Sporogenesis of an extracellular cell chain from the spheroidal radiolarian host Haliommilla capillaceum (Haeckel), Polycystina, Protista

We present the first record of a living solitary spherical polycystine radiolarian, Haliommilla capillaceum (Haeckel), with a chain of extracellular cells. The extracellular cells divided seven times over about 4 h from the 8-cell to the 1024-cell stage and then swam away. At the 1024-cell stage, the cells were very small isospores (ca. 12 μm in length). After final cytokinesis, the cells remained essentially immobile, with only occasional movement. After this immobile stage, they abruptly swam out in a burst into seawater. The cell division patterns and cell shape are similar to the sporogenesis of the host-specific parasitic dinoflagellate Duboscquella sp. infecting a tintinnid. Precise identification, however, needs molecular analysis, transmitted scanning images and proliferation test.