Huh. Today I learned that Acantharian protozoans make their skeletons out of Celestine, strontium sulphate, rather than from more common skeletal materials like calcite, phosphates or silica!
Morphology Celestine crystal Acantharian skeletons are composed of strontium sulfate, SrSO4,[3] 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.[4] 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.[5][6][4] The strontium sulfate crystals[3] are secreted by vacuoles surrounding each spicule or spine. Acantharians are unique among marine organisms for their ability to biomineralize strontium sulfate as the main component of their skeletons.[7] 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 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.
