Chlamydomonadales
(A) Gonium pectorale, (B) Eudorina elegans, (C) Pleodorina californica, and (D) Volvox carteri
Scientific classification
Clade:	Viridiplantae
Division:	Chlorophyta
Class:	Chlorophyceae
Order:	Chlamydomonadales Fritsch in West & Fritsch, 1927
Families
See text.
Synonyms
Volvocida Francé, 1894, orth. zool. Phytomonadina Blochmann, 1895 Volvocales Oltmanns, 1904

Chlamydomonadales, also known as Volvocales, are an order of flagellated or pseudociliated green algae, specifically of the Chlorophyceae.^[1] Chlamydomonadales can form planar or spherical colonies. These vary from Gonium (four to 32 cells) up to Volvox (500 cells or more). Each cell has two flagella, and is similar in appearance to Chlamydomonas, with the flagella throughout the colony moving in coordination.^{[citation needed]}

Both asexual and sexual reproduction occur. In the former, cells divide until they form new colonies, which are then released. In the smaller forms, typically all cells are involved, but larger forms have anterior vegetative and posterior reproductive cells. Sexual reproduction varies from isogamy (both genders produce flagellated gametes of equal size) to oogamy (one gender produces a much larger, nonmotile gamete).

The classification of the Chlamydomonadales varies. Very often they are taken to include the orders Volvocales and Dunallielales, which contain closely related unicellular flagellates, as suborders. Colony inversion is believed to have arisen twice in this order. Spheroidal colony formation differs between the two lineages: rotation of daughter protoplasts during successive cell divisions in Astrephomene, and inversion after cell divisions in the Volvocaceae.^[2]

As of May 2023, AlgaeBase accepted the following families:^[3]

Traditionally, families and genera within Chlamydomonadales have been classified based on their morphology. However, molecular phylogenetic studies have shown that the evolutionary relationships do not correspond to the traditional taxonomic classifications. Despite this, the families and genera are still in use, because the differences have not been reconciled into a single, useful classification system.^[4]

In 2008, Nakada et al. defined a number of well-supported clades within Chlamydomonadales using PhyloCode. Their relationships, as well as a few representative genera and species, are shown below.^[4]