Zooflagellate

Zooflagellates are single-celled eukaryotes with flagella (i.e., flagellates). They are heterotrophic flagellates, as opposed to phytoflagellates, which are photosynthetic.^[1] The term "zooflagellate" is also used to refer to reproductive cells or zoospores belonging to multicellular organisms, such as fungi.^[2]

History

Zooflagellates
An illustration of choanoflagellates
Scientific classification (obsolete)
Domain:	Eukaryota
Kingdom:	Protista
Phylum:	Sarcomastigophora
Subphylum:	Mastigophora (flagellates)
Class:	Zoomastigophorea Calkins, 1909

In historical systems of classification during the 20th century, zooflagellates were grouped as a single taxon Zoomastigophora or Zoomastigophorea within the kingdom Protista.^[3] It included protists that lack chloroplasts and cell walls and have one or more flagella, but not many as in ciliates or opalines,^[4][5] namely:

• Bicosoecids, composing the order Bicosoecida, distinguished by two unequal flagella.^[4] They are currently known as a lineage of Stramenopiles, more closely related to algae such as diatoms than to other zooflagellates.^[6]

• Choanoflagellates, then treated as order Choanoflagellida, distinguished by one anterior flagellum surrounded by a collar.^[4][3] They are now known as the lineage most closely related to animals.^[7]

• Various orders of metamonads,^[8] treated separately at that time: Hypermastigida, Diplomonadida, Oxymonadida, Retortamonadida, Trichomonadida.^[4][3]

• Kinetoplastids, known as order Kinetoplastida at the time, distinguished by one or two flagella and the presence of the kinetoplast.^[3] They are closely related to euglenophytes, a group of phytoflagellates.^[9]

Over time, with the advancements in phylogenetics and ultrastructure studies, the term "zooflagellate" became obsolete as a formal taxon.^[1] Instead, it was adopted as an informal name to refer to any given group of heterotrophic flagellates, regardless of evolutionary relationships.^[10][11][12]