Gymnosphaerid

Genera

There are only three genera, each with a single species: Gymnosphaera albida, Hedraiophrys hovassei, and Actinocoryne contractilis.

• 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. In order to move, it collapses the stalk and head into an amoeboid form which is capable of migration. 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.