Capsaspora is a single-celled eukaryote

Eukaryote

A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

 which is a symbiont in the haemolymph of the tropical freshwater snail

Freshwater snail

A freshwater snail is one kind of freshwater mollusc, the other kind being freshwater clams and mussels, i.e. freshwater bivalves. Specifically a freshwater snail is a gastropod that lives in a watery non-marine habitat. The majority of freshwater gastropods have a shell, with very few exceptions....

 Biomphalaria glabrata

Biomphalaria glabrata

Biomphalaria glabrata is a species of air-breathing freshwater snail, an aquatic pulmonate gastropod mollusk in the family Planorbidae, the ram's horn snails....

.

Taxonomy

Although it is an 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...

, it seems not to fit in any of the established opisthokont lineages such as nucleariid

Nucleariid

The nucleariids are a group of amoebae with filose pseudopods, known mostly from soils and freshwater. They are distinguished from the similar vampyrellids mainly by having mitochondria with discoid cristae.-Classification:...

s or Mesomycetozoea

Mesomycetozoea

The Mesomycetozoea are a small group of protists, mostly parasites of fish and other animals.-Significance:...

. It is part of its own group, possibly together with Ministeria

Ministeria

Ministeria is a small single-celled eukaryote which feeds on bacteria. Although it is an opisthokont, it seems not to fit in any of the established opisthokont lineages such as choanoflagellates or animals. It might form a group with Capsaspora, another opisthokont of uncertain placement....

. This group is related to 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 and choanoflagellate

Choanoflagellate

The choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals...

s.

Species

How animals, metazoans originated from their single-celled ancestors may still remain a major question in biology. The research team from University of Barcelona

University of Barcelona

The University of Barcelona is a public university located in the city of Barcelona, Catalonia in Spain. It is a member of the Coimbra Group, LERU, European University Association, Mediterranean Universities Union, International Research Universities Network and Vives Network...

has however, come up with new findings and Capsaspora owczarzaki has recently emerged as the prime candidate.