The Romantic Discovery of Radiolaria in the Ocean

Radiolaria are unicellular marine organisms (protists) that have been drifting in oceanic plankton for hundreds of millions of years. These mineral architects can build extraordinarily complex skeletons, which fascinated and puzzled naturalists observing water samples through rudimentary microscopes. In the 19th century, the discovery and study of Radiolaria are associated with scientific voyages and human adventures. Naturalists who studied medicine and anatomy in European universities were captivated by the morphology of Radiolaria and expressed a profound wanderlust to collect them in the ocean. These intrepid and workaholic adventurers devoted their restless lives to studying microscopic life, while also actively engaging in teaching and sharing their observations and hypotheses with students. This article aims to retrace the discovery of Radiolaria through the lives of prominent naturalists and marine biologists, primarily Christian Gottfried Ehrenberg, Thomas Henry Huxley, Johannes Müller, and Ernst Haeckel. It also highlights the intellectual and geographic influences that shaped their research, including figures such as Johannes Wolfgang von Goethe and Alexander von Humboldt, as well as places like Jena, Helgoland, Villefranche-sur-Mer and Italy, which served as sampling locations and sources of romantic and artistic inspiration. Pioneering work on Radiolaria played a central role in shaping several emerging concepts (e.g., cell theory, individuality) and fields (e.g., taxonomy, evolution of morphology, symbiosis). The discovery of Radiolaria therefore reveals that even the most elusive marine microorganisms can deeply transform our understanding of life.