|Ammonia tepida (Foraminifera)|
The Rhizaria are a species-rich supergroup of mostly unicellular eukaryotes. Except for the Chlorarachniophytes and three species in the genus Paulinella in the phylum Cercozoa, they are all non-photosynthethic, but many foraminifera and radiolaria have a symbiotic relationship with unicellular algae. A multicellular form, Guttulinopsis vulgaris, a cellular slime mold, has also been described. This supergroup was proposed by Cavalier-Smith in 2002. Being described mainly from rDNA sequences, they vary considerably in form, having no clear morphological distinctive characters (synapomorphies), but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. The Rhizaria’s pseudopodial mineral skeleton network, which is made up of opal (SiO₂), celestite (SrSO₄), or calcite (CaCO₃), is what differentiates it from the amoebae. It can attain sizes of more than a centimeter with some species being able to form cylindrical colonies approximately 1 cm in diameter and greater than 1 m in length. They feed by capturing and engulfing prey within the extensions of their pseudopodial skeletons, and some even become homes for algae, which aid in boosting the total primary production of the ocean.
The three main groups of Rhizaria are:
- Cercozoa – various amoebae and flagellates, usually with filose pseudopods and common in soil
- Foraminifera – amoeboids with reticulose pseudopods, common as marine benthos
- Radiolaria – amoeboids with axopods, common as marine plankton
A few other groups may be included in the Cercozoa, but some trees appear closer to the Foraminifera. These are the Phytomyxea and Ascetosporea, parasites of plants and animals, respectively, and the peculiar amoeba Gromia. The different groups of Rhizaria are considered close relatives based mainly on genetic similarities, and have been regarded as an extension of the Cercozoa. The name Rhizaria for the expanded group was introduced by Cavalier-Smith in 2002, who also included the centrohelids and Apusozoa.
Another order that appears to belong to this taxon is the Mikrocytida. These are parasites of oysters. This includes the causative agent of Denman Island Disease, Mikrocytos mackini a small (2−3 μm diameter) amitochondriate protistan.
Historically, many rhizarians were considered animals because of their motility and heterotrophy. However, when a simple animal-plant dichotomy was superseded by a recognition of additional kingdoms, taxonomists generally placed Rhizarians in the kingdom Protista. When scientists began examining the evolutionary relationships among eukaryotes using molecular data, it became clear that the kingdom Protista was paraphyletic. Rhizaria appear to share a common ancestor with Stramenopiles and Alveolates forming part of the SAR (Stramenopiles+Alveolates+Rhizaria) super assemblage. Rhizaria has been supported by molecular phylogenetic studies as a monophyletic group. Biosynthesis of 24-isopropyl cholestane precursors in various rhizaria suggests a relevant ecological role already during the Ediacaran.
In 2019, the Cercozoa were recognized as a basal Rhizaria group, as sister of the Retaria.
Ebria sp. (Cercozoa: Ebridea)
Rhipidodendron sp. (Cercozoa: Spongomonadea)
Euglypha sp. (Cercozoa: Euglyphida)
Phaeodarians (Cercozoa: Phaeodarea)
Vampyrella sp. (Cercozoa: Vampyrellidae)
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