A group of heliozoan protists, found in marine environments and including both mobile and sessile forms. Gymnosphearids tend to be roughly spherical, with radially directed pseudopods arising from an amorphous central granule, supported by hexagonally arranged microtubules, as in the Centrohelids. They are distinguished by having tubular rather than flat cristae in the mitochondria. Flagellate stages and sexual reproduction are unknown. There are three genera in the group, with a single species each: Gymnosphaera albida, Hedraiophrys hovassei, and Actinocoryne contractilis.
Gymnosphaera are round, naked cells about 70-100 μm across, with a distinct ectoplasm containing large vesicles, somewhat similar to the unrelated Actinosphaerum. There is a single nucleus, lying away from the center of the cell. These are usually benthic in shallow water.
Hedraiophrys are larger, and covered with long siliceous spicules. In these cells the outer cytoplasm (ectoplasm) is distinct and frothy, and typically contains bacterial and algal symbionts. Adult cells live attached to algae or other objects by a conical base.
Actinocoryne are differentiated into a multinucleate base, a contractile stalk up to 150 μm in length, and a relatively small uninucleate head sporting the axopods. They reproduce by budding of the head or by fragmentation of the base after it absorbs the head, producing small free-living cells similar to Gymnospheara, which then attach themselves and regrow the stalk and base. Actinocoryne are again usually benthic in shallow water, and typically move about in the headless amoeboid form.