Two new species of Aricidea Webster, 1879 (Paraonidae), Aricidea (Acmira) anusakdii sp. nov. and Aricidea (Aricidea) thammapinanae sp. nov. were collected from 10–26.5 m depth, in shallow soft bottoms with mud mixed with sand and shell substrates at Songkhla Sea, the Gulf of Thailand between 2011–2018. Aricidea (Acmira) anusakdii sp. 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) thammapinanae sp. 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.

Specimens were collected between 2011 and 2018 in the southern Gulf of Thailand (7°14'21"–7°49'21"N; 100°24'42"–100°49'0"E) (Fig. 1), with a Van Veen grab (0.1 m²) at depths ranging from 10 to 26.5 m. The collected samples were sieved with 2.0 mm, 1.0 mm and 0.5 mm mesh screens in the field. Later, water and sediment from the sieved grab samples were passed through a 300 µm filter bag. Specimens retained by both separation methods were separately fixed with a 4% formaldehyde in seawater solution. In the laboratory, samples were washed with fresh water and transferred to 70% ethanol. The polychaetes were sorted into taxonomic groups using a stereomicroscope and those belonging to the proposed new species were examined under dissection and compound light microscopes. SEM images were produced with a JEOL JSM-5800LV microscope and Apreo-Field Emission Scanning Electron Microscope (FESEM) at the Office of Scientific Instrument and Testing (OSIT), Prince of Songkla University, Hatyai, Thailand. Light microscope photographs were taken with a Leica digital camera in the SEC, Prince of Songkla University. The morphological measurements of the holotype are indicated in the taxonomic description. Information on character variability found in the paratypes is included in parentheses. The confirmation of the taxonomic status of the new species was based on the excellent revision and compilation of the diagnostic characteristics of all recognized species of the subgenera Aricidea (Acmira) and Aricidea (Aricidea) by Blake (1996), Lovell (2002) and Arriaga-Hernández et al. (2013). For comparative purposes, a table with the main diagnostic characters of the new species and closely related species was also included.

            The type material was deposited in the Princess Maha Chakri Sirindhorn Natural History Museum, Prince of Songkla University (PSUZC) and the Australian Museum (AM). Additional material is maintained in the personal collections of Jintana and Sakanan Plathong at MEM (Marine Ecosearch Management Co., Ltd.).