Review of the genus Zygothuria Perrier, 1898 and the Atlantic group of species of the genus Mesothuria Ludwig, 1894 (Synallactidae: Holothuroidea) with description of the new species Mesothuria millerisp. nov.

Species of the genus Mesothuria (fam. Synallactidae) occurring in the Atlantic Ocean are revised. The genus includes 25 valid species, nine of them known from the Atlantic. The new species Mesothuria milleri sp. nov. is described from the north-east Atlantic. It is argued that the genus Zygothuria, established by Perrier in 1898 and recognized later only by Deichmann, should be maintained as a separate genus. Mesothuria and Zygothuria differ in body form, arrangement of ambulacral appendages, and structure of segments of calcareous ring and ossicles from the body wall. The six known species of Zygothuria are reviewed. Zygothuria oxysclera, a former variation of Zygothuria lactea, is proposed as a valid species. Identification keys to species of the genus Zygothuria and Atlantic species of the genus Mesothuria are provided. Data on the life history and reproductive biology of Mesothuria milleri and Zygothuria lactea are given. Both species are common in the north-east Atlantic.