Single cells that build glass skeletons shaped like geometry lessons
A radiolarian is one cell, a fifth of a millimetre across at most, yet it builds an intricate mineral skeleton, often of silica, pierced with holes and bristling with spines. Some species resemble perfect polyhedra. Their remains carpet stretches of seabed and preserve a fossil diary of the oceans going back to the Cambrian.
These plankton drift through every ocean. A central capsule divides each cell into an inner zone holding the nucleus and an outer zone packed with frothy vacuoles and oil droplets that keep it afloat. Long, needle-like extensions stiffened by bundles of microtubules also aid buoyancy and let the cell reach through its shell to catch prey. Many harbour symbiotic algae that supply most of their energy, so they both hunt and photosynthesise, secondhand. One species, Circogonia icosahedra, is shaped like a twenty-sided solid.
When they die, their shells sink. Over time these accumulate as siliceous ooze, and because species change quickly, geologists use the microfossils to date rocks and reconstruct past sea conditions. Records show that during the late Neogene some species vanished from high latitudes but held on in the tropics. Cold-water species of the Southern Ocean may not have that escape route as their habitat shrinks with warming, while tropical species may push toward the midlatitudes.
Radiolarians sit in the supergroup Rhizaria, close to the shelled foraminifera. The main lineages include the silica-building polycystines and the acantharians, whose skeletons are made of strontium sulfate instead. Environmental DNA has turned up whole groups nobody has yet seen, so the real diversity is probably far larger than the catalogue.
Their beauty has long attracted scientists. Ernst Haeckel drew them in exquisite detail in 1887. Decades later, Bernard Richards, one of Alan Turing's last students at Manchester, studied radiolarian forms to help test Turing's 1952 theory of morphogenesis, a line of thinking Turing had wanted to extend from D'Arcy Thompson's On Growth and Form of 1917.
Source: Radiolaria
