The gymnosphaerids (or Gymnosphaerida)
are a small group of
heliozoa
Heliozoa, commonly known as sun-animalcules, are microbial eukaryotes (protists) with stiff arms ( axopodia) radiating from their spherical bodies, which are responsible for their common name. The axopodia are microtubule-supported projections fro ...
n protists found in marine environments. They tend to be roughly spherical with radially directed
axopods, supported by
microtubule
Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 an ...
s in a triangular-hexagonal array arising from an amorphous central
granule.
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 actinophryids are an order of heliozoa, a polyphyletic array of stramenopiles, having a close relationship with pedinellids and ''Ciliophrys''. They are common in fresh water and occasionally found in marine and soil habitats. Actinophryids a ...
''. 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