Two new species of Aricidea Webster, 1879 (Paraonidae), Aricidea (Acmira) anusakdiisp. nov. and Aricidea (Aricidea) thammapinanaesp. nov. were collected from 10–26.5 m depth, in soft bottoms with mud mixed with sand and shells at Songkhla Sea, the Gulf of Thailand between 2011–2018. Aricidea (Acmira) anusakdiisp. nov. is clearly distinguished from other species of the subgenus Acmira by having a rounded bilobed prostomium divided by a slight notch on the anterior margin; red pigments on the subdistal to the tip of each branchia (new character); two prebranchial chaetigers; 48–68 pairs of branchiae; and modified neurochaetae as strong curved spines with blunt shafts surrounded by pubescence from chaetigers 19–44. On the other hand, Aricidea (Aricidea) thammapinanaesp. nov. can be separated from other members of the subgenus Aricidea by the presence of a biarticulated median antenna; distinctive notopodial lobes as broad triangular with short distal protuberances on chaetiger 3, 4–8 pairs of branchiae; and modified neurochaetae as bidentate neurochaetae with a long pubescent subterminal arista on the concave side. All data have been archived and are freely available from the Dryad Digital Repository (https://doi.org/10.5061/dryad.hqbzkh1cn).

Four new species belonging to the Family Pilargidae, Ancistrosyllis kornkanokae sp. nov., A. nakkaritae sp. nov., Cabira saithipae sp. nov., and C. thailandica sp. nov. were collected from 9–27 m depth in soft sediments in the Songkhla Sea, Gulf of Thailand. Ancistrosyllis kornkanokae sp. nov. can be distinguished from other species in the genus by the presence of a small median antenna on the prostomium; two pairs of elongate tentacular cirri; and short limbate neurochaetae with unidentate curved pointed tips. Ancistrosyllis nakkaritae sp. nov. is characterized by having a conspicuous median antenna and elongate lateral antennae longer than palps; and short limbate neurochaetae with bifid tips. While Cabira saithipae sp. nov. is characterized by the presence of the first notopodial hook at chaetiger eight, large, conical papillae ont chaetigers 7–17 and the presence of both short, recurved tipped and short, curved tip chaetae in anterior chaetigers. Cabira thailandica sp. nov. is distinguished from other species in the genus by the first notopodial hooks occurring at chaetiger seven and the presence of long, non-limbate capillary neurochaetae with bidentate tips. Updated keys to the genera Ancistrosyllis and Cabira are also provided.

In this contribution, three new species of sternaspids collected from sediments along the coast of southern Thailand are described: Petersenaspis apinyae sp. nov. from a depth of 50–80 m offshore in the Gulf of Thailand; P. narisarae sp. nov. from 9 m depth in the Songkhla Sea, Gulf of Thailand; and P. pakbaraensis sp. nov. from tidal mudflats on the Andaman Coast. All three species resemble P. palpallatoci Sendall & Salazar-Vallejo, 2013. They differ mainly in the pigmentation and shape of the shield, the number of ventral chaetae and chaetae at the posterior shield, body papillae, and size of the abdomen. Further, P. apinyae sp. nov. is clearly distinguished from other species of the genus by having dark orange to red butterfly wing-shaped shields, with strongly curved anterior margins. P. narisarae sp. nov. differs from other species by having concentric colored bands over shields and an expanded oval abdomen. P. pakbaraensis sp. nov. can be distinguished from other species by its dark brown-purple shield and laterally expanded fan. These three species have a unique character in their branchial plates: long brownish filaments. A key to the identification of all species of Petersenaspis is included.

Despite being a great species-rich area, the Gulf of Thailand has been relatively poorly studied and new species are awaiting to be described. In the present study two new species of Sphaerodoridae (Annelida), Geminofilum thailandica sp. nov. and Sphaerodoridium songkhlaensis sp. nov. were collected in shallow soft bottoms at Songkhla Sea. Geminofilum thailandica sp. nov. is characterised by bearing sessile and elongated dorsal macrotubercles, arranged in two transverse rows per segment, lacking other dorsal papillae, having parapodia with a single papilla at the base and compound chaetae with blades 4–6 times as long as wide and serration showing variation within fascicles. Sphaerodoridium songkhlaensis sp. nov. can be distinguished from other congeners by the singular morphology of the short stalked dorsal macrotubercles as inverted cones, with a conspicuous rim encircling the flattened distal surface, and two longitudinal rows of smaller sessile tubercles, as inverted cones, along ventrum, with four pairs of tubercles per segment. Chaetae show variation in the serration within fascicles. The present study highlights the need for further biodiversity studies on benthic communities in this area.

Two new species of Pilargidae, Ancistrosyllis eidimtaiteae sp. nov. and A. suksani sp. nov., are described from the Gulf of Thailand. These Ancistrosyllis are characterized by having a median and two lateral antennae. Ancistrosyllis eidimtaiteae sp. nov. differs from all other species in the group by having brown pigmentation and by having both short antennae and tentacular cirri, first notopodial hooks from chaetiger 3, and a single type of neurochaetae (long non-limbate capillaries with bifid tips). Ancistrosyllis suksani sp. nov. can be distinguished from other species of Ancistrosyllis by the presence of the first notopodial hooks from chaetiger 4 and three types of neurochaetae (short, stout, falcates with a subdistal small guard, short limbates, and long bifid non-limbates). An updated key to the Ancistrosyllis species of this group is also provided.