The nucleariids are a group of amoeba

Amoeboid

Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...

e with filose pseudopod

Pseudopod

Pseudopods or pseudopodia are temporary projections of eukaryotic cells. Cells that possess this faculty are generally referred to as amoeboids. Pseudopodia extend and contract by the reversible assembly of actin subunits into microfilaments...

s, known mostly from soils and freshwater. They are distinguished from the similar vampyrellid

Vampyrellid

The Vampyrellidae are a group of cercozoans with filose pseudopods and lacking shells. Vampyrella is typical of the group, which also contains genera such as Platyreta and Arachnula.The term "Aconchulinida" is sometimes used for vampyrellid...

s mainly by having mitochondria

Mitochondrion

In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 1.0 micrometers in diameter...

 with discoid crista

Crista

Cristae are the internal compartments formed by the inner membrane of a mitochondrion. They are studded with proteins, including ATP synthase and a variety of cytochromes. The maximum surface for chemical reactions to occur is within the mitochondria...

e.

Classification

Nucleariids are opisthokont

Opisthokont

The opisthokonts or "Fungi/Metazoa group" are a broad group of eukaryotes, including both the animal and fungus kingdoms, together with the eukaryotic microorganisms that are sometimes grouped in the paraphyletic phylum Choanozoa...

s, the group which includes animal

Animal

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s, fungi

Fungus

A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

 and several smaller groups. Several studies place the nucleariids as a sister group to the fungi.

Characteristics

Nucleariids are usually small, up to about 50 μm in size.

• Nuclearia
  Nuclearia
  Nuclearia is a nucleariid genus.Species include:* Nuclearia delicatula* Nuclearia moebiusi* Nuclearia pattersoni Dyková, Veverková, Fiala, Macháčková & Pecková, 2003* Nuclearia simplex* Nuclearia thermophila...
  
  and Micronuclearia
  Micronuclearia
  Micronuclearia is a Nucleariid genus.It includes the species Micronuclearia podoventralis....
  
  .
• The other genera, Rabdiophrys
  Rabdiophrys
  Rabdiophrys is a genus of nucleariid. It has 19 species, including the species Rabdiophrys anulifera....
  
  , Pinaciophora
  Pinaciophora
  Pinaciophora is an amoeboid genus of Nucleariid.It includes the species Pinaciophora fluviatilis....
  
  , and Pompholyxophrys
  Pompholyxophrys
  Pompholyxophrys is a genus of Nucleariid.It includes the species Pompholyxophrys punicea....
  
  , are freshwater forms with hollow siliceous scales or spines. These were formerly included among the heliozoa
  Heliozoa
  Heliozoa are phagotrophs. They are roughly spherical amoeboids with many stiff, microtubule-supported projections called axopods radiating outward from the cell surface. These give them the characteristic sun-like appearance for which they are named, and are variously used for capturing food,...
  
   as the Rotosphaerida.

According to a 2009 paper, Fonticula

Fonticula

Fonticula is a cellular slime mold which forms a fruiting body in a volcano shape. As long ago as 1979 it has been known to not have a close relationship with either the Dictyosteliida or the Acrasidae, the two well-established groups of cellular slime molds...

, a cellular slime mold, is an opisthokont and more closely related to Nuclearia than to fungi.