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Chrompodellids are a phylum of single-celled protists belonging to the Alveolata supergroup. It comprises two different polyphyletic groups of flagellates: the colpodellids, phagotrophic , and the chromerids, photosynthetic algae that live as symbionts of corals. These groups were independently discovered and described, but molecular phylogenetic analyses demonstrated that they are intermingled in a clade that is the closest relative to Apicomplexa, and they became collectively known as chrompodellids. Due to the history of their research, they are variously known in biological classification as Chromerida or Colpodellida (ICZN)/ Colpodellales (ICN).
Some species, considered , do not ingest prey cells, but rather fully or partially "suck" their contents, a process known as myzocytosis, common among . They feed on bacteria and other protozoa, such as , , , and . After feeding, they internalize their flagella, become microbial cyst and divide into tetrads, similarly to the development of zoospores in Chromera. The cells conjugate after leaving the cyst, which could imply a sexual stage.
The two genera are markedly different from each other, both in phylogeny and life cycles. Chromera autospores are 5–7 μm in diameter. They asexually reproduce through binary division to develop autosporangia, which in turn harbor 2–4 autospores under an additional membrane. They also form Zoospore, up to 15 μm in diameter, capable of generating 2–10 flagellated that strongly resemble colpodellids. This dispersal process is similar to the schizogony of apicomplexans. Sexual reproduction has not been observed. Under adverse environmental conditions, they form resistant microbial cyst that remain viable for years. Similarly to apicomplexans, they undergo closed mitosis, without dissolving the nuclear envelope. In addition, Chromera produces high amounts of an exclusive type of isofucoxanthin.
Vitrella autospores, by contrast, start measuring 3 μm and grow up to 40 μm before transforming into sporangia that generate dozens of autospores or zoospores. There are two types of Vitrella zoospores: one is generated by budding from the mother cell and exhibits flagella outside the cytoplasm, the other develops axonemes and flagella within their cytoplasm and are ejected from the mother cell after maturing, though both types lack a pseudo-conoid. Some zoospores fuse, possibly representing a sexual stage in the life cycle. In addition, Vitrella produces vaucheriaxanthin.
The first chromerid alga, Chromera, was discovered and isolated from in 2001. It was described in 2008 as the first member of a new phylum Chromerida, followed by Vitrella in 2012. They showed morphological resemblance to colpodellids and other . In the following years, Phylogenetics studies reported the evolutionary proximity between colpodellids and chromerid algae. This was supported by the discovery of retained vestigial in some colpodellid species. In 2015 there was strong support for a clade containing the two groups, phylogenetically mixed with each other, which rendered both as Polyphyly. The clade was given the provisional name " chrompodellids", later referred to as Chrompodellida by posterior studies.
Between 2004 and 2017, Cavalier-Smith retained the classification scheme of Apicomonadea, from which he excluded Perkinsida, leaving only colpodellids and chromerids across multiple orders. In addition, several genera of Flagellate were added on the basis of morphological data: Algovora, Microvorax and Dinomonas. Due to lacking Sequencing, these genera have been excluded from later classifications. Two genera, Chilovora and Alphamonas, were eventually rejected in his classification, but later revisions by other authors maintain them as independent genera supported by molecular data.
The treatment of chrompodellids as a subgroup of Apicomplexa, under the name of Apicomonadea, was rejected by the International Society of Protistologists. In a 2019 revision of eukaryotic classification, Protistology emended the previous name Colpodellida to contain all chrompodellids, and treated it as a direct subgroup of Alveolata, independent from Apicomplexa. Later, Phycology advocated for this treatment as a separate phylum, and regularized it under the name of Chromerida or Chromeridophyta, composed of a single class Colpodellophyceae and a single order Colpodellales, in accordance to the nomenclatural rules of the ICN. However, other authors consider them a subgroup of the phylum Myzozoa, together with apicomplexans, Perkinsea and .
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