Two new species of Ancistrosyllis McIntosh, 1878 (Annelida: Pilargidae) from the Gulf of Thailand, Western Pacific.

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.