{{Short description|Group of protists}} {{Automatic taxobox | name = Centrohelids | taxon = Centroplasthelida | authority = Febvre-Chevalier & Febvre, 1984<ref name="Adl-2019"/> | image = Raphidiophrys contractilis.jpg | image_caption = ''Raphidiophrys contractilis'' | subdivision_ranks = Subgroups | subdivision_ref = <ref name="Shishkin-2023"/> | subdivision = *'''Pterocystida''' **Clypiferidae **''Meringosphaera'' **Raphidophryidae **Pterista *'''Panacanthocystida''' **Chthonida **Acanthocystida ---- {{small|'''''Incertae sedis'''''}} *Spiculophryidae *''Parasphaerastrum'' *''Heteroraphidiophrys'' *''Heterophrys'' *''Choanocystis'' | synonyms = * Centroheliozoa <small>Cushman & Jarvis 1929 sensu Durrschmidt & Patterson 1987</small><ref name="Dürrschmidt-1987"/> * Centrohelida <small>Kühn 1926</small><ref>Kühn, A. (1926). ''Morphologie der Tiere in Bildern''. Heft 2: Protozoen. Teil 2. Rhizopoden. Gebrüder Borntraeger: Berlin.</ref> * Centrohelea {{au|Kühn 1926 emend. Cavalier-Smith 1993}}<ref>{{cite journal|last1=Cavalier-Smith|first1=Thomas|author-link=Thomas Cavalier-Smith|title=Kingdom Protozoa and its 18 phyla|journal=Microbiological Reviews|date=1993|volume=57 |pages=953–994|pmc=372943|pmid=8302218|doi= 10.1128/mr.57.4.953-994.1993|issue=4}}</ref> }}
The '''centrohelids''' or '''centroheliozoa''' are a group of heliozoan protists, single-celled eukaryotes with stiff radiating arms (known as axopodia) supported by microtubules and bearing extrusomes (known as kinetocysts). Their cells are spherical, ranging from 3 to 150 μm. Unlike other heliozoa, centrohelids lack flagella, have flat ribbon-shaped mitochondrial cristae, and arrange their microtubules in hexagons or triangles. Their microtubule-generating organelle, the centroplast, has a unique shape with a central trilamellar disc surrounded by two hemispherical caps. Some are naked or covered in a mucous coat, but most centrohelids produce cell coverings, namely organic spicules and siliceous scales of various species-specific shapes. Several species form colonies.
Centrohelids are passive predators with a cosmopolitan distribution. They feed on bacteria, other protists, and invertebrate larvae by phagocytosis; they can merge several cells around a larger prey to ingest it. Although they have been studied in aquatic (mostly freshwater) environments, they are more diverse in soil habitats. They include both free-floating and benthic forms, some of which attach to the substrate by a stalk.
Centrohelids are the closest relatives of the haptophyte algae, together forming the clade Haptista. Both groups have mineralized scales and at least one thin microtubule-based appendage. The common ancestor of centrohelids lost flagella and probably produced the two known types of cell coverings, organic spicules and complex siliceous scales. Some species can still produce both, while others secondarily lost or simplified them.
Centrohelids compose the class '''Centroplasthelida''', in reference to their centroplast. Around 130 species have been described, but they only represent about 10% of the total estimated diversity, according to environmental DNA surveys. They are classified in 11 families grouped into two major clades, Pterocystida and Panacanthocystida.
==Characteristics==
Centrohelids are heliozoans or "sun animalcules", a type of single-celled organisms that have axopodia, narrow stiff projections radiating from the cell and granting a sun ray appearance.<ref name="Gast-2017"/> These axopodia are internally supported by bundles of microtubules (axonemes) and bear visible extrusomes,<ref name="Patterson-1999">{{cite journal|last1=Patterson|first1=David J.|author-link1=David J. Patterson|title=The Diversity of Eukaryotes|journal=The American Naturalist|date=October 1999|volume=154|issue=S4|url=https://www.journals.uchicago.edu/doi/epdf/10.1086/303287|url-access=subscription|doi=10.1086/303287|pages=S96–S124|pmid=10527921|s2cid=4367158}}</ref> organelles involved in the capture of prey, known as kinetocysts in centrohelids.<ref name="Gast-2017"/> Their cells are spherical, ranging in size from 3 μm (as in ''Choanocystis minima'') to 150 μm (in ''Acanthocystis turfacea'').<ref name="Zagumyonnyi-2021"/>
Centrohelid heliozoa are distinguished from other heliozoa by the absence of flagella<ref name="Cavalier-Smith-2003"/><ref name="Cavalier-Smith-2012"/><ref name="Cavalier-Smith-2015">{{cite journal|last1=Cavalier-Smith|first1=Thomas|author-link1=Thomas Cavalier-Smith|last2=Chao|first2=Ema E.|last3=Lewis|first3=Rhodri|title=Multiple origins of Heliozoa from flagellate ancestors: New cryptist subphylum Corbihelia, superclass Corbistoma, and monophyly of Haptista, Cryptista, Hacrobia and Chromista|journal=Molecular Phylogenetics and Evolution|volume=93|date=31 July 2015|doi=10.1016/j.ympev.2015.07.004|doi-access=free|pages=331–362}}</ref> and by several cellular features. One is their flat ('lamellate'), ribbon-shaped mitochondrial cristae.<ref name="Gast-2017"/><ref name="Patterson-1999"/> Another is a uniquely shaped microtubule-organizing center (MTOC), the organelle responsible for generating the axopodia.<ref name="Gerasimova-2023"/> The MTOC of centrohelids, known as the centroplast, is located at the center of the cell and has a unique inner differentiation of a central trilamellar disc sandwiched between two dense hemispherical structures ('caps'), each around 0.1–1.5 μm in diameter. Centrohelids are also distinct in the arrangement of their axoneme-forming microtubules into hexagons and triangles.<ref name="Gast-2017"/><ref name="Patterson-1999"/><ref name="Shishkin-2023">{{cite journal|last1=Shɨshkin-Skarð|first1=Y.|last2=Drachko|first2=D.|last3=Zlatogursky|first3=V.V.|title=Shedding light on the origin of Acanthocystidae: ''Ricksol blepharistes'' gen. n., sp. n. (Ricksolidae fam. n., Panacanthocystida, Centroplasthelida), with notes on the evolution of the genera ''Acanthocystis'', ''Ozanamia'' gen. n. (Ozanamiidae fam. n.), and "''Heterophrys''-like organisms"|journal=Organisms Diversity and Evolution|volume=23|pages=263–274|date=26 December 2022|doi=10.1007/s13127-022-00595-3}}</ref>
thumb|upright=2|center|Diagram of a centrohelid showing the different cellular compartments
A few centrohelids are naked (like ''Oxnerella'') or with a mucous coat. The majority are able to produce two kinds of cell coverings: organic spicules and siliceous scales,<ref name="Shishkin-2023"/> which are generated in diverse shapes.<ref name="Gast-2017">{{cite book|last=Gast|first=Rebecca J.|chapter=Centrohelida and Other Heliozoan-Like Protists|doi=10.1007/978-3-319-28149-0_28|pages=955–971|chapter-url=https://tuaulavirtual.educatic.unam.mx/pluginfile.php/1747259/mod_resource/content/1/Gast2016_ReferenceWorkEntry_CentrohelidaAndOtherHeliozoan-.pdf|access-date=9 June 2025|title=Handbook of the Protists|editor-last1=Archibald|editor-first1=John M.|editor-last2=Simpson|editor-first2=Alastair G.B.|editor-last3=Slamovits|editor-first3=Claudio H.|edition=2nd|publisher=Springer International Publishing|location=Cham|url=https://ficoherb.fciencias.unam.mx/Recursos/Bibl/Handbookoftheprotists.pdf|isbn=978-3-319-28149-0|lccn=2017945328|date=2017|volume=2}}</ref> The specific shape of the siliceous scales of many centrohelids helps identify the species.<ref name="Zagumyonnyi-2025">{{cite journal|last1=Zagumyonnyi|first1=Dmitry G.|last2=Gong|first2=Yingchun|last3=Huo|first3=Da|last4=Tikhonenkov|first4=Denis V.|title=Morphology and phylogeny of the centrohelid heliozoans Raphidocystidae and their ability to consume cyanobacteria|journal=PLOS One|volume=20|issue=5|date=9 May 2025|issn=1932-6203|pmid=40344036|pmc=12063867|doi=10.1371/journal.pone.0322585|doi-access=free|article-number=e0322585}}</ref> Some centrohelids bear both scales and spicules, either simultaneously (as in ''Raphidiophrys heterophryoidea'') or throughout their life cycles (like ''Triangulopteris lacunata'').<ref name="Shishkin-2023"/>
Some centrohelids have a mucous stalk to attach to the substrate (as in ''Raphidocystis arborescens'').<ref>{{cite journal|last1=Drachko|first1=Daria|last2=Zlatogursky|first2=Vasily V.|title=Morphology, Systematics and Life Cycle of ''Ozanamia fimbriatus'' (Haptista: Centroplasthelida), With Notes on Evolution of Organic Skeleton in Centrohelids|journal=Journal of Eukaryotic Microbiology|volume=72|issue=4|date=2025|issn=1066-5234|doi=10.1111/jeu.70022|article-number=e70022}}</ref> Some form colonies, with individual cells connected by cytoplasmic bridges, each cell with their own layer of spicules or scales. An exception is ''Yogsothoth'': its colonies lack any bridges, and instead contain a roundish mass of densley packed cells surrounded by a thick outer layer of scales that differ from the inner plate scales covering each cell.<ref name="Shishkin-2018"/>
Although centrohelids have only one cell nucleus in normal conditions,<ref>{{cite book|chapter=Phylum Actinopoda: Class Heliozoa|first=Colette|last=Febvre-Chevalier|title=Handbook of Protoctista: the structure, cultivation, habitats and life histories of the eukaryotic microorganisms and their descendants exclusive of animals, plants and fungi. A guide to the algae, ciliates, foraminifera, sporozoa, water molds, slime molds and the other protoctists|date=1990|chapter-url=https://archive.org/details/handbookofprotoc0000unse/page/346/mode/2up|url-access=registration|url=https://archive.org/details/handbookofprotoc0000unse/page/34|editor-first1=Lynn|editor-last1=Margulis|editor-first2=John O.|editor-last2=Corliss|editor-first3=Michael|editor-last3=Melkonian|editor-first4=David J.|editor-last4=Chapman|publisher=Jones and Bartlett Publishers|location=Boston|isbn=0-86720-052-9|oclc=1358644054}}</ref> multiple centrohelid cells have been observed fusing their cytoplasms to engulf a larger prey, resulting in a multinucleated cell.<ref name="Gaponova-2021"/>
== Nutrition ==
[[File:Raphidocystis contractilis ingesting Actinophrys sol JJP 2021 fig4.png|thumb|upright=2|Various centrohelids (r) preying on an actinophryid (a) by extending their cytoplasms to form a common food vacuole around it. Scale bars: 10 μm.]]
Centrohelids feed on bacteria, other protists, and even larvae of invertebrates, through phagocytosis. They are considered passive feeders, capturing prey as they pass by.<ref name="Gast-2017"/><ref name="Gaponova-2021">{{cite journal|last1=Gaponova|first1=Liudmyla|last2=Suzaki|first2=Toshinobu|last3=Islam|first3=Md Shafiqul|last4=Kolosiuk|first4=Andrii|title=Interaction between centrohelid and actinophryid heliozoans: Field and laboratory studies|journal=Journal of Protistology|publisher=Japan Society of Protistology|doi-access=free|date=2021|doi=10.18980/jop.e004|volume=53|article-number=e004}}</ref> Experiments studying the feeding behavior of centrohelids are rare, but they have been observed consuming environmentally impactful strains of cyanobacteria, such as ''Microcystis aeruginosa'' and ''Aphanizomenon''. This predation is interesting due to its potential to regulate harmful algal blooms caused by such cyanobacteria.<ref name="Zagumyonnyi-2025"/> In mixed cultures of centrohelids and actinophryids, another group of heliozoan passive feeders with the same ecological niche, the centrohelids outcompete actinophryids, causing a sharp decline in their population. Centrohelids can also fuse with more centrohelid cells to ingest actinophryid cells. Actinophryids actively avoid centrohelids, suggesting that they perceive centrohelids as threats.<ref name="Gaponova-2021"/>
One marine centrohelid genus, ''Meringosphaera'', practices kleptoplastidy, i.e., temporarily seizes chloroplasts from its prey cells to use them for photosynthesis. In particular, it retains plastids from different green-colored algae of the class Dictyochophyceae. Despite the retainment being temporary, some of the genes associated with the retained plastids have been transferred to the centrohelid host's nucleus (endosymbiotic gene transfer); this feature is a major step in the transition towards acquiring permanent plastids.<ref>{{cite journal|last1=Sørensen|first1=Megan E.S.|last2=Zlatogursky|first2=Vasily V.|last3=Onuţ-Brännström|first3=Ioana|last4=Walraven|first4=Anne|last5=Foster|first5=Rachel A.|last6=Burki|first6=Fabien|title=A novel kleptoplastidic symbiosis revealed in the marine centrohelid ''Meringosphaera'' with evidence of genetic integration|journal=Current Biology|volume=33|issue=17|date=2023|pmid=37536342|pmc=7615077|doi=10.1016/j.cub.2023.07.017|pages=3571–3584.e6}}</ref>
== Ecology ==
Centrohelids are free-living predatory protists with a ubiquitous distribution.<ref name="Gerasimova-2023">{{cite journal|last1=Gerasimova|first1=Elena A.|last2=Mindolina|first2=Yulia V.|last3=Tikhonenkov|first3=Denis V.|last4=Kataev|first4=Vladimir Y.|last5=Balkin|first5=Alexander S.|last6=Mikhailov|first6=Kirill V.|last7=Zagumyonnyi|first7=Dmitry G.|last8=Plotnikov|first8=Andrey O.|last9=Zlatogursky|first9=Vasily V.|display-authors=5|title=Unexpected ubiquity of heart-shaped scale morphotype in Centroplasthelida (Haptista): Ancestral trait or multiple acquisitions?|journal=Journal of Eukaryotic Microbiology|volume=70|issue=6|date=14 July 2023|issn=1066-5234|doi=10.1111/jeu.12992|article-number=e12992}}</ref> They are found abundantly in global freshwater environments, and also occur widely in marine and soil habitats, where they are comparatively understudied.<ref name="Cavalier-Smith-2012">{{cite journal|last=Cavalier-Smith|first=Thomas|author2=Chao, Ema E.|year=2012|title=''Oxnerella micra'' sp. n. (Oxnerellidae fam. n.), a Tiny Naked Centrohelid, and the Diversity and Evolution of Heliozoa|journal=Protist|volume=163|issue=4|pages=574–601|doi=10.1016/j.protis.2011.12.005|pmid=22317961}}</ref> Despite their ubiquity, little is known about their biogeography.<ref name="Zagumyonnyi-2021"/> Most reports of centrohelids are done in temperate zones due to insufficient studies in tropical regions. Within temperate regions, the species diversity of freshwater centrohelids appears to be influenced by the type of body of water: highest diversity occurs in terrace forest lakes, while ''Sphagnum'' peat bogs have the lowest diversty.<ref>{{cite journal|last1=Prokina|first1=Kristina I.|last2=Zagumyonnyi|first2=Dmitry G.|last3=Tikhonenkov|first3=Denis V.|title=Centrohelid Heliozoans (Centroplasthelida Febvre-Chevalier et Febvre, 1984) from Different Types of Freshwater Bodies in the Middle Russian Forest-steppe|journal=Acta Protozoologica|volume=57|issue=4|date=2018|doi=10.4467/16890027AP.18.018.10094|doi-access=free|pages=245–268}}</ref> According to environmental DNA analyses, soil-dwelling centrohelids are twice as diverse as their freshwater counterparts, and ten times more than marine ones.<ref name="Zagumyonnyi-2021"/><ref>{{cite journal|last1=Singer|first1=David|last2=Seppey|first2=Christophe V.W.|last3=Lentendu|first3=Guillaume|last4=Dunthorn|first4=Micah|last5=Bass|first5=David|last6=Belbahri|first6=Lassâad|last7=Blandenier|first7=Quentin|last8=Debroas|first8=Didier|last9=de Groot|first9=G. Arjen|last10=de Vargas|first10=Colomban|last11=Domaizon|first11=Isabelle|last12=Duckert|first12=Clément|last13=Izaguirre|first13=Irina|last14=Koenig|first14=Isabelle|last15=Mataloni|first15=Gabriela|last16=Schiaffino|first16=M. Romina|last17=Mitchell|first17=Edward A.D.|last18=Geisen|first18=Stefan|last19=Lara|first19=Enrique|display-authors=5|title=Protist taxonomic and functional diversity in soil, freshwater and marine ecosystems|journal=Environment International|volume=146|date=2021|doi=10.1016/j.envint.2020.106262|article-number=106262|doi-access=free|hdl=10261/265020|hdl-access=free}}</ref>
Like other heliozoans, most known species are found in aquatic benthic environments, where they prey on a variety of other microbes. Some float in the water column, while others attach to substrates by a stalk. Free-floating (planktonic) forms are well known, but the ecological niche of centrohelids is considered to be the benthos, inhabiting the superficial layer of detritus and interstitial spaces.<ref name="Gast-2017"/>
== Evolution ==
{{cladogram| {{clade|style=font-size:80%;line-height:90%; |1={{clade |1={{clade|label1=Diaphoretickes |1={{clade |1={{clade |1={{clade|label1=SAR|1={{clade|1=Stramenopiles|2=Alveolata|3=Rhizaria}} |label2=?|2=Telonemia*|state2=dotted |label3=Haptista*|3={{clade |1=Haptophyta |2='''Centroplasthelida''' }} }} |2=Disparia* }} |2={{clade |1=Pancryptista* |2=Archaeplastida{{br}}(plants ''sensu lato'') }} }}}} |2=Other eukaryotes }} }} |caption=Cladogram of eukaryotes focused on the position of centrohelids and related groups. The topology follows phylogenomic analyses published in the 2020s;<ref>{{cite journal|last1=Irisarri|first1=Iker|last2=Strassert|first2=Jürgen F H|last3=Burki|first3=Fabien|title=Phylogenomic Insights into the Origin of Primary Plastids|journal=Systematic Biology|volume=71|issue=1|date=16 December 2021|issn=1063-5157|doi=10.1093/sysbio/syab036|pages=105–120}}</ref> telonemids may branch inside Haptista,<ref>{{cite journal |last1=Eglit |first1=Yana |last2=Shiratori |first2=Takashi |last3=Jerlström-Hultqvist |first3=Jon |last4=Williamson |first4=Kelsey |last5=Roger |first5=Andrew J. |last6=Ishida |first6=Ken-Ichiro |last7=Simpson |first7=Alastair G.B. |title=''Meteora sporadica'', a protist with incredible cell architecture, is related to Hemimastigophora |journal=Current Biology |volume=34 |issue=2 |date=22 January 2024 |doi=10.1016/j.cub.2023.12.032 |doi-access=free |pages=451–459.e6 |pmid=38262350 |url=https://www.cell.com/article/S0960982223016834/pdf |access-date=26 December 2025|url-access=subscription }}</ref><ref>{{cite journal|last1=Zlatogursky|first1=Vasily|last2=Boscaro|first2=Vittorio|last3=Lax|first3=Gordon|last4=Wanntorp|first4=Matias|last5=Pohl|first5=Nina|last6=Burki|first6=Fabien|last7=Keeling|first7=Patrick J.|display-authors=5|title=Phylogenetic position and mitochondrial genome evolution of "orphan" eukaryotic lineages|journal=iScience|volume=28|issue=8|date=15 August 2025|pmid=40948565|pmc=12432456|doi=10.1016/j.isci.2025.113184|article-number=113184}}</ref> or may be sister to SAR (as 'TSAR').<ref>{{cite journal|last1=Strassert|first1=Jürgen F. H.|last2=Irisarri|first2=Iker|last3=Williams|first3=Tom A.|last4=Burki|first4=Fabien|title=A molecular timescale for eukaryote evolution with implications for the origin of red algal-derived plastids|journal=Nature Communications|volume=12|issue=1|date=25 March 2021|issn=2041-1723|pmid=33767194|pmc=7994803|doi=10.1038/s41467-021-22044-z|doi-access=free|url=https://www.nature.com/articles/s41467-021-22044-z.pdf|access-date=13 May 2025|article-number=1879|bibcode=2021NatCo..12.1879S}}</ref><ref name="Valt-2025">{{Cite journal |last1=Valt |first1=Marek |last2=Pánek |first2=Tomáš |last3=Mirzoyan |first3=Seda |last4=Tice |first4=Alexander K. |last5=Jones |first5=Robert E. |last6=Dohnálek |first6=Vít |last7=Doležal |first7=Pavel |last8=Mikšátko |first8=Jiří |last9=Rotterová |first9=Johana |last10=Hrubá |first10=Pavla |last11=Brown |first11=Matthew W. |last12=Čepička |first12=Ivan |date=19 November 2025 |title=Rare microbial relict sheds light on an ancient eukaryotic supergroup |journal=Nature |language=en |doi=10.1038/s41586-025-09750-0 |issn=0028-0836 |url = https://www.researchsquare.com/article/rs-5245440/v1.pdf |access-date=25 November 2025}}</ref> Groups included in the paraphyletic Hacrobia are marked *.<ref name="Cavalier-Smith-2015"/> }}
Before molecular phylogenetics, centrohelids were grouped with other axopodial protists in the polyphyletic taxon Heliozoa.<ref>{{cite journal|last=Udalov|first=Ilya A.|title=Taxonomy of amoeboid protists: a brief history of research from C. Gessner to T. Cavalier-Smith|journal=Protistology|volume=16|issue=2|date=2022|doi=10.21685/1680-0826-2022-16-2-2|url=https://www.zin.ru/journals/protistology/num16_2/Udalov_protistology_16-2.pdf|access-date=27 December 2025|pages=68–86}}</ref> Over the 1990s and 2000s decades, smaller heliozoan groups were removed from this artificial taxon and into their true evolutionary lineages, particularly Stramenopiles and Rhizaria.<ref name="Nikolaev-2004">{{cite journal|last1=Nikolaev|first1=Sergey I.|last2=Berney|first2=Cédric|last3=Fahrni|first3=José F.|last4=Bolivar|first4=Ignacio|last5=Polet|first5=Stephane|last6=Mylnikov|first6=Alexander P.|last7=Aleshin|first7=Vladimir V.|last8=Petrov|first8=Nikolai B.|last9=Pawlowski|first9=Jan|title=The twilight of Heliozoa and rise of Rhizaria, an emerging supergroup of amoeboid eukaryotes|journal=Proceedings of the National Academy of Sciences|volume=101|issue=21|date=25 May 2004|issn=0027-8424|pmid=15148395|pmc=419558|doi=10.1073/pnas.0308602101|doi-access=free|pages=8066–8071|display-authors=5}}</ref><ref name="Burki-2009">{{cite journal|last1=Burki|first1=Fabien|last2=Inagaki|first2=Yuji|last3=Bråte|first3=Jon|last4=Archibald|first4=John M.|last5=Keeling|first5=Patrick J.|last6=Cavalier-Smith|first6=Thomas|last7=Sakaguchi|first7=Miako|last8=Hashimoto|first8=Tetsuo|last9=Horak|first9=Ales|last10=Kumar|first10=Surendra|last11=Klaveness|first11=Dag|last12=Jakobsen|first12=Kjetill S.|last13=Pawlowski|first13=Jan|last14=Shalchian-Tabrizi|first14=Kamran|display-authors=5|title=Large-Scale Phylogenomic Analyses Reveal That Two Enigmatic Protist Lineages, Telonemia and Centroheliozoa, Are Related to Photosynthetic Chromalveolates|journal=Genome Biology and Evolution|volume=1|date=27 July 2009|issn=1759-6653|pmid=20333193|pmc=2817417|doi=10.1093/gbe/evp022|pages=231–238}}</ref> The centrohelids, the largest heliozoan group, remained difficult to resolve, and could not be placed within any other phyla.<ref name="Cavalier-Smith-2003">{{cite journal|last1=Cavalier-Smith|first1=Thomas|author-link1=Thomas Cavalier-Smith|last2=Chao|first2=Ema E. -Y.|title=Molecular Phylogeny of Centrohelid Heliozoa, a Novel Lineage of Bikont Eukaryotes That Arose by Ciliary Loss|journal=Journal of Molecular Evolution|volume=56|issue=4|date=April 2003|issn=0022-2844|doi=10.1007/s00239-002-2409-y|pages=387–396}}</ref><ref name="Cavalier-Smith-2007"/>
With the first 18S rRNA gene sequences of centrohelids, weakly supported analyses indicated some affinities to bikonts (organisms ancestrally with two flagella), and they were suggested to have evolved from bikont flagellates such as ancyromonads.<ref name="Cavalier-Smith-2003"/> Based on further low-support 18S rRNA analyses and some cellular similarities, a relationship with haptophytes was later suggested,<ref name="Cavalier-Smith-2007">{{cite journal|first1=Thomas|last1=Cavalier-Smith|first2=Sophie|last2=von der Heyden|author-link1=Thomas Cavalier-Smith|title=Molecular phylogeny, scale evolution and taxonomy of centrohelid heliozoa|journal=Molecular Phylogenetics and Evolution|volume=44|date=September 2007|issue=3 |pages=1186–1203|doi=10.1016/j.ympev.2007.04.019|pmid=17588778}}</ref> but phylogenetic trees were generally inconsistent. They also branched with telonemids and cryptomonads in a clade known as CCTH,<ref name="Burki-2009"/> later renamed Hacrobia.<ref name="Cavalier-Smith-2015"/>
In the 2010s, the monophyly of Haptista (centrohelids and haptophytes) was further confirmed with maximal support in larger-scale phylogenomic analyses,<ref name="Burki-2016">{{cite journal|last1=Burki|first1=Fabien|last2=Kaplan|first2=Maia|last3=Tikhonenkov|first3=Denis V.|last4=Zlatogursky|first4=Vasily|last5=Minh|first5=Bui Quang|last6=Radaykina|first6=Liudmila V.|last7=Smirnov|first7=Alexey|last8=Mylnikov|first8=Alexander P.|last9=Keeling|first9=Patrick J.|display-authors=5|title=Untangling the early diversification of eukaryotes: a phylogenomic study of the evolutionary origins of Centrohelida, Haptophyta and Cryptista|journal=Proceedings of the Royal Society B: Biological Sciences|volume=283|issue=1823|date=27 January 2016|issn=0962-8452|pmid=26817772|pmc=4795036|doi=10.1098/rspb.2015.2802|doi-access=free|article-number=20152802}}</ref> and Hacrobia was found to be paraphyletic.<ref name="Zhao-2012">{{Cite journal |last1=Zhao |first1=Sen |last2=Burki |first2=Fabien |last3=Bråte |first3=Jon |last4=Keeling |first4=Patrick J. |last5=Klaveness |first5=Dag |last6=Shalchian-Tabrizi |first6=Kamran |year=2012 |title=''Collodictyon''—An Ancient Lineage in the Tree of Eukaryotes |journal=Molecular Biology and Evolution |doi=10.1093/molbev/mss001 |pmid=22319147 |pmc=3351787 |volume=29 |issue=6 |pages=1557–68}}</ref><ref name="Burki-2016"/> Haptista branches closer to the SAR supergroup (Stramenopiles, Alveolata, Rhizaria), while the clade containing cryptomonads and their relatives, Cryptista, is closer to plants.<ref name="Burki-2012">{{cite journal|last1=Burki|first1=Fabien|last2=Okamoto|first2=Noriko|last3=Pombert|first3=Jean-François|last4=Keeling|first4=Patrick J.|title=The evolutionary history of haptophytes and cryptophytes: phylogenomic evidence for separate origins|journal=Proceedings of the Royal Society B: Biological Sciences|volume=279|issue=1736|date=7 June 2012|issn=0962-8452|pmid=22298847|pmc=3321700|doi=10.1098/rspb.2011.2301|pages=2246–2254}}</ref> An elusive microheliozoan, ''Microheliella maris'', previously suggested to be a possible centrohelid relative,<ref name="Nikolaev-2004"/><ref name="Yabuki-2012">{{cite journal|last=Yabuki|first=Akinori|last2=Chao|first2=Ema E.|last3=Ishida|first3=Ken-Ichiro|last4=Cavalier-Smith|first4=Thomas|author-link4=Thomas Cavalier-Smith|title=''Microheliella maris'' (Microhelida ord. n.), an Ultrastructurally Highly Distinctive New Axopodial Protist Species and Genus, and the Unity of Phylum Heliozoa|journal=Protist|volume=163|issue=3|date=2012|doi=10.1016/j.protis.2011.10.001|pages=356–388}}</ref><ref name="Zlatogursky-2015">{{cite journal|last=Zlatogursky|first=Vasily V.|title=There and Back Again: Parallel Evolution of Cell Coverings in Centrohelid Heliozoans|journal=Protist|volume=167|issue=1|date=30 December 2015|doi=10.1016/j.protis.2015.12.002|pages=51–66|pmid=26828628}}</ref> branches next to Cryptista (forming Pancryptista) instead.<ref name="Valt-2025"/> All these groups belong to the large eukaryotic clade Diaphoretickes,<ref name="Adl-2019"/><ref name="Valt-2025"/> from which all heliozoa are presumed to have evolved.<ref>{{cite journal |last1=Shishkin |first1=Yegor |last2=Drachko |first2=Daria |last3=Zlatogursky |first3=Vasily V. |title=The smallest known heliozoans are the ''Erebor'' lineage (nom. clad. n.) inside ''Microheliella maris'' (Eukaryota, Diaphoretickes), with the amendation of M. maris diagnosis and description of ''Berkeleyaesol magnus'' gen. nov., comb. nov. (Eukaryota, incertae sedis) |journal=International Journal of Systematic and Evolutionary Microbiology |volume=71 |issue=4 |date=22 April 2021 |issn=1466-5026 |doi=10.1099/ijsem.0.004776 |pmid=33886450 |doi-access=free}}</ref>
Traits present in haptophytes (specifically Prymnesiophyceae) are inferred as the ancestral state of centrohelids. Both have an outer coat of complex mineralized scales:<ref name="Cavalier-Smith-2007"/> calcareous in haptophytes, siliceous in centrohelids. Another common feature is the presence of a thin microtubule-based appendage used for feeding: the axopodia in centrohelids, and the haptonema in haptophytes.<ref name="Adl-2019"/> Centrohelids evolved from a common flagellate ancestor that secondarily lost its flagella,<ref name="Cavalier-Smith-2003"/> and was probably capable of producing both siliceous scales and organic spicules.<ref name="Shishkin-2023"/>
== Taxonomy == === Nomenclature ===
The centrohelids compose the taxon '''Centroplasthelida''', described by protistologists Colette Febvre-Chevalier and Jean Febvre in 1984, in reference to the centroplast that characterizes them among other heliozoa.<ref name="Febvre-Chevalier-1984">{{cite journal|last1=Febvre-Chevalier|first1=Colette|last2=Febvre|first2=Jean|title=Axonemal microtubule pattern ofCienkowskya mereschkovskyi and a revision of heliozoan taxonomy|journal=Origins of Life|volume=13|issue=3-4|date=1984|issn=0302-1688|doi=10.1007/BF00927180|pages=315–338|url=http://link.springer.com/10.1007/BF00927180|access-date=28 December 2025}}</ref> A similar earlier name '''Centrohelida''', established by Kühn in 1926, is not preferred because it includes both centrohelids and gymnosphaerids.<ref name="Adl-2019"/> A modified version of this name, '''Centrohelea''', was used by some authors, especially Thomas Cavalier-Smith.<ref name="Cavalier-Smith-2015"/> The synonym '''Centroheliozoa''' was coined by Monika Dürrschmidt and David J. Patterson in 1987, and is used as an alternative common name for the group.<ref name="Dürrschmidt-1987">{{cite journal|last1=Dürrschmidt|first1=Monika|last2=Patterson|first2=David J.|title=A light and electron microscopic study of a new species of centroheliozoon, Chlamydaster fimbriatus|journal=Tissue and Cell|volume=19|issue=3|date=1987|doi=10.1016/0040-8166(87)90032-2|pages=365–376}}</ref>
=== History === Traditionally, the classification and species identification of centrohelids has been based upon the morphology of their cell coverings (spicules and scales). According to the main cell covering types, three families were distinguished before molecular phylogenetics: "Heterophryidae", either naked or covered in organic spicules, proposed as the most primitive family; "Raphidiophryidae", interpreted as more derived, with tangential siliceous plate-scales; and "Acanthocystidae", interpreted as the most derived, with the most complex coverings, including a double layer of siliceous scales.<ref name="Zlatogursky-2015"/>
With the first molecular analyses using the 18S rRNA gene, and the first sequence of a naked centrohelid (''Oxnerella''), centrohelids were divided into two orders based on the presence of (at least seven) insertions in this gene: Acanthocystida (with insertions) and Pterocystida (without),<ref name="Yabuki-2012"/><ref name="Zlatogursky-2015"/> both containing species with and without silica scales.<ref name="Shishkin-2018"/> The hypothesis for the ancestral centrohelid changed from a naked or spicule-bearing form to a form capable of producing complex coverings, secondarily lost or simplified. As such, "Acanthocystidae" represented the paraphyletic ancestral state, and "Heterophryidae" the polyphyletic derived state. Only Raphidiophryidae remained monophyletic, while the remaining centrohelids were grouped in new monophyletic families, including a modified Heterophryidae and Acanthocystidae.<ref name="Cavalier-Smith-2007"/><ref name="Zlatogursky-2015"/>
{{cladogram| {{clade|style=font-size:80%;line-height:80%;|1={{clade |label1=Pterocystida|1={{clade |state=dotted |2=Clypiferidae |state1=dotted|1={{clade |2=Raphidiophryidae |state=dotted|1={{clade |2=Spiculophryidae |state=dotted|1={{clade|state=dotted |1={{clade|1=Pterocystidae*|2=Oxnerellidae}} |2={{clade|state=dotted|1=''Meringosphaera''|2=Ozanamiidae}} }} }} }} }} |label2=Panacanthocystida|2={{clade |label1=Chthonida|1=Yogsothothidae |label2=Acanthocystida|2={{clade |label1=Ricksolina|1=Ricksolidae* |label2=Chalarothoracina|2={{clade |1=Acanthocystidae* |2=Raphidocystidae* }} }} }} }}}} |caption=Cladogram of centrohelid families based on a 2023 phylogenetic analysis with the 18S rRNA gene. Environmental sequences are omitted. Dotted lines represent clades below 50% phylogenetic support. Families containing HLOs ("''Heterophrys''"-like organisms) are marked *.<ref name="Shishkin-2023"/> }}
The discovery and sequencing of new centrohelids led to updates in their higher classification. The circumscription of several families was modified to match molecular data. New relatives of Acanthocystida (such as ''Yogsothoth'') were grouped in a larger clade Panacanthocystida. In the clade Pterocystida, families were briefly grouped into two orders: Raphidista, later abandoned due to the lack of phylogenetic support, and Pterista, still accepted.<ref name="Shishkin-2018"/><ref name="Shishkin-2023"/> The two major clades, Pterocystida and Panacanthocystida, are often considered superorders.<ref name="Shishkin-2021">{{cite journal|last1=Shishkin|first1=Yegor|last2=Drachko|first2=Daria|last3=Zlatogursky|first3=Vasily V.|title=''Clypifer cribrifer'' gen. nov., sp. nov. (Clypiferidae fam. nov., Pterocystida, Centroplasthelida), with notes on evolution of centrohelid siliceous coverings|journal=International Journal of Systematic and Evolutionary Microbiology|volume=71|issue=7|date=1 July 2021|issn=1466-5026|doi=10.1099/ijsem.0.004856|article-number=004856}}</ref><ref name="Shishkin-2023"/><ref name="Zagumyonnyi-2025"/>
=== Described taxa ===
New species of centrohelids are continuously described in new environments,<ref name="Zagumyonnyi-2021"/><ref>{{cite journal|last=Nicholls|first=Kenneth H.|title=Marine and freshwater centrohelid heliozoans (Haptista: Centroplasthelida) in Canada, including taxonomic revisions and descriptions of 22 new species and subspecies|journal=Canadian Journal of Zoology|volume=101|issue=5|date=27 March 2023|doi=10.1139/cjz-2022-0114|pages=327–375}}</ref> and the total number of described species has increased from around 85 species in 1999<ref name="Patterson-1999"/> to about 130 in 2021.<ref name="Zagumyonnyi-2021"/> Environmental DNA sampling suggests that over 90% of species remain undescribed.<ref name="Cavalier-Smith-2007"/><ref>{{cite journal|last1=Gerasimova|first1=Elena A.|last2=Plotnikov|first2=Andrey O.|last3=Khlopko|first3=Yuri A.|last4=Zlatogursky|first4=Vasily V.|title=Multiple Euryhaline Lineages of Centrohelids (Haptista: Centroplasthelida) in Inland Saline Waters Revealed with Metabarcoding|journal=Journal of Eukaryotic Microbiology|volume=67|issue=2|date=17 December 2019|issn=1066-5234|doi=10.1111/jeu.12776|pages=223–231}}</ref> Listed below are the accepted centrohelid genera, grouped into 11 families:<ref name="Shishkin-2018"/><ref name="Shishkin-2023"/>
*Superorder Pterocystida {{au|Cavalier-Smith in Yabuki et al. 2012}} **Family Clypiferidae {{au|Shɨshkin in Shɨshkin et al. 2021}} — ''Clypifer''. **''Meringosphaera'' {{au|Lohmann 1902}} (no family formally described). **Family Ozanamiidae {{au|Shɨshkin-Skarð 2023}} — ''Ozanamia''. **Family Raphidiophryidae {{au|Febvre-Chevalier & Febvre 1984 emend. Shɨshkin & Zlatogursky 2018}} — ''Raphidiophrys''. **Order Pterista {{au|Shɨshkin & Zlatogursky 2018}} ***Family Oxnerellidae <small>Cavalier-Smith & Chao 2012</small> — ''Oxnerella''. ***Family Pterocystidae <small>Cavalier-Smith & von der Heyden 2007</small> — ''Chlamydaster'', ''Pseudoraphidocystis'', ''Pseudoraphidiophrys'', ''Pterocystis'', ''Raineriophrys'' (=''Raineria'', ''Echinocystis''), ''Triangulopteris'',<ref name="Zagumyonnyi-2021">{{cite journal|last1=Zagumyonnyi|first1=Dmitry G.|last2=Radaykina|first2=Liudmila V.|last3=Tikhonenkov|first3=Denis V.|title=''Triangulopteris lacunata'' gen. et sp. nov. (Centroplasthelida), a New Centrohelid Heliozoan from Soil|journal=Diversity|volume=13|issue=12|date=11 December 2021|issn=1424-2818|doi=10.3390/d13120658|doi-access=free|article-number=658}}</ref> ''Sphaerastrum''. *Superorder Panacanthocystida {{au|Shɨshkin & Zlatogursky 2018}} **Order Chthonida {{au|Shɨshkin & Zlatogursky 2018}}, suborder Yogsothothina {{au|Shɨshkin & Zlatogursky 2018}}, family Yogsothothidae {{au|Shɨshkin & Zlatogursky 2018}} — ''Yogsothoth''. **Order Acanthocystida <small>Cavalier-Smith 2011</small> ***Suborder Ricksolina {{au|Shɨshkin-Skarð 2023}}, family Ricksolidae {{au|Shɨshkin-Skarð 2023}} — ''Ricksol''. ***Suborder Chalarothoracina <small>Hertwig & Lesser 1874 emend. Cavalier-Smith in Yabuki et al. 2012</small> ****Family Raphidocystidae {{au|Zlatogursky in Zlatogursky et al. 2018}} — ''Raphidocystis''. ****Family Acanthocystidae <small>Claus 1874 emend. Cavalier-Smith & von der Heyden 2007</small> — ''Acanthocystis''. The following taxa have an uncertain status among centrohelids: * ''Heterophrys'' and other spicule-bearing genera — The genus ''Heterophrys'' was established to group species covered in needle-like spicules. Spicule-bearing forms are scattered across the centrohelid tree, and some have been described within the life cycle of scale-bearing species.<ref name="Zagumyonnyi-2021" /> Therefore, potentially all centrohelids have "''Heterophrys''"-like stages. Because of this, genera that have been typified on "''Heterophrys''"-like organisms (HLOs) are now considered ''nomina dubia'': ''Heterophrys'' {{au|Archer 1869}}, ''Sphaerastrum'' {{au|Greef 1873}}, and ''Marophrys'' {{au|Cavalier-Smith & von der Heyden 2007}}.<ref name="Shishkin-2023"/> * ''Parasphaerastrum'' <small>Mikrjukov 1996</small> — This genus has not been sequenced<ref name="Zagumyonnyi-2022"/> and is considered ''incertae sedis'' among centrohelids.<ref name="Adl-2019"/> * ''Heteroraphidiophrys'' <small>Mikrjukov & Patterson 2000</small> — This genus was first mentioned by Mikrjukov in 2002, but was never formally described. The organism needs to be re-isolated, carefully studied and provided with a formal taxonomic description.<ref name="Adl-2019"/> *Spiculophryidae {{au|Shɨshkin & Zlatogursky 2018}} — The phylogenetic position of this family, containing only the genus ''Spiculophrys'' <small>Zlatogursky 2015</small>, is not yet firmly resolved. Depending on the parameters of phylogenetic analysis, it branched either as sister to all other centrohelids or as sister to one of the two superorders. It is considered ''incertae sedis'' among centrohelids.<ref name="Shishkin-2018"/><ref name="Zagumyonnyi-2022">{{cite journal|last1=Zagumyonnyi|first1=Dmitry G.|last2=Radaykina|first2=Liudmila V.|last3=Keeling|first3=Patrick J.|last4=Tikhonenkov|first4=Denis V.|title=Centrohelid heliozoans of Ukraine with a description of a new genus and species (Haptista: Centroplasthelida)|journal=European Journal of Protistology|volume=86|date=2022|doi=10.1016/j.ejop.2022.125916|article-number=125916}}</ref> * ''Choanocystis'' {{au|Penard 1904}} — The initial description of this genus did not report any feature indicative of centrohelids, except possibly external spine scales and internal plate scales (reported as "''sticks and scales''"), which he mentioned without specifying their precise shape. These were already the diagnostic features of ''Acanthocystis'', a genus described earlier, and he did not provide a comparison of both.<ref name="Shishkin-2023"/> In 1988, an organism was identified as "''Choanocystis lepidula''"<ref>{{cite journal|last1=Siemensma|first1=F.J.|last2=Roijackers|first2=R.M.M.|title=A Study of New and Little-known Acanthocystid Heliozoeans, and a Proposed Division of the Genus Acanthocystis (Actinopoda, Heliozoea)|journal=Archiv für Protistenkunde|volume=135|issue=1-4|date=1988|doi=10.1016/S0003-9365(88)80069-1|pages=197–212}}</ref> with no explanation for its identity, and its diagnostic feature (a cardioid-shaped basal plate of spine scales) also leaves its affinity uncertain, as it appears in several centrohelid clades. The organism known as "''C. lepidula''" is similar to a sequenced strain belonging to Panacanthocystida, but cannot be traced to the "''Choanocystis''" genus. Two additional species with cardioid-shaped basal plates, initially placed under "''Choanocystis''"<ref name="Cavalier-Smith-2007"/> and branching in Pterocystida, were eventually transferred to a newer genus ''Ozanamia'' in 2023.<ref name="Shishkin-2023"/>
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}} == External links == {{Eukaryota|D.}} {{Cryptophyta and haptophyta}} {{Taxonbar|from1=Q1185812|from2=Q21446759|from3=Q22112974|from4=Q21446752}}
Category:Haptista Category:Amoebas