Abstract
Rhoicosphenia michali sp. nov., described from the shallow sublittoral zone in Antarctica, is the second species in the genus with just one raphe slit on its convex valve. The first species, Rhoicosphenia flexa, was also described from marine coastal habitats in the Southern Hemisphere. Here, the morphology and ecology of R. flexa and R. michali are compared. The new species described herein may be endemic to Antarctica and can be found as free living cells on the shallow seabed, although it mainly occurs on the red alga Georgiella confluens, which is endemic to Antarctica.