paratypes: 12 individuals (NSMT- Pol P-767–770), data as for holotype (fixed in 80 or 99% ethanol); two females (NSMT -Pol P-771, 772), locality same as holotype, 24 March 2007, coll. K. Satake, fixed in 10% formalin; three individuals (NSMT -Pol P-773), locality same as holotype, 25 March 1999, coll. K. Satake, fixed in 10% formalin.

Non-type materials examined. One individual, 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 throughout, lacking heterogomph spinigers. Lower neurochaetae comprising of heterogomph spinigers and heterogomph falcigers throughout, lacking homogomph or sesquigomph falcigers. Oral ring greatly enlarged in full-everted proboscis.

Description. Holotype (Figs 1A, 2 B–H), complete female, 12 mm BL, 1.0 mm BW, with 53 chaetigers. Paratypes 9–17mm BL, 0.8–1.5 mm BW, with 49–64 chaetigers.

Body stout almost throughout, tapering around pygidium (Fig. 1A). Dorsum convex, venter relatively flat with longitudinal midventral groove. Colour in live specimens brown with greenish pigmentation on anterior dorsum (Fig. 1B). Colour in preserved specimens whitish cream with brownish or greenish pigmentation on anterior dorsum (Fig. 1A, C).

Prostomium pear-shaped. Antennae short, tapered, separated from each other (Figs 1B, C, 2A). Palps with massive palpophores and short subconical palpostyles. Both pairs of eyes arranged trapezoidally, anterior pair more separated and as large as (or slightly larger than) posterior pair; anterior pair reniform, posterior pair round (Figs 1B, C, 2A). Mid-longitudinal white cleft present on dorsal anterior surface of prostomium (Figs 1B, C, 2A).

Apodous segment (peristomium) slightly longer than subsequent chaetigers, with four pairs of tentacular cirri of unequal length; posterior dorsal tentacular cirri longest, reaching back to chaetiger 10 in holotype (chaetigers 6–14 in paratypes, usually chaetigers 10–14).

Proboscis with pair of amber jaws, each with around 6 marked teeth (Fig. 1D). Brown paragnaths with usually sharply pointed tip present only on maxillary ring (Fig. 2B, C). Paragnath numbers in holotype (range for all type series in parentheses): area I: 1 (0–2, n =18); area II: 24 on each side in two or three arched rows, total 48 (40–59, n =17); area III: 14 (13–25, n =17) in ovoid patch along base of maxillary ring; area IV: 20 on left and 19 on right, in triangular patch, total 39 (21–52, n =16). Oral ring greatly enlarged into trapezoidal shape in full-everted proboscis, 2.2 times longer and 1.6 times wider than maxillary ring in holotype, without any paragnaths or papillae (Figs 1A, 2B, C).

Parapodia most enlarged around chaetigers 5–10 (Fig. 2E, F). Sub-biramous parapodia of first 2 chaetigers with thin notoacicula (Fig. 2D). Notopodial dorsal ligule conical with tapering tip throughout. Notopodial prechaetal lobe absent throughout. Notoacicular process absent throughout. Notopodial ventral ligule conical with tapering tip throughout, subequal to or slightly smaller than notopodial dorsal ligule throughout. Dorsal cirri slender, tapering, as long as or longer than notopodial dorsal ligule throughout. 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. 2 E–H).

Neuropodial postchaetal lobe with tapering tip present in first 8 chaetigers in holotype (6–10 chaetigers in paratypes), absent in following chaetigers. Superior lobe in acicular ligule absent throughout. Inferior lobe conical in anterior parapodia, diminishing in middle parapodia, absent in posterior parapodia (Fig. 2D, F–H). Ventral ligule conical with tapering tip throughout, diminishing from middle parapodia, shorter than neuroacicular ligule. Ventral cirrus slender with tapering tip, not beyond ventral ligule throughout.

Notochaetae all homogomph spinigers, having long blades with finely serrated edge (Figs 3A, 4A); in holotype, 7, 4, and 3 spinigers present in chaetigers 5, 20, and 41, respectively; up to 11 spinigers in paratypes.

Upper neurochaetae consisting of homogomph spinigers and heterogomph falcigers (Fig. 4A) throughout. Heterogomph falcigers with short finely-serrated blades located at superior/anterior position; in holotype, 4, 3, 2, and 2 falcigers present in chaetigers 1, 5, 20, and 41, respectively; up to 6 falcigers in paratypes. Homogomph spinigers with long finely-serrated blades located at posterior position; in holotype, 3, 5, 4, and 3 spinigers present in chaetigers 1, 5, 20, and 41, respectively; up to 12 spinigers in paratypes.

Lower neurochaetae consisting of heterogomph spinigers and heterogomph falcigers (Fig. 4A) throughout. Heterogomph falcigers with short finely-serrated blades (Fig. 3C) located at inferior/anterior position; in holotype, 6, 10, 4, and 1 falcigers present in chaetiger 1, 5, 20, and 41, respectively; up to 11 falcigers in paratypes. Heterogomph spinigers with finely-serrated blades (Fig. 3B) located at posterior position; in holotype, 7, 7, 4, and 3 spinigers present in chaetiger 1, 5, 20, and 41, respectively; up to 12 spinigers in paratypes.

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

Small oocytes (50–75 µm in diameter) present in coelom of holotype.

Variations. In our subsequent extensive surveys, additional specimens of this species were collected from 10 additional sites on five islands in the Ryukyu Islands. 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 two paratype specimens (NSMT-Pol P-771, 772) collected in March in 2007 was filled with large oocytes (about 250 µm and 150 µm, respectively, in maximum diameter). None of the specimens show epitokous metamorphosis.

Habitat. Sandy bottom with pebbles in a small creek originating from a freshwater spring within the upper intertidal zone of the uplifted coral reef, surrounded by saltmarsh vegetation (Fig. 5). Based on my field survey on 22 and 23 November 2013 in the type locality in Kume-jima island and daily tidal records of observed sea level in Naha, Okinawa-jima island, close to Kume-jima (Japan Meteorological Agency 2019), the habitat condition was judged as follows: living usually under fresh-water conditions, which drastically changes to a marine regime during the most extreme spring high tides for a few days in a month (around 6 days in November 2013), with salinities ranging from 0.2 to 33.1 psu and temperatures from 19.2°C to 22.6°C.

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