The gymnosphaerids are a group of heliozoan protists, found in marine environments, and including both mobile and sessile forms. Gymnosphaerids tend to be roughly spherical, with radially directed axopods arising from an amorphous central granule. These are cell projections supported by hexagonally arranged microtubules, as in the centrohelids, from which the gymnosphaerids are distinguished by having tubular rather than flat cristae in the mitochondria. Both flagellate stages and sexual reproduction are unknown. There are three genera in the group, with a single species each.

Gymnosphaera albida is free-living, usually benthic in shallow water. The cells are round and naked, around 70-100 μm in diameter, and resemble the unrelated Actinosphaerium. The outer cytoplasm, or ectoplasm, forms a distinct layer containing large vesicles.

Hedraiophrys hovassei is larger and lives attached to algae and other objects. The cells have a conical base, and are covered with long siliceous spicules. The ectoplasm is distinct and frothy, and typically contains bacterial and algal endosymbionts.

Actinocoryne contractilis is benthic. When feeding, it has a multinucleate base and a contractile stalk up to 150 μm in length, supporting a relatively small uninucleate head, where the central granule and axopods are located. It may move about in a headless amoeboid form. Reproduction is either by budding off the head or fragmentation of the headless form, producing small free-living cells similar to Gymnosphaera, which then attach themselves and regrow the stalk and base.