Actinophryid

The actinophryids are a small group of heliozoan protists, comprising a few species common in fresh water and especially in lakes and rivers. They are unicellular and spherical in shape, without any shell or test, with radial axopods projecting from the body. These are granular projections, supported by microtubules arranged in a unique double coil pattern, that adhere to passing prey and allow the creature to float or roll about. It appears that the actinophryids belong to a specialized group of algae called the actinodines, which are included among the stramenopiles.

Reproduction takes place by fission. Under unfavorable conditions, the organism will form a cyst, which is multi-walled and covered in spikes. While encysted it may undergo a peculiar process of autogamy[?] or self-fertilization, where it goes through meiosis and divides to form two gametes, which then fuse together again. This is the only form of sexual reproduction that occurs within the group, though it is really more genetic reorganization than reproduction.

There are two genera of actinophryids. Actinophrys are small, around 50 μm in diameter with axopods around 100 μm in length, and have a single central nucleus. Some are found in marine and soil habitats. Actinosphaerium are several times larger, around 200-1000 μm in diameter, with many nuclei, and are found exclusively in fresh water. Two more genera, Echinosphaerium and Camptonema, have been described but are now considered equivalent to Actinosphaerium.