Fine structure and physiological studies of major biological

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processes in solitary and colonial radiolarians have begun to elucidate some of the adaptive mechanisms that may account for the remarkable success of these holoplankters in diverse oceanic environments. The diversity of algal symbionts including dinoflagellates, prasinophytes, and a brown-pigmented alga has been documented by light and electron microscopy, and the role of the symbionts in host nutrition has been determined for some solitary and colonial species using cytochemical and radioisotopic techniques. Fine structure analysis of predatory behavior and food vacuole contents shows that some larger solitary spumellarian species are omnivores consuming crustacean prey and other microzooplankton in addition to algae. This is in contrast to prior assumptions that most solitary radio arians are microherbivores and therefore changes our conception of their niche in oceanic environments.

Fine structure examination of skeletal morphogenesis in a number of solitary and colonial species has clarified the mechanism of skeletal deposition and pattern of skeletal ontogeny. These studies contribute to our understanding of cellular specialization in radiolarians, their physiological adaptive mechanisms to a holoplanktonic existence, and the biological factors correlated with patterns of skeletal morphogenesis that may be useful in interpreting radiolarian evolution and paleoecology.

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