Three new species and one new record of Deimatidae (Echinodermata, Holothuroidea, Synallactida) discovered in the South China Sea and the Mariana fore-arc area using integrative taxonomic methods.

Deep-sea holothurian specimens were collected during five scientific expeditions (2018-2023) using the submersible vehicle 'Shenhaiyongshi'. Our examination of specimens of Deimatidae from the South China Sea and the Mariana fore-arc area revealed three new species, which were described as Oneirophantaidsseicasp. nov., Oneirophantabrunneannulatasp. nov., and Oneirophantalucernasp. nov. These species were distinguished from each other and from congeners by the arrangement, and number of ventrolateral tube feet and ossicle types. We also reported Oneirophantamutabilismutabilis Théel, 1879 for the first time from the Mariana fore-arc area, and we recorded Deimavalidumvalidum for the second time from the South China Sea. The taxonomy of these new species and new records is discussed, and a phylogenetic analysis based on a concatenated dataset of 16S and COI genes was conducted. Additionally, the inter- and intraspecific genetic divergences we calculated among deimatid species. The results support the assignment of these new species to the genus Oneirophanta and their separation from congeners. A description of the main morphological characters of Oneirophanta species is also provided. The data were collected from geographically diverse areas and suggest that species of Deimatidae were abundant in the Pacific Ocean and occupied a wide range of depths.