A new cirratulid species, Timareteposteriasp. n., is described from the intertidal habitats of the eastern coast of South Korea. The new species is closely related to Timareteluxuriosa (Moore, 1904) from southern California based on morphological similarity of the branchial and tentacular finalents and the noto- and neuropodial spines. However, T.posteriasp. n. differs from the latter based on the following characteristics: 1) evenly divided peristomium into three annulations; 2) 2–4 neuropodial spines originating in the posterior chaetigers alternated by a few capillaries; and 3) complete shift in branchial finalents located about one-third between the notopodium and the dorsal midline. The new species has a methyl green staining pattern (MGSP) distinct from other Timarete species. Detailed description and illustrations of the new species are provided with molecular information based on the partial sequences of the mitochondrial cytochrome c oxidase subunit I (COI) and 16S ribosomal RNA (16S). This study also includes a key and discussion of known Timarete species from East Asia.

A capitellid species, Heteromastus filiformis (Claparède, 1864) is newly reported from Korean waters with the description and illustrations. Korean materials of the present study show several characteristics generally agreed well with the previous descriptions of H. filiformis as follows: the thorax is composed of the first achaetigerous peristomium and following 11 thoracic chaetigers; the capillary chaetae are present on the thoracic chaetigers 15 and narrowly bilimbate; the thoracic hooded hooks are appeared on the chaetigers 611 and have indistinct nodes on the shaft and several teeth above the main fang; the abdominal hooded hooks possess distinct nodes on the shaft and a few teeth above the main fang; the branchiae appearing on the posterior abdominal segments are the broadlybased and rounded lamellae projecting posteriorly. The authors reviewed the taxonomy of Heteromastus with a comparison of morphological characteristics among worldwide species, and provided a key of them.

Three amphipods of the family Photidae collected from Korean waters are reported here with detailed descriptions and illustrations. The Korean materials of Gammaropsis examined in this study are readily assigned to G. longipropodi by the characteristic shape of gnathopod 2 in males: not slenderly produced posterior margin of the carpus, uniform width of very elongate propodus, and the transverse palm. Among the species of genus Photis, P. fischmanni is only one species bearing stridulation ridges of the basis on gnathopod 2 and coxa 3 simultaneously in females, until now. However, Photis stridulus sp. nov. also show this characteristic but is clearly distinguishable from P. fischmanni by the weak setation of the appendages, smaller eye on the anterior cephalic lobe, and the slender carpus and propodus on gnathopod 1. The genus Podoceropsis is recorded from Korean waters for the first time with the discovery of P. clavapes sp. nov. This new species is characterized by its elongate propodus and dactylus on gnathopod 2 and the shape of the posterior lobe of the basis on pereopod 5 in mature males.

A terebellid polychaete identified as Pista shizugawaensis Nishi and Tanaka, 2006, whose species validity has been doubted, is newly reported from the southwest coast of Korea. Korean materials of the present study have several characteristics that agree well with the original description of P. shizugawaensis as follows: two pairs of branchiae on the 2nd and 3rd segments have tufts composed of many dichotomously branched filaments; the nephridial papillae are present on the 6th and 7th segments; the uncini on the anterior and middle thoracic segments possess only long-handled shafts while those on the posterior ones have additional short-handled shafts or lacking shafts; the notosetae are broadly or narrowly winged capillary. The authors examined the taxonomic value of the presence of lateral lobes on the 5th and 6th segments, which has been known as a key characteristic feature of P. shizugawaensis in the classification of Pista species. In the present study, several characteristics such as the shape of notosetae, uncinial shafts in the thoracic segments, and the presence of thin narrow lateral lobes on the 4th segment are suggested as the specific characteristics that help to distinguish P. shizugawaensis from its congeners. A key to Pista species from Korean waters is also provided.

A new eunicid species, Leodice duplexa sp. n., from intertidal and subtidal habitats in the eastern coast of South Korea is described. The new species is assigned to the C-2 group, and is similar to Leodice antennata, the type species of the genus, in having the following combination of characteristics: moniliform antennae and palps, bidentate compound falcigers, articulated peristomial and notopodial cirri, pectinate branchiae showing bimodal distribution of branchial filaments, and yellow aciculae. However, L. duplexa sp. n. is readily distinguished from L. antennata by the following features: the aciculae are 2–4 in number, with blunt or pointed tips and hammer-headed or bifid tips, and the subacicular hooks are paired in some chaetigers. A detailed description and illustrations are provided for the new species. The validity of the new species is also supported by a genetic comparison using sequences of the mitochondrial cytochrome c oxidase subunit I (COI). A revised key to known Leodice species is provided with a comparison of their morphological characteristics.