Specific Host-Symbiont Relationship in Larger Foraminifera

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Abstract
Fine structure in diatom symbionts of larger foraminifera indicates an exclusive relationship of host and symbiont species regardless of water depth, location or season. In Amphistegina lobifera, A. lessonii and A. bicirculata, the cell ultrastructure of the symbionts examined in situ uniformly corresponds to that of the diatom Fragilaria shiloi, which is one of the algal species that recently was extracted from these hosts and identified [J. J. Lee et al.] after frustules developed in culture. The symbionts in A. papillosa are larger than F. shiloi but have a similar pyrenoid ultrastructure (simple internal pyrenoid). The symbiont in Heterostegina depressa is characterized by invaginated pyrenoids. Although not yet identified, it is not the same as either of the 2 diatom species that apparently were isolated from this host, i.e., Nitzschia panduriformis, from Red Sea specimens, which has simple internal pyrenoids or N. valdestriata, from Hawaiian specimens, which has compound internal pyrenoids. Morphological studies of symbionts in situ indicate a higher specificity of the foraminiferal-algal associations than investigation of material isolated from hosts.