Acantharea
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Date: 2021-12-04

Acantharea, by Wikipedia https://en.wikipedia.org/wiki?curid=2502 / CC BY SA 3.0

#Acantharea
The Acantharea （Acantharia） are a group of radiolarian protozoa, distinguished mainly by their strontium sulfate skeletons.
Acantharians are heterotrophic marine microplankton that range in size from about 200 microns in diameter up to several millimeters.
Some acantharians have photosynthetic endosymbionts and hence are considered mixotrophs.
Acantharian radiolarians have shells made of celestine crystal, the heaviest mineral in the ocean Acantharian skeletons are composed of strontium sulfate, SrSO4, in the form of mineral celestine 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 have been observed at times in the Iceland Basin and the Southern Ocean, as much as half of the total gravitational organic carbon flux.
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 unique.
However, 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 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 cell cy...