The monotypic polynoid genus Arctonoella Buzhinskaja, 1967 comprises solely the type species A. sinagawaensis (Izuka, 1912), which was originally described from Tokyo Bay (central Japan), and subsequently recorded from China and the Russian Far East. The six specimens we collected together with the burrowing filter-feeding spoon worm Urechis unicinctus (Drasche, 1880) from three intertidal-flat sites in the Seto Inland Sea represent a new report for the western Japan, and the second for the country. Our morphological observations reveal that the shape of the cephalic peaks in the frontal prostomial margin is variable even within a local population, although this character has been considered as diagnostic for Arctonoella. This genus is closely related with Hesperonoe Chamberlin, 1919, both morphologically and phylogenetically. Hesperonoe urechis Marin and Antokhina, 2020, collected inside a burrow of U. unicinctus in the Russian Far East, is hereby deemed a junior synonym of A. sinagawaensis. This species morphologically resembles Hesperonoe adventor (Skogsberg in Fisher and MacGinitie, 1928), which inhabits spoon worm (Urechis caupo Fisher and MacGinitie, 1928 and Echiurus echiurus alaskanus Fisher, 1946) burrows along the northeastern Pacific coast of America. Therefore, we conclude that the Arctonoella-Hesperonoe species complex requires to be reviewed to determine the proper generic arrangement. Additionally, we revise the available data on the symbiotic fauna living inside the burrows of U. unicinctus and U. caupo.

Seven specimens of Paloa villadolidi Roxas and Ablan, 1940, were collected from Okinawa and Ishigaki islands in the Ryukyu Archipelago, Japan, and their morphologies are described herein. Although this species has been considered as a synonym of Odonteleotris macrodon (Bleeker, 1853) which is redescribed in this study, the former is distinguished from the latter by having a steeper jaw (upper jaw tip not reaching orbit vs. ending below orbit center in O. macrodon), a contrasting number of canine teeth on the jaws (more upper-jaw canines than lower-jaw ones vs. more lower-jaw canines), the absence of distinct fin markings (vs. many small black spots on fin rays), no spots on the caudal-fin base (vs. with a red spot), and a higher number of cephalic sensory canal pores (17 vs. 13 or 14). This study also noted a unique canine teeth arrangement in P. villadolidi, which has canine teeth only in the anterior half of the lower jaw, unlike the canine teeth arrangement of four other butid genera (Incara Rao, 1971, Odonteleotris Gill, 1863, Ophiocara Gill, 1863, and Oxyeleotris Bleeker, 1874), which have an inner row of canine teeth in the posterior half of the lower jaw. Although further study is needed to determine the taxonomic status of P. polylepis Herre, 1927, the other nominal species of the genus, the present study tentatively considered it valid based on the original description, which described it as having a deeper body than P. villadolidi. The seven Japanese specimens were identified as P. villadolidi, as they have slenderer bodies than P. polylepis, and they represent the first Japanese records of the species.

A single specimen of gecarcinid crab, Discoplax gracilipes Ng and Guinot, 2001, was collected from an anchialine pool in a cave on Hateruma Island, southern Ryukyu Archipelago, southwestern Japan. This species was previously known only from the central Philippines. The present specimen represents the first record of the species from Japan, extending its distributional range to about 1400 km northward. Additionally, the fauna of the cave on Hateruma Island briefly noted. A key to the Japanese species of the family Gecarcinidae is provided.

A species of cyclopoid copepod Anthessius cucullatus Lee and Kim, 2021 (Anthessiidae) is newly recorded based on both sexes collected from three species of the sea slug genus Aplysia Linnaeus, 1767, i.e., Ap. dactylomela Rang, 1828, Ap. juliana Quoy and Gaimard, 1832, and Ap. kurodai Baba, 1937, from coastal waters of Kagoshima Prefecture, southern Japan. The copepod is characterized by having the following characters in the female: 1) the genital double somite covered with a hood-like dorsal lobe; 2) the antenna with four unequal claws and three setae at the distal tip; 3) the maxilla has a spinulose basal lobe; 4) leg 5 with irregular marginal rows of spinules. Furthermore, the male has 5) the third endopodal segment of leg 4 bearing a curved third inner spine. The findings of An. cucullatus in this study represent the second record of the copepod from all around the world as well as the first record from Japanese waters with the detailed description of the morphological characters based on the specimens. Aplysia dactylomela and Ap. juliana are new host records of the copepod.

Monostiliferous nemerteans are mostly benthic and free-living, but a few are recognized as commensal and parasitic species. We describe Tetrastemma carneum sp. nov. based on five specimens found from demosponges collected by the use of remotely operated vehicle at a depth of 398 m, off Ofunato, Japan. The new species resembles T. appendiculatum Chernyshev, 1998 described based on specimens discovered from demosponge Esperiopsis digitata (Miklucho-Maclay, 1870) (now accepted as Amphilectus digitatus) trawled at depths of 40–60 m in Peter the Great Bay, in having a pale-colored body with four conspicuously large eyes (approximately 50 µm in the maximum width). However, it is differentiated from T. appendiculatum in possessing a mid-dorsal line on the dorsal surface of the head as well as the number of proboscis nerves. In our molecular phylogenetic tree based on partial sequences of 16S rRNA, cytochrome c oxidase subunit I, 18S rRNA, 28S rRNA, and histone H3 genes, Tetrastemma carneum sp. nov. was nested in the clade of Tetrastemma Ehrenberg, 1828 as a sister taxon to Tetrastemma sp. IP Iturup reported from Peter the Great Bay.