Large, spongiose skeletal Radiolaria occur abundantly in tropical and some subtropical oceanic locations. They are particularly conspicuous in plankton tows owing to their brightly coloured central capsular region (red to purplish-brown) and the numerous dinoflagellate, prymnesiophyte, or prasinophyte symbionts that impart a distinctly golden-brown or greenish hue to the peripheral cytoplasm. There is substantial variability in skeletal and cytoplasmic morphology and the major skeletal features used by Haeckel and others to discriminate among species intergrade to such a degree that it is not possible in most circumstances to make a clear taxonomic distinction. Furthermore, there is no correlation between the kind of symbiont associated with the host and its skeletal organization. Moreover, there is a nearly continuous variation in cytoplasmic organization of the central capsules varying from loosely organized radially arranged lobes surrounding a centrally located nucleus to a more spongiose and compact cytoplasm (consisting of interconnected masses of nucleated cytoplasm) with large, densely staining reserve bodies. In the extreme development of spongiose cytoplasmic organization, the reserve substance is dispersed throughout the cytoplasm. There is no correlation between the degree of spongiose quality of the cytoplasm and the organization of the skeleton. The variations in cytoplasmic organization suggest an ontogenetic sequence progressing from a large, centrally located nucleus with loose, radially-arranged cytoplasmic lobes toward increasingly dispersed nuclear lobes distributed into the peripheral cytoplasm containing abundant reserve bodies. Further research is needed to evaluate this hypothesis. In overall perspective, the variability among this group is so large and so intergraded that due caution is advised in assigning them to different species. Indeed, given the lack of clear skeletal and cytoplasmic demarcation within this group, and the absence of molecular genetic information to clarify the species affinities, it may be misleading to establish species based only on skeletal evidence. Moreover, further research is required to determine how much of the variation in skeletal structure can be attributed to genetic differences (taxonomic differences) versus ecophenotypic variation that may provide information about environmental variables recorded in the skeletal morphology.
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