The mention of Radiolaria always brings to mind Ernst Haeckel's astonishing hand-drawn images, compiled in his classic plate collection Art Forms in Nature. I'm always mesmerized by the symmetry and detail of his drawings of microscopic skeletons. Haeckel's ability to combine science and art is one aspect of his remarkable career. His life presents a rather bizarre mixture of brilliance and controversy, very much worthy of a future post here. But when it comes to drawing and cataloguing Radiolaria, he is second to none. He described nearly 4000 different organisms and grouped them as Radiolaria. These abundant marine microbes, encased in silica shells, catch prey by extending parts of their cell body through holes in the shells. Many of them also obtain some of their energy from photosynthetic symbionts such as zooxanthellae. But relatively little else is known about the biology of these beautiful ocean dwellers. In the end, what is best understood is their evolutionary history as recorded in the ocean sediments. The fossil record shows them appearing early in the Cambrian, some 540 million years ago. As such, radiolarian remains are used in geological dating and in the determination of ancient climates.
Who are the Radiolaria? In the big scheme of things they are protists, unicellular eukaryotes. Within Eukarya, they belong to the supergroup Rhizaria that also includes Cercozoa and Foraminifera, which Elio described in a prior post. As it turns out, molecular phylogenetic analyses now show that the organisms that Haeckel grouped as Radiolaria are polyphyletic, with some belonging to the Cercozoa. As one set of authors put it the title of a 2013 paper, this meant "a farewell to Haeckel's Radiolaria." For anyone interested in exploring radiolarian taxonomy in great detail, here is a good starting place.
What on Earth do these beautiful yet mysterious creatures do? In researching material for my recent post on the biological pump (the transport of carbon from the ocean surface to great depths and the ocean floor) I came across what I considered a most surprising finding. A paper with authors from six countries, with Andres Gutierrez-Rodriguez as first author, describes the microbial components of the particulate organic matter (POM) that passively sinks through the water column to bring carbon that is fixed at the surface into the ocean interior. Their method was to set traps to catch sinking POM and then analyze the nucleic acids present to determine microbial community composition. Surprisingly, Radiolaria dominated among the eukaryotes present. Who would have guessed it? Silica enclosed microbes bringing down most of the carbon. We know so little about Radiolaria that seem to be doing so much for Earth.