A cantharia

Abstract Acantharia are marine planktonic unicellular eukaryotes within the Radiolaria and currently encompass nearly 50 genera and 150 species. They are distributed worldwide from subsurface to deep waters and appear to be most numerous in oligotrophic surface waters. Their main distinctive features are an internal star‐shaped skeleton made of strontium sulfate mineral and an amoeboid cell that possesses multiple nuclei, extensive axopods and contractile myonemes. Acantharia feed on a large variety of prey, but some species are mixotrophs through symbiosis with intracellular eukaryotic microalgae, typically the haptophyte Phaeocystis . As part of their life cycle, some taxonomic groups form heavy cysts that rapidly sink to deep waters creating a biogeochemically relevant vertical flux of carbon, strontium and barium. Overall, Acantharia remain poorly studied, and our knowledge of their biology and ecology is still in its infancy. Key Concepts Acantharia are marine skeleton‐bearing planktonic protists that form a monophyletic taxon among radiolarians within the super‐group Rhizaria. Acantharia have a complex cellular organisation (e.g. presence of myonemes and axopods) and exhibit an internal skeleton made of strontium sulfate (celestite), enriched in barium and composed of spicules that follow a specific geometric pattern (i.e. Müller's law). There are nearly 50 genera and 150 species, distributed in several suborders, and most taxonomic criteria are based on skeleton morphology (e.g. central junction, form and shape of spicules). Acantharia are ubiquitous in the world's oceans, and particularly abundant in surface oligotrophic waters at tropical and subtropical latitudes, but remain largely overlooked due to rapid skeleton dissolution upon cell death and inappropriate sampling methods. Acantharia are primarily grazers with the use of their axopods, but several species are actually mixotrophs as they host intracellular symbiotic microalgae, making them significant primary consumers and producers in pelagic ecosystems. Some acantharian species undergo encystment that is part of the reproduction process in deep waters, creating a biogeochemically significant vertical flux of carbon, nitrogen and strontium in the mesopelagic and bathypelagic zones.