Rhizaria

Abstract

Abstract Rhizaria are a diverse group of amoeboid protists characterised by the presence of pseudopodia, cytoplasmic extensions used for locomotion and/or predation. Pseudopodial morphology includes broad and fan‐shaped anterior extensions, finger‐like tubular extensions or a web‐like network. Some Rhizaria build complex tests using inorganic/organic material or a variety of minerals (e.g. opaline, calcium carbonate). The shape of the pseudopodia, and the morphology of the enclosing test or skeleton, when present, are major defining taxonomic characteristics. They are distributed worldwide from surface to deep waters, but also in freshwater and terrestrial ecosystems. Therefore, Rhizaria are important marine and terrestrial protists at the base of food webs and thus provide a major link in the transfer of energy to higher‐order consumers. While most are capable of preying on diverse organisms (from inorganic marine snow to bacteria and metazoans), others are obligatory parasites (of land plants or marine invertebrates), and only a few contribute to primary production via photosymbiosis (e.g. Radiolaria) or acquired phototrophy (e.g. Chlorarachnea). Key Concepts The shape and organisation of pseudopodia were among major characteristics used to classify members of the Rhizaria. However, pseudopodial shape is not a conserved feature; and additional evidence, including the fine structure and molecular genetic data, is needed to create a natural classification scheme. The diversity of Rhizaria is broad, including a wide range in size from small naked amoebae (several microns) to giant single cells (up to several centimetres or larger for Xenophyophores). A vast majority of Rhizaria are heterotrophs, feeding on a large diversity of prey, but two groups (Chlorarachnea and Paulinella ) include photosynthetic species. Many of the Rhizaria are free‐living, forming important links in food chains, but others are parasites of algae, aquatic or terrestrial plants, or even marine invertebrates. Some marine Rhizaria (e.g. Radiolaria or Foraminifera) are mixotrophs, hosting a diversity of algal photosymbionts (e.g. green algae, dinoflagellates, etc.). Some testate Rhizaria, bearing mineralised skeletons, settle into aquatic sediments upon death and contribute to the palaeontological record of life and the environment over vast geological time periods.