Haeckel, 1881, emend. Mikrjukov, 2000
Acantharian skeletons are composed of strontium sulfate crystals secreted by vacuoles surrounding each spicule or spine. Acantharians are the only marine organisms known to biomineralize strontium sulfate as the main component of their skeletons, making them quite unique. Unlike other radiolarians, whose skeletons are made of silica, acantharian skeletons do not fossilize, primarily because strontium sulfate is very scarce in seawater and the crystals dissolve after the acantharians die. 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.
The cell is divided into two regions: the endoplasm and the ectoplasm. The endoplasm, at the core of the cell, contains the main organelles, including many nuclei, and is delineated from the ectoplasm by a capsular wall made of a microfibril mesh. In symbiotic species, the algal symbionts are maintained in the endoplasm. 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, also made up of microfibrils, 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 control by allowing the ectoplasm to expand and contract, increasing and decreasing the total volume of the cell.
The arrangement of the spines is very precise, and is described by what is called the Müllerian law, which can be described 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 way that the spines are joined together at the center of the cell varies and is one of the primary characteristics by which acantharians are classified. Acantharians with diametric spicules or loosely attached radial spicules are able to rearrange or shed spicules and form cysts.
The morphological classification system roughly agrees with phylogenetic trees based on the alignment of ribosomal RNA 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.
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, acantharians are essentially mixotrophs: 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 dinoflagellates (Pelagodinium, Heterocapsa, Scrippsiella, Azadinium) and a haptophyte (Chrysochromulina). Clade E & F acantharians have a more specific symbiosis and primarily host symbionts from the haptophyte genus Phaeocystis, 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.
Adults are usually multinucleated. Reproduction is thought to take place by formation of swarmer cells (formerly referred to as "spores"), which may be flagellate. Not all life cycle stages have been observed, and study of these organisms has been hampered mainly by an inability to maintain these organisms in culture through successive generations.
Acantharia may refer to:
Acantharia Theiss. & Syd., a genus of fungi.
Acantharia Rojas 1897, a genus name in the Faboideae that is a nomen dubium.
Acantharia, an alternative spelling of Acantharea, a class of protozoa.Acanthometridae
Acanthometridae is a family of radiolarians.Acanthophractida
Acanthophractida is an order of marine radiolarians in the subclass Acantharia; skeleton includes a latticework shell and skeletal rods". They have a latticework shell, which can be spherical or ovoid and fused with the skeletal rods. The shell is concentric with the central capsule. "The body is usually covered with a single or double gelatinous sheath through which the skeletal rods emerge".Arthracanthida
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 (strontium sulfate). While mostly found in the upper areas of the ocean, they are able to move vertically by using microfilaments attached to the spicules to expand and contract the sheath. They are plentiful in the Gulf Stream during the summer months, but little is known about their overall distribution.Celestine (mineral)
Celestine or celestite is a mineral consisting of strontium sulfate (SrSO4). The mineral is named for its occasional delicate blue color. Celestine and the carbonate mineral strontianite are the principal sources of the element strontium, commonly used in fireworks and in various metal alloys.Cell nucleus
In cell biology, the nucleus (pl. nuclei; from Latin nucleus or nuculeus, meaning kernel or seed) is a membrane-bound organelle found in eukaryotic cells. Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others including osteoclasts have many.
The cell nucleus contains all of the cell's genome, except for a small fraction of mitochondrial DNA, organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these chromosomes are structured in such a way to promote cell function. The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression—the nucleus is, therefore, the control center of the cell. The main structures making up the nucleus are the nuclear envelope, a double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm, and the nuclear matrix (which includes the nuclear lamina), a network within the nucleus that adds mechanical support, much like the cytoskeleton, which supports the cell as a whole.
Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope. The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions. Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes. Although the interior of the nucleus does not contain any membrane-bound subcompartments, its contents are not uniform, and a number of nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes. The best-known of these is the nucleolus, which is mainly involved in the assembly of ribosomes. After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA.Cercozoa
The Cercozoa are a group of single-celled eukaryotes. They lack shared morphological characteristics at the microscopic level, being defined by molecular phylogenies of rRNA and actin or polyubiquitin.Myoneme
A myoneme (or spasmoneme) is a contractile structure found in some eukaryotic single-celled organisms, particularly Vorticella. It consists of a series of protein filaments that shorten rapidly upon exposure to calcium. Although the shortening can be up to 100 lengths per second, faster than any muscle, the relaxation time is several seconds (compared to approximately one tenth of a second for muscle). The myonemes of Acantharea also display slow contraction and undulation movements.Picoeukaryote
Picoeukaryotes are picoplanktonic eukaryotic organisms 3.0 µm or less in size. They are distributed throughout the world’s marine and freshwater ecosystems and constitute a significant contribution to autotrophic communities. Though the SI prefix pico- might imply an organism smaller than atomic size, the term was likely used to avoid confusion with existing size classifications of plankton.Protista taxonomy
Kingdoms animal, plant and fungi are in bold. Protists are a large and diverse group of eukaryotic microorganisms which belong to the kingdom Protista.Radiolaria
The Radiolaria, also called Radiozoa, are protozoa 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 ectoplasm.The elaborate mineral skeleton is usually made of silica. They are found as zooplankton throughout the ocean, and their skeletal remains make up a large part of the cover of the ocean floor as siliceous ooze. Due to their rapid turn-over of species, they represent an important diagnostic fossil found from the Cambrian onwards. Some common radiolarian fossils include Actinomma, Heliosphaera and Hexadoridium.Rhizaria
The Rhizaria are a species-rich supergroup of mostly unicellular eukaryotes. A multicellular form has also been described.
This supergroup was proposed by Cavalier-Smith in 2002. Being described mainly from rDNA sequences, they vary considerably in form, having no clear morphological distinctive characters (synapomorphies), but for the most part they are amoeboids with filose, reticulose, or microtubule-supported pseudopods. Many produce shells or skeletons, which may be quite complex in structure, and these make up the vast majority of protozoan fossils. Nearly all have mitochondria with tubular cristae.Sticholonche
Sticholonche is a genus of radiolarians with a single species, Sticholonche zanclea, found in open oceans at depths of 99–510 metres. It is generally considered a heliozoan, placed in its own order, called the Taxopodida. However it has also been classified as an unusual radiolarian, and this has gained support from genetic studies, which place it near the Acantharea.Sticholonche are usually around 200 μm, though this varies considerably, and have a bilaterally symmetric shape, somewhat flattened and widened at the front. The axopods are arranged into distinct rows, six of which lie in a dorsal groove and are rigid, and the rest of which are mobile. These are used primarily for buoyancy, rather than feeding. They also have fourteen groups of prominent spines, and many smaller spicules, although there is no central capsule as in true radiolarians.Strontium
Strontium is the chemical element with symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to air. Strontium has physical and chemical properties similar to those of its two vertical neighbors in the periodic table, calcium and barium. It occurs naturally mainly in the minerals celestine and strontianite, and is mostly mined from these. While natural strontium is stable, the synthetic 90Sr isotope is radioactive and is one of the most dangerous components of nuclear fallout, as strontium is absorbed by the body in a similar manner to calcium. Natural stable strontium, on the other hand, is not hazardous to health.
Both strontium and strontianite are named after Strontian, a village in Scotland near which the mineral was discovered in 1790 by Adair Crawford and William Cruickshank; it was identified as a new element the next year from its crimson-red flame test color. Strontium was first isolated as a metal in 1808 by Humphry Davy using the then-newly discovered process of electrolysis. During the 19th century, strontium was mostly used in the production of sugar from sugar beet (see strontian process). At the peak of production of television cathode ray tubes, as much as 75 percent of strontium consumption in the United States was used for the faceplate glass. With the replacement of cathode ray tubes with other display methods, consumption of strontium has dramatically declined.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 nitric acid and appreciably soluble in alkali chloride solutions (e.g. sodium chloride).Vladimir Shevyakov
Vladimir Timofeyevich Shevyakov, in Russian Владимир Тимофеевич Шевяков (29 October 1859, St. Petersburg – 18 October 1930, Irkutsk) was a Russian biologist who worked on Protozoa.
Shevyakov studied under Konstantin Mereschkowski in St. Petersburg and Otto Bütschli at the University of Heidelberg. He was married to Lydia Kowalevsky, the youngest daughter of Alexander Kovalevsky. He was a professor at St. Petersburg University until 1911 when he left science and became a vice-minister in the government of Tzar Nicholas.During the revolution he and his family moved first to Perm in Ural and in 1920 he became professor in Irkutsk.
He is mainly known for his work on Radiolaria, Ciliata and Acantharea. He described many taxa. Most of his publications are under the German spelling of his name which is Schewiakoff.