Linopherus littoralis, Shimooka & Koyama & Jimi, 2025

Linopherus littoralis sp. nov.

[New Japanese name: Hama-doro-umikemushi]

( Figs 2–4 View Fig View Fig View Fig )

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. GoogleMaps 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. GoogleMaps

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 View Fig , 3 View Fig ). Color pinkish when alive, pale yellow in alcohol, without color pattern ( Fig. 2 View Fig ). 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 View Fig , 3A View Fig ). 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 View Fig , 3A View Fig ). 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 View Fig , 3A View Fig ). Median antenna conical, as long as lateral antennae, located two-thirds from the front of posterior lobe ( Figs 2C View Fig , 3A View Fig ). Caruncle short, rounded, inconspicuous, completely concealed by second chaetiger ( Figs 2C View Fig , 3A View Fig ). Buccal opening occupying two chaetigers, upper lip bilobed, expanded distally ( Fig. 3B View Fig ).

All parapodia biramous ( Figs 3C, D View Fig , 4A View Fig ). Chaetiger 1 greatly reduced, not forming dorsally complete ring ( Fig. 3A View Fig ). Parapodia with slender cirri, both notopodia and neuropodia well-developed, neuropodia larger than notopodia, chaetal sac forming rounded lobe, cirrophores smooth ( Figs 3C, D View Fig , 4A View Fig ). First two parapodia with slightly articulated cirri, dorsal and ventral similar-sized ( Fig. 2A, B View Fig ). 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 View Fig ).

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 View Fig ); chaetiger 14 with 21 filaments ( Fig. 3C View Fig ); chaetiger 28 with six filaments ( Fig. 3D View Fig ); chaetiger 38 with 1–3 filaments.

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

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 View Fig ).

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 View Materials ). 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.