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.

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 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.

Mitogenome sequence of Cirriformia cf. tentaculata (Terebellida, Cirratulidae), a species of marine polychaete worm, was determined in this study. This is the first mitogenome reported for genus Cirriformia and family Cirratulidae. This complete mitogenome is 15,495 bp in length, containing 13 protein coding genes (PCGs), two ribosomal RNAs (rRNAs), and 23 transfer RNAs (tRNAs). The mitogenome of Cirriformia cf. tentaculata has high A + T content of 59.5% (A, 29.5%; C, 26.9%; G, 13.6%; T, 30.0%). In neighbour-joining (NJ) analysis, Cirriformia cf. tentaculata is clustered with the components of order Terebellida.

Two new cirratulid polychaetes of the genus Aphelochaeta collected from Korean waters are reported here along with detailed descriptions and illustrations. Aphelochaeta petila n. sp. is readily distinguished from its congeners by the following characteristics: non-expanded anterior and posterior ends of the body; non-moniliform segments in the abdomen; peristomium with three annulations and an indistinct dorsal crest; both the dorsal tentacles and the first branchiae attached on the peristomium; presence of only a pair of branchiae on chaetiger 1; and the ventrum of the thoracic region with transverse dark blue stripes on the chaetigers 6–16 with methyl green staining. Aphelochaeta intinctoria n. sp. is characterized by a combination of the following features: a rounded prostomium; a peristomium with a dorsal crest; a non-expanded thoracic region; dorsal tentacles on the peristomium and the first pair of branchiae on chaetiger 1; and the thoracic region lacking a distinct methyl green staining pattern. These two new species from Korean waters also differ in details of chaetal ultrastructure.

The mitogenome sequence of the epitokous Nereis species (Phyllodocida, Nereididae) was determined first in the genus Nereis . The complete mitogenome is 15,667 bp in length, containing 13 protein-coding genes, two ribosomal RNAs, 22 transfer RNAs and a control region, and their gene order and structure is identical to those of other nereidid species. The mitogenome consists of 33.5% A, 20.0% C, 13.3% G, 33.1% T, showing a high content of A + T similar to the other phyllodocid polychaetes. These results will be useful for inferring the phylogenetic relationships among the members of Nereididae within the phyllodocids.

The genera Nannopus Brady, 1880 and Ilyophilus Lilljeborg, 1902 are reappraised based on the characteristic features of a new species, Nannopus parvipilis sp. nov., collected from the brackish mud flat on Jindo Island, South Korea. Nannopus species have long been known as having a two-segmented P3 endopod. However, we discovered N. parvipilis sp. nov. having a one-segmented P3 endopod in accordance with the description of the genus by Brady. It implies that Brady’s generic diagnosis was appropriate and tentatively correct, and such species having a two-segmented P3 endopod but still being placed in Nannopus should be excluded from the genus. The genus Ilyophilus Lilljeborg, 1902, which has been regarded as a junior synonym of Nannopus, is revived to accommodate these Nannopus species. Thus, Nannopus is now composed of only two species, N. palustris and N. parvipilis sp. nov. and Ilyophilus comprises eight species: I. flexibilis Lilljeborg, 1902, I. perplexus Sars, 1909b comb. nov., I. unisegmentatus (Shen & Tai, 1964) comb. nov., I. didelphis (Fiers & Kotwicki, 2013) comb. nov., I. hirsutus (Fiers & Kotwicki, 2013) comb. nov., I. procerus (Fiers & Kotwicki, 2013) comb. nov., I. scaldicola (Fiers & Kotwicki, 2013) comb. nov., and I. ganghwaensis (Vakita, Kihara & Lee, 2016). In addition, N. palustris sensu Canu, 1892, which was commonly considered as representing the criterion of N. palustris, is established as a new species, I. canui sp. nov., based on the characteristic feature of the P3. A detailed description and illustrations of N. parvipilis sp. nov. are provided.