Heliozoa

Heliozoa are a now largely obsolete grouping of protists, formerly classified within the phylum Actinopoda. They are characterized by their spherical bodies and radiating axopodia, stiff, needle-like pseudopodia supported by microtubules. These axopodia are used for prey capture and locomotion.

Heliozoans are primarily freshwater organisms, though some marine and soil-dwelling species exist. They are generally solitary and free-floating, but some species may form temporary colonies.

Characteristics:

• Shape: Typically spherical.
• Axopodia: Numerous radiating axopodia, providing a "sun-like" appearance (hence the name, derived from the Greek "helios" meaning sun and "zoon" meaning animal).
• Microtubules: The axopodia are supported by an internal array of microtubules.
• Extrusomes: Many species possess extrusomes, organelles that discharge rapidly to capture prey or defend against predators.
• Nutrition: Primarily carnivorous, feeding on bacteria, algae, and other protists.
• Reproduction: Reproduction occurs primarily through asexual means, such as binary fission or budding. Sexual reproduction has been observed in some species.
• Shell (Optional): Some heliozoans secrete a shell or test composed of siliceous scales or plates.

Taxonomy and Classification:

The classification of Heliozoa has been significantly revised due to molecular phylogenetic studies. They are no longer considered a formal taxonomic group. Instead, the organisms previously classified as Heliozoa are now distributed among various groups within the Stramenopiles, Alveolates, and Rhizaria (SAR) supergroup and other eukaryotic lineages. This reflects the polyphyletic nature of the traditional Heliozoa classification, meaning the group did not share a single common ancestor. Examples of former heliozoan groups now placed elsewhere include the Centrohelids, Taxopodida and Desmothoracida.