The Acantharia, often written Acantharea, are composed of approximately 140 species taxon of unicellular eukaryotic organisms that the radiolarians ( Radiolaria ) belong. All species live in the sea as an extremely common part of the zooplankton in the upper layers of the sea. They have a mathematically regularly built skeletal from the mineral celestite ( strontium sulfate = ) and have great importance for the strontium cycle in the sea.
Ecology, biology, and in particular the life cycle of animals are known so far only insufficiently.
- 3.1 dissemination
- 3.2 prey
- 3.3 zooxanthellae
- 3.4 Role in the marine element circuit
Acantharia are flattened spherical or oblong- round, occasionally and have a diameter of 0.05 to 5 millimeters.
The cell has a skeleton of 20 (at the Holacanthida 10) spikes which consist of single crystal, rhombic strontium sulfate, a unique among the Protista building material.
The always same arrangement of the spines is done with mathematical regularity, from the imaginary "sphere" of the round cell enter the tips of the respective rows of four, then to 60 ° N, 60 ° S, 30 ° N, 30 ° S and 0 ° latitude and 0 °, 90 °, 180 ° and 270 ° ( polar or equatorial peaks ) or 45 °, 135 °, 225 ° and 315 ° (tropical peaks) longitude forth ( see figure).
This regularity recognized for the first time Johannes Peter Müller and formulated it in 1858 in " Müller'schen Law" named after him:
Therefore " gives [ ... ] for the Acanthometren with 20 spines the same formula that between two stingless Poland 5 Belt of spines are, each of four spines, all directed by the Community Center around the sphere, and that the spines of each belt with the previous old Irish. "
The cell body consists of the central endoplasm and peripheral ectoplasm. That the cell nucleus and most organelles containing endoplasm is usually brown, red or black pigmented, with all types except those of Arthracantida is the concentration of the pigmented endoplasm in the center particularly dense, the cell is filled with an increasing proportion of the transparent ectoplasm outwardly always clearer.
Endo- and ectoplasm are separated by a capsule wall having a fibrillar mesh having openings for cytoplasmic projections and Axopodien. In Arthracantida the capsule wall is significantly thicker than in the other systems, in which the capsule wall is partially hardly recognizable.
As a " skin " of the ectoplasm and so also the entire cell serves the periplasmic cortex, also a fibrillar meshwork. It consists of twenty by means of elastic connecting pieces interconnected polygons that are each disposed around one of the skeleton spines.
Capsule wall and periplasmic cortex lying on the thorns of another. In the small space between the capsule wall and cortex are the Myoneme, anchored to the capsule wall and the barbed fibrillar bundles of proteins, which are flat, ribbon-like, short cylindrical or triangular, and 5 to 90 microns long. Depending on the type found 40-1200 Myoneme, ie from 2 to 60 pieces per sting. They serve as motion producing organelles and can trigger three movements: A slow wavy, then an abrupt contraction, which can push the periplasmic cortex up to the top of the spike and a slow subsequent relaxation. When an individual all Myoneme activated simultaneously, which leads to a sudden increase in cell volume. These all ten to twenty minutes observable movement probably promoted the lift.
As organelles found in the endoplasm large mitochondria with tubular cristae, a rough endoplasmic reticulum, ribosomes, dictyosomes, peroxisomes and the Kinetozysten the heliozoan similar Extrusome. The nuclei are usually numerous, small and round or oblong - round basis, except some species of the order of Symphyacanthida with a single very large and probably polyploid nucleus.
In almost all species except those of Arthracanthida, the endoplasm with increasing age Lithosomen. These are small, oval-shaped, birefringent plates, which are produced from the Golgi apparatus. Transported to the cell surface, is formed from them during the encystment of gametogenesis before the outer skin of the cyst, the Myoneme are secreted there.
Like all radiolarians and the heliozoan have the Acantharia Axopodien, especially long, thin and straight cell extensions that protrude from the cell surface. They are made of a thin cytoplasmic layer and the cell membrane and strengthened by a special structure in Holacanthida dodekagonal, otherwise hexagonal to Axonemen arranged microtubules, small, located in the endoplasm MTOCs spring. Cytoplasmic organelles such Extrusome, mitochondria, as well as different types of vesicles are present. The Axopodien serve the prey capture and respond to stimuli, they pull back as in chemical physical (temperature, touch) stimuli and then slowly build up again.
The life cycle of Acantharia is not fully known. Problems in monitoring prepares that you can cultivate under laboratory conditions nor obtain longer time alive the animals.
In addition to the described Trophontenstadium are known from single observations freshly caught animals only strong einskelettierte cysts and swarmer. By the latter thousands of mononuclear, flagellated swarmers are released. About other life stages are no findings. However, it was surprisingly at depths below 900 m, so live well below the zone in which Acantharia, both in samples of water, as well as the seabed DNA are detected by Acantharia. It is thought that this is to remove traces of previously unknown life stages of Acantharia because the trophonts not let there microscopic evidence.
Way of life
Acantharia belong to the so-called zooplankton, that are not self- photosynthetic parts of the plankton in the sea. They are found worldwide in all oceans, but especially in tropical and subtropical waters, only scattered, however, they are found in temperate or even polar latitudes. Coastal areas are largely avoided, as eutrophic waters. In particular, due to their photosynthetic symbionts they live mainly in the light-flooded a few hundred meters near the surface on calm days to great Acantharia few meters gather in large groups among them. The highest stocking densities are found in 50 to 200 meters water depth, for the purpose of gametogenesis some species can, however, decrease at depths of 300 to 400 meters. Rarely, a few individuals also found several thousand feet deep on the sea floor.
Clear seasonal dependencies there are probably not, some studies provided to clusters in spring and summer, other observations, clusters were observed in the spring, while in summer they seemed to be rare. In coastal regions apparent seasonal accumulations in connection with the periodic exchange eutrophic coastal water could be detected by oligotrophic water of the open sea, in oligotrophic waters throughout the tropics and subtropics, they are a year-round frequent component of the micro- plankton.
The animals are very common in near-surface water samples in the North Atlantic were for the upper 20 meters a density of up to 16 copies per liter, between 40 and 120 meters there are still about 10 copies per liter. Thus, they are about 10 - to 16- times more likely than, say, planktonic foraminifera. For some samples Acantharia accounted for more than 30, sometimes even more than 70% of all living beings of the sample from.
By means of a very dynamic and changing stimulus-responsive high void network of anastomosing Zytoplasmafortsätzen and with the Axopodien be captured mainly micro-organisms such as diatoms, Silikoflagellaten, coccolithophores and Tintinniden, but also small mollusks. In addition, traces of extremely small prey such as cyanobacteria or other bacteria were detected, it is unclear, however, whether these prey is targeted for grazing or is only " catch ".
In many species there are Acantharia optional zooxanthellae symbionts, which provide the Acantharia by photosynthesis energy. This exhibit in the stocks only in size to the middle 50 % of all individuals zooxanthellae to represent the largest as the smallest individuals they are missing. However, all Arthracanthida species they have in certain phases of the life cycle. The zooxanthellae are absent in gamonts and young trophonts. They are taken during or immediately prior to gametogenesis, during trophonts Stage their number to then. Before reaching the reproductive stage but they are repelled.
The zooxanthellae are mostly to Haptophyta or dinoflagellates. In a host here may also occur simultaneously multiple symbiont species. Also close to the surface they can continue to fix carbon, partially in amounts that significantly exceed the requirements of the host. Frequently found in the endoplasm also parasitic dinoflagellates of the genus Amoebophrya.
Role in the marine element circuit
Since the skeleton related to the Acantharia strontium sulfate is soluble in water, the animals must continuously record material for it from the sea water. With her death, the shell sinks and dissolves rapidly in water layers around 900 m. Due to the high number of pets as a continuous transfer of strontium from higher to deeper water layers takes place, so that the higher are depleted compared to the deeper layers downright with strontium. Acantharia therefore considered as the most important biological component of the marine strontium cycle. The same is true - although in a smaller amount - for the element barium, which accounts for approximately 0.4 % of the skeletons. During the skeleton structure Acantharia accumulate additional trace elements such as lead, zinc, copper and iron in a significant amount to and " carry " them. Likewise by resolution of the skeleton in middle layers of the sea
Since Ernst Haeckel monographic treatment of the radiolarians based on the Challenger discoveries in 1887 were the Acantharia as one of three sub-groups of Radiolaria. Your exact systematic position was at times controversial. Vladimir Schewiakoff they separated in 1926 completely out of the Radiolaria, but this view did not prevail itself.
Molecular genetic studies strengthened the association of the group to the Radiolaria. The traditional line-up as a separate group was essentially confirmed. The internal system of the Group is, however, to be no longer tenable and needs to be revised. ,
The group includes the traditional classification, around 140 species in 50 genera and is divided into four orders with 18 families. As a test for distinguishing the four orders serve different compounds of sting approaches.:
- Order Arthracanthida Subordination Sphaenacantha Acanthometridae
As dissolve the mineral skeletons of Acantharia in sea water quickly, there is little fossils of the group. The few surviving finds date back only to the Eocene ( Chiastolus amphicopium ).