Nucleariid

The nucleariids, or nucleariid amoebae, are a group of amoebae that compose the sister clade of the fungi. Together, they form the clade Holomycota. They are aquatic organisms found in freshwater and marine habitats, as well as in faeces. They are free-living phagotrophic predators that mostly consume algae and bacteria.

Nucleariids are characterized by simple, spherical or flattened single-celled bodies with filopodia (fine, thread-like pseudopods), covered by a mucous coat. They lack flagella and microtubules. Inside the cytoplasm of some species are endosymbiotic proteobacteria. Some species are naked, with only the mucous coat as cover, while others (known as 'scaled' nucleariids) have silica-based or exogenous particles of various shapes.

An exceptional nucleariid, Fonticula alba, develops multicellular fruting bodies (sorocarps) for spore dispersal. It is one of several cases of independently evolved multicellularity within Opisthokonta, the clade that houses both Holozoa (which includes animals) and Holomycota.

Initially, nucleariids were grouped with other filose amoebae (i.e., with filopodia) based on their superficial similarity. Silica-scaled and naked nucleariids were classified into separate families from one another, Pompholyxophryidae and Nucleariidae, respectively. Due to its nature as a slime mold, the genus Fonticula has also been classified separatedly, particularly with acrasids and other slime molds. With advancements in electron microscopy and molecular phylogenetics, the three groups were revealed to belong to the same clade as sister to the fungi. Due to lack of molecular data, the three groups are treated as one family, under the name of Nucleariidae.

Various conflicting systems of above-family classification exist for nucleariids, with older systems grouping them as a class Cristidiscoidea composed of two orders: one for Fonticula and another for the remaining species. Mycologists regard them as an independent kingdom of life, Nucleariae, with two phyla that mirror those two orders. They are generally accepted by protistologists as a single order Rotosphaerida, which is the oldest taxonomic name for these organisms.