The Acantharia are a group of

radiolaria The Radiolaria, also called Radiozoa, are unicellular eukaryotes of diameter 0.1–0.2 mm that produce intricate mineral skeletons, typically with a central capsule dividing the cell into the inner and outer portions of endoplasm and ect ...

protozoa Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically ...

, distinguished mainly by their

strontium sulfate Strontium sulfate (SrSO4) is the sulfate salt of strontium. It is a white crystalline powder and occurs in nature as the mineral celestine. It is poorly soluble in water to the extent of 1 part in 8,800. It is more soluble in dilute HCl and nitr ...

skeletons. Acantharians are

heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...

ic marine

microplankton Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microbe) is any microscopic liv ...

that range in size from about 200 microns in diameter up to several millimeters. Some acantharians have

photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

s and hence are considered

mixotroph A mixotroph is an organism that uses a mix of different sources of energy and carbon, instead of having a single trophic mode, on the continuum from complete autotrophy to complete heterotrophy. It is estimated that mixotrophs comprise more than ...

Morphology

Acantharian skeletons are composed of

, SrSO₄, in the form of mineral

celestine Celestine is a given name and a surname. People Given name * Pope Celestine I (died 432) * Pope Celestine II (died 1144) * Pope Celestine III (c. 1106–1198) * Pope Celestine IV (died 1241) * Pope Celestine V (1215–1296) * Antipope Cel ...

crystal. Celestine is named for the delicate blue colour of its crystals, and is the heaviest mineral in the ocean. The denseness of their celestite ensures acantharian shells function as mineral ballast, resulting in fast sedimentation to bathypelagic depths. High settling fluxes of acantharian

cysts A cyst is a closed Wikt:sac, sac, having a distinct Cell envelope, envelope and cell division, division compared with the nearby Biological tissue, tissue. Hence, it is a cluster of Cell (biology), cells that have grouped together to form a sac ...

have been observed at times in the Iceland Basin and the

Southern Ocean The Southern Ocean, also known as the Antarctic Ocean, comprises the southernmost waters of the world ocean, generally taken to be south of 60th parallel south, 60° S latitude and encircling Antarctica. With a size of , it is the seco ...

, as much as half of the total gravitational

organic carbon Total organic carbon (TOC) is an analytical parameter representing the concentration of organic carbon in a sample. TOC determinations are made in a variety of application areas. For example, TOC may be used as a non-specific indicator of wa ...

flux. Material was copied from this source, which is available under
Creative Commons Attribution 4.0 International License
The

crystals are secreted by

vacuoles A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic mo ...

surrounding each spicule or spine. Acantharians are unique among marine organisms for their ability to

biomineralize Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often resulting in hardened or stiffened '' mineralized tissues''. It is an extremely widespread phenomenon: all six taxonomic kingd ...

strontium sulfate as the main component of their skeletons. However, unlike other radiolarians whose skeletons are made of silica, acantharian skeletons do not

fossilize A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved i ...

, primarily because strontium sulfate is very scarce in seawater and the crystals dissolve after the acantharians die. The arrangement of the spines is very precise, and is described by what is called the

Müllerian law Müllerian may refer to: *Müllerian mimicry, a type of mimicry or convergence named after Fritz Müller *Müllerian ducts, which enter the cloaca of an embryo (named after Johannes Peter Müller) * Mullerian anomalies are structural anomalies cau ...

in terms of lines of latitude and longitude – the spines lie on the intersections between five of the former, symmetric about an equator, and eight of the latter, spaced uniformly. Each line of longitude carries either two ''tropical'' spines or one ''equatorial'' and two ''polar'' spines, in alternation. The cell

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

is divided into two regions: the

endoplasm Endoplasm, also known as entoplasm, generally refers to the inner (often granulated), dense part of a cell's cytoplasm. This is opposed to the ectoplasm (cell biology), ectoplasm which is the outer (non-granulated) layer of the cytoplasm, which ...

and the ectoplasm. The endoplasm, at the core of the cell, contains the main

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s, including many nuclei, and is delineated from the ectoplasm by a capsular wall made of a

microfibril A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently o ...

mesh. In symbiotic species, the

algal Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, s ...

symbionts Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term

The ectoplasm consists of cytoplasmic extensions used for prey capture and also contains food vacuoles for prey digestion. The ectoplasm is surrounded by a periplasmic

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

, also made up of

s, but arranged into twenty plates, each with a hole through which one spicule projects. The cortex is linked to the spines by contractile myonemes, which assist in

buoyancy Buoyancy (), or upthrust, is the force exerted by a fluid opposing the weight of a partially or fully immersed object (which may be also be a parcel of fluid). In a column of fluid, pressure increases with depth as a result of the weight of t ...

control by allowing the ectoplasm to expand and contract, increasing and decreasing the total volume of the cell.

Taxonomy

The way that the spines are joined at the center of the cell varies and is one of the primary characteristics by which acantharians are classified. The skeletons are made up of either ten diametric or twenty radial spicules. Diametric spicules cross the center of the cell, whereas radial spicules terminate at the center of the cell where they either form a tight or flexible junction depending on species. Acantharians with diametric spicules or loosely attached radial spicules are able to rearrange or shed spicules and form cysts. * Holacanthida – 10 diametric spicules, simply crossed, no central junction, capable of encystment * Chaunacanthida – 20 radial spicules, loosely attached, capable of encystment * Symphiacanthida – 20 radial spicules, tight central junction *

Arthracanthida Radiolaria (Challenger) Plate. Arthracanthida, a subclass of Acantharea, is a group of marine protozoans. They consist mainly of a gelatinous sheath filled with cytoplasm and a skeleton of up to 20 radially placed spicules made of celestine (stro ...

– 20 radial spines, tight central junction The morphological classification system roughly agrees with

phylogenetic tree A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In ...

s based on the alignment of

ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...

genes, although the groups are mostly polyphyletic. Holacanthida seems to have evolved first and includes molecular clades A, B, and D. Chaunacanthida evolved second and includes only one molecular clade, clade C. Arthracanthida and Symphacanthida, which have the most complex skeletons, evolved most recently and constitute molecular clades E and F.

Symbiosis

Many acantharians, including some in clade B (Holacanthida) and all in clades E & F (Symphiacanthida and Arthracanthida), host single-celled algae within their inner cytoplasm (endoplasm). By participating in this

photosymbiosis Photosymbiosis is a type of symbiosis where one of the organisms is capable of photosynthesis. Examples of photosymbiotic relationships include those in lichens, plankton, ciliates, and many marine organisms including corals, fire corals, giant ...

, acantharians are essentially

s: they acquire energy through both heterotrophy and autotrophy. The relationship may make it possible for acantharians to be abundant in low-nutrient regions of the oceans and may also provide extra energy necessary to maintain their elaborate strontium sulfate skeletons. It is hypothesized that the acantharians provide the algae with nutrients (N & P) that they acquire by capturing and digesting prey in return for sugar that the algae produces during photosynthesis. It is not known, however, whether the algal symbionts benefit from the relationship or if they are simply being exploited and then digested by the acantharians. Symbiotic Holacanthida acantharians host diverse symbiont assemblages, including several genera of

dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...

s ('' Pelagodinium, Heterocapsa, Scrippsiella, Azadinium'') and a

haptophyte The haptophytes, classified either as the Haptophyta, Haptophytina or Prymnesiophyta (named for '' Prymnesium''), are a clade of algae. The names Haptophyceae or Prymnesiophyceae are sometimes used instead. This ending implies classification at ...

(''

Chrysochromulina ''Chrysochromulina'' is a genus of haptophytes. This phytoplankton is distributed globally in brackish and marine waters across approximately 60 known species. All ''Chrysochromulina'' species are phototrophic, however some have been shown to b ...

''). Clade E & F acantharians have a more specific symbiosis and primarily host symbionts from the haptophyte genus ''

Phaeocystis ''Phaeocystis'' is a genus of algae belonging to the Prymnesiophyte class and to the larger division of Haptophyta. It is a widespread marine phytoplankton and can function at a wide range of temperatures ( eurythermal) and salinities (euryhali ...

'', although they sometimes also host ''Chrysochromulina'' symbionts. Clade F acantharians simultaneously host multiple species and strains of ''Phaeocystis'' and their internal symbiont community does not necessarily match the relative availability of potential symbionts in the surrounding environment. The mismatch between internal and external symbiont communities suggests that acantharians can be selective in choosing symbionts and probably do not continuously digest and recruit new symbionts, and maintain symbionts for extended periods of time instead.

Life cycle

Adults are usually multinucleated. Earlier diverging clades are able to shed their spines and form cysts, which are often referred to as reproductive cysts. Reproduction is thought to take place by formation of swarmer cells (formerly referred to as "spores"), which may be

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

, and cysts have been observed to release these swarmers. Non-encysted cells have also been seen releasing swarmers in laboratory conditions. Not all life cycle stages have been observed, however, and no one has witnessed the fusion of swarmers to produce a new acantharian. Cysts are often found in

sediment trap Sediment traps are instruments used in oceanography and limnology to measure the quantity of sinking particulate Organic material, organic (and inorganic) material in aquatic ecosystem, aquatic systems, usually oceans, Lake, lakes, or Reservoir, ...

s and it is therefore believed that the cysts help acantharians sink into deep water. Genetic data and some imaging suggests that non-cyst-forming acantharians may also sink to deep water to release swarmers. Releasing swarmer cells in deeper water may improve the survival chances of juveniles. Study of these organisms has been hampered mainly by an inability to "close the lifecycle" and maintain these organisms in culture through successive generations.

References

{{Authority control