Holozoa

Holozoa () is a clade of organisms that includes animals and their closest single-celled relatives, but excludes fungi and all other organisms. Together they amount to more than 1.5 million species of purely heterotrophic organisms, including around 300 unicellular species. It consists of various subgroups, namely Metazoa (or animals) and the protists Choanoflagellata, Filasterea, Pluriformea and Ichthyosporea. Along with fungi and some other groups, Holozoa is part of the Opisthokonta, a supergroup of eukaryotes. Choanofila was previously used as the name for a group similar in composition to

The Filozoa are a monophyletic grouping within the Opisthokonta. They include animals and their nearest unicellular relatives (organisms which are more closely related to animals than to fungi or Mesomycetozoa).

Choanozoa is a clade of opisthokont eukaryotes consisting of the choanoflagellates (Choanoflagellatea) and the animals (Animalia). The sister-group relationship between the choanoflagellates and animals has important implications for the origin of the animals. The clade was identified in 2015 by Graham Budd and Sören Jensen, who used the name Apoikozoa. The 2018 revision of the classification first proposed by the International Society of Protistologists in 2012 recommends the use of the name Choanozoa.

Corallochytrium limacisporum is a species of unicellular holozoan eukaryote. It is the only species in genus Corallochytrium and the class Corallochytrea. Most research concerning this genus has been done to uncover the evolution of animals and fungi, as Corallochytrium is related to these groups and possess characteristics of both, such as animal and fungal enzymatic trademarks (C-14 reductase and α-aminoadipate reductase (α-AAR) respectively).

Syssomonas is a monotypic genus of unicellular flagellated protists containing the species Syssomonas multiformis. It is a member of Pluriformea inside the lineage of Holozoa, a clade containing animals and their closest protistan relatives. It lives in freshwater habitats. It has a complex life cycle that includes unicellular amoeboid and flagellated phases, as well as multicellular aggregates, depending on the growth medium and nutritional state.

Bicellum is a genus of fossil holozoans containing the single species Bicellum brasieri. It is one billion years old and could be the oldest example of complex multicellularity in the evolutionary lineage leading to the animals, and has been described as bridging "the gap between the very first living creatures — single-celled organisms — and more complex multicellular life." It was discovered in 2021, and is posthumously named after the late Martin Brasier, a paleontologist who was a co-author of the paper that first described it.

Pluriformea is a sibling clade of the Filozoa, and consists of Syssomonas multiformis and the Corallochytrea. Together with the Ichthyosporea and the Filozoa, they form the Holozoa.