Linopherus littoralis sp. nov.

[New Japanese name: Hama-doro-umikemushi]

(Figs 2–4)

Material examined. Holotype (NSMT-Pol H-973): 22.5 mm long, 1.9 mm wide (without chaetae, in widest chaetiger), 100 chaetigers, Nagaihama beach (33°43′24.4″N 131°01′27.2″E), sandy-mud, collected by using a yabbie pump, 0–1 m in depth, 15 October 2023, collected by Satoshi Shimooka and Akihiko Koyama. Paratypes (NSMT-Pol P-974): three specimens (one of them was used for SEM observation), 28.1–30.9 mm long, 1.5–2.2mm wide (without chaetae, in widest chaetiger), 97–105 chaetigers, Nagaihama beach (33°43′39.5″N 131°01′33.2″E), sandy-mud, collected by using a yabbie pump, 0–1 m in depth, 16 October 2023, collected by Satoshi Shimooka and Akihiko Koyama.

Diagnosis. Linopherus with prostomium globular, two pairs of indistinct eyes. Caruncle short, inconspicuous. Parapodia biramous, rami widely separated, with single dorsal and single ventral cirrus. Branchiae dendritic, present from chaetiger 3 onwards, with more than 33 pairs.

Description. Body elongate, nearly rectangular in cross section, tapering posteriorly (Figs 2, 3). Color pinkish when alive, pale yellow in alcohol, without color pattern (Fig. 2). Prostomium pear-shaped, longer than wide, divided into two parts by transverse groove, located two-fifths of the way from the front end (Figs 2C, 3A). Anterior lobe two-thirds as long as posterior lobe, rounded, with pair of cirriform antennae dorsally and similar pair of palps laterally, palps with three distinct articulations. Antennae nearly as long as palps (Figs 2C, 3A). Posterior lobe quadrangular, with median antenna and two pairs of indistinct rounded eyes not visible after fixation. Eyes appear as pale dots arranged in square around median antenna (Figs 2B, C, 3A). Median antenna conical, as long as lateral antennae, located two-thirds from the front of posterior lobe (Figs 2C, 3A). Caruncle short, rounded, inconspicuous, completely concealed by second chaetiger (Figs 2C, 3A). Buccal opening occupying two chaetigers, upper lip bilobed, expanded distally (Fig. 3B).

All parapodia biramous (Figs 3C, D, 4A). Chaetiger 1 greatly reduced, not forming dorsally complete ring (Fig. 3A). Parapodia with slender cirri, both notopodia and neuropodia well-developed, neuropodia larger than notopodia, chaetal sac forming rounded lobe, cirrophores smooth (Figs 3C, D, 4A). First two parapodia with slightly articulated cirri, dorsal and ventral similar-sized (Fig. 2A, B). Chaetiger 2 with longest cirri among all. Succeeding parapodia with dorsal cirri about same length as cirri of chaetiger 1, with ventral cirri one-third as long as dorsal cirri (Fig. 3A, B).

Branchiae present from chaetiger 3, located on posterior side of each parapodia, arising as tufts, dendritically branched, decreasing along body, disappearing at chaetiger 38, chaetiger 3 with four filaments (Fig. 3A); chaetiger 14 with 21 filaments (Fig. 3C); chaetiger 28 with six filaments (Fig. 3D); chaetiger 38 with 1–3 filaments.

Notochaetae of four types: thick, smooth aciculars located in the lower part of the fascicle (Figs 3E, 4B); stout harpoonchaetae with over 20 denticles (Figs 3F, 4C); long, slender smooth capillaries; long, slender, basally spurred capillaries with denticulate cutting margins (Fig. 3G). Neurochaetae of three types: long, slender, basally spurred capillaries with denticulate cutting margins (Fig. 3H); furcate chaetae, shortest tine two times longer than wide, longest tine denticulate, cutting margins various length located in upper part of fascicle (Fig. 3I); smaller furcate chaetae with thick shafts, longest tine smooth located in lower part of fascicle (Fig. 3J).

Anus opening dorsally on terminal chaetiger. Pygidium rounded, without appendages.

Variations. Four specimens are examined (NSMT-Pol H-973, NSMT-Pol P-974). Specimen size varies from 22.5 to 30.9 mm length, 1.5 to 2.2 mm maximum width. Branchial chaetiger range varies, branchiae always start in chaetiger 3 to and extends along body to chaetigers 36–46.

Etymology. The specific name is derived from the Latin adjective littoralis (coastal), referring to the habitat of the specimens, which were found in the littoral zone of a sandy-mud beach.

Phylogenetic analysis. Linopherus littoralis sp. nov. and Paramphinome M. Sars in G. Sars, 1872 formed a clade with low bootstrap support (BS: 58). Linopherus is included in the subfamily Amphinominae with good support (BS: 94) (Fig. 5).

Distribution and habitat. This species was found in shallow water of a sandy-mud beach at depths of 0–1 m in the Seto Inland Sea, Japan. In terms of COI sequences, Linopherus littoralis sp. nov. shows a high similarity (99.54%) to a specimen identified as Amphinomidae sp. C, collected from China (GenBank accession number OL964293). This suggests the potential distribution of L. littoralis sp. nov. in China. However, due to the lack of detailed morphological descriptions or additional information on the Chinese specimen, further verification is needed to confirm whether it is conspecific with L. littoralis sp. nov. Therefore, the precise distribution of this species outside Japan remains unclear.