Rhizaria

Rhizaria is a supergroup of mostly unicellular eukaryotic organisms. Most are amoeboid protists characterized by their slender, thread-like pseudopodia, known as filopodia, which they use to capture food and move. These filopodia often form intricate networks. Rhizaria is a highly diverse group, both morphologically and genetically.

Classification and Phylogeny:

Rhizaria belongs to the SAR supergroup (Stramenopiles, Alveolates, and Rhizaria), a major lineage within eukaryotes. The phylogenetic relationships within Rhizaria have been refined through molecular studies. The group typically includes three major lineages:

• Cercozoa: A diverse group of amoeboid and flagellated protists. Many are heterotrophic, feeding on bacteria, algae, or other protists, while some are photosynthetic. Some Cercozoa have shells (tests).
• Foraminifera: Primarily marine amoeboid protists characterized by their reticulopodial networks (complex, branching pseudopodia) and the presence of a test, or shell, composed of calcium carbonate or agglutinated particles. Foraminifera are important components of marine ecosystems and are widely used in paleoceanography and biostratigraphy.
• Radiolaria: Marine amoeboid protists with intricate siliceous skeletons. They possess axopodia (stiff, needle-like pseudopodia supported by microtubules) that radiate outwards from the cell body. Radiolaria are planktonic and are important contributors to marine sediments.

Characteristics:

• Filopodia/Reticulopodia/Axopodia: Rhizarians are defined by their thread-like pseudopodia used for feeding and movement. Cercozoa use filopodia, Foraminifera use reticulopodia, and Radiolaria use axopodia.
• Tests/Shells: Many rhizarians, particularly Foraminifera and Radiolaria, produce elaborate external tests or skeletons. These structures provide protection and support. The composition and morphology of these tests are important taxonomic characteristics.
• Habitat: Rhizarians are found in a wide range of aquatic environments, including marine, freshwater, and soil habitats.
• Nutrition: Most rhizarians are heterotrophic, feeding on bacteria, algae, and other protists. Some Cercozoa are photosynthetic.
• Ecological Importance: Rhizarians play important roles in marine and freshwater ecosystems as consumers and decomposers. Foraminifera and Radiolaria are also important components of marine sediments and are used as paleoenvironmental indicators.

Fossil Record:

The fossil record of Rhizaria, particularly Foraminifera and Radiolaria, is extensive. Their mineralized skeletons are readily preserved in sediments, providing valuable insights into the evolution of these groups and past environmental conditions. The fossil record extends back to the Cambrian period.