Gymnosphaerid
From Free net encyclopedia
{{Taxobox
| color = khaki
| name = Gymnosphaerids
| regnum = Protista
| classis = Centrohelea
| ordo = Axoplasthelida
| familia = Gymnosphaeridae
| subdivision_ranks = Species
| subdivision =
Gymnosphaera elbida
Hedraiophrys hovassei
Actinocoryne contractilis
}}
The gymnosphaerids are a small group of heliozoan protists found in marine environments. They tend to be roughly spherical with radially directed axopods, whose ultrastructure is typical of the centrohelids. They differ from other centrohelids in having mitochondria with tubular rather than flat cristae, and as such have been considered a separate group, but it is now known this trait may vary among closely related genera. They also have an amorphous central granule or axoplast instead of a tripartite centroplast. There are three genera, each with a single species:
- 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.