A new troglobitic sigillid genus, Kasella, is found in the submarine caves of the Ryukyu Islands. Sigillids are primitive podocopid ostracods having ancient characters such as unequal valves and many small adductor muscle scars. Kasella is possibly derived from the sigillid Cardobairdia, a deep-sea muddy bottom inhabitant, or its relatives. Submarine caves are characterized as both ‘safe places’, where biological factors such as competition and predation are reduced, and as ‘crypts’, representing dark and oligotrophic environments. It is likely that Kasella has successfully colonized hard substrates in caves, particularly exploiting the crevices on cave walls and ceilings. Kasella may have adapted to this cryptic habitat in response to strong cytheroidean competition and predation by, for example, gastropods that do not survive well in dark and oligotrophic submarine caves. In adapting to an oligotrophic cave environment, Kasella has undergone significant divergence in carapace morphology, possibly accompanying functional and behavioural changes.