The Indo-West Pacific pinnotherid crab genus, Ostracotheres H. Milne Edwards, 1853, was recognised for species with a simple, unadorned carapace without marginal lamellae, and a spatulate maxilliped 3 propodus with a rounded or subtruncate apex. Prior to the present study, the genus included: O. tridacnae (Red Sea; type species), O. affinis (Mauritius, Djibouti), O. cynthiae Nobili, 1906a (Red Sea); O. spondyli Nobili, 1905 (Persian Gulf); O. subglobosus (Baker, 1907) (southern Australia); O. holothuriensis Baker, 1907 (southern Australia); O. subquadratus Sakai, 1939 (Japan); and O. tomentipes Takeda & Konishi, 1994 (Japan). Reconsideration of species assigned to Ostracotheres shows they differ considerably in other aspects such as carapace physiognomy, pereopod article proportions, and gonopod structure (where known), suggesting the genus is not monophyletic. Thus, Ostracotheres is revised based on type and other material. Ostracotheres sensu stricto is restricted to O. tridacnae and O. cynthiae. Ostracotheres tridacnae is confirmed as a senior synonym of O. savignyi H. Milne Edwards, 1853, and appears to be restricted to the Red Sea as a commensal of tridacnine clams; records of O. tridacnae from southeast Africa are referred to Afropinnotheres dofleini (Lenz in Lenz & Strunck, 1914). Similarly, O. cynthiae appears to be restricted to the Red Sea as a commensal of ascidians; the single record of O. cynthiae from Indonesia requires confirmation but its published account is not consistent with O. cynthiae sensu stricto. A new genus, Austrotheres, is proposed for O. holothuriensis and a new species from eastern Australia. Ostracotheres subglobosus and O. subquadratus, considered by some to be conspecific, are shown to be separate valid species, with the identity of the latter fixed by a neotype selection; both are transferred to a new genus, Discorsotheres, together with O. spondyli and a new species from the tropical western Pacific. Ostracotheres affinis and O. tomentipes are referred to a new genus Latatheres, with the identity of the former fixed by a neotype selection. Keys to the species of the genera studied herein and to pinnotherid genera with a 2-segmented maxilliped 3 palp are provided.

The Indo-West Pacific pinnotherid crab genus, Ostracotheres H. Milne Edwards, 1853, was recognised for species with a simple, unadorned carapace without marginal lamellae, and a spatulate maxilliped 3 propodus with a rounded or subtruncate apex. Prior to the present study, the genus included: O. tridacnae (Red Sea; type species),O. affinis (Mauritius, Djibouti), O. cynthiae Nobili, 1906a (Red Sea); O. spondyli Nobili, 1905 (Persian Gulf); O.subglobosus (Baker, 1907) (southern Australia); O. holothuriensis Baker, 1907 (southern Australia); O. subquadratusSakai, 1939 (Japan); and O. tomentipes Takeda & Konishi, 1994 (Japan). Reconsideration of species assigned toOstracotheres shows they differ considerably in other aspects such as carapace physiognomy, pereopod articleproportions, and gonopod structure (where known), suggesting the genus is not monophyletic. Thus, Ostracotheresis revised based on type and other material. Ostracotheres sensu stricto is restricted to O. tridacnae and O. cynthiae.Ostracotheres tridacnae is confirmed as a senior synonym of O. savignyi H. Milne Edwards, 1853, and appears tobe restricted to the Red Sea as a commensal of tridacnine clams; records of O. tridacnae from southeast Africa arereferred to Afropinnotheres dofleini (Lenz in Lenz & Strunck, 1914). Similarly, O. cynthiae appears to be restrictedto the Red Sea as a commensal of ascidians; the single record of O. cynthiae from Indonesia requires confirmationbut its published account is not consistent with O. cynthiae sensu stricto. A new genus, Austrotheres, is proposedfor O. holothuriensis and a new species from eastern Australia. Ostracotheres subglobosus and O. subquadratus,considered by some to be conspecific, are shown to be separate valid species, with the identity of the latter fixedby a neotype selection; both are transferred to a new genus, Discorsotheres, together with O. spondyli and a newspecies from the tropical western Pacific. Ostracotheres affinis and O. tomentipes are referred to a new genusLatatheres, with the identity of the former fixed by a neotype selection. Keys to the species of the genera studiedherein and to pinnotherid genera with a 2-segmented maxilliped 3 palp are provided.

Opinions and declarations Rendered by the International Commission on Zoological Nomenclature