Symbiosis in benthic foraminifera; specificity and host adaptations

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Abstract
Ultrastructure of foraminiferal symbionts in situ indicates exclusive host-symbiont specificity, independent of locality, depth or season. Rhodophyceans are harbored by 3 ribbed species of Peneroplinae; chlorophyceans occur in Peneroplis proteus and 2 spp. of Archaiasinae. Dinophyceans are present in 5 spp. of Soritinae. Four groups of diatoms are associated with different taxa of foraminifera, i.e., group B1 with 3 Calcarinidae species, B2 (2 types) with 4 Amphisteginidae species, B3 (5 types) with 2 Alveolinidae species, 5 Nummultidae species and Amphistegina sp., and B4 with A. radiata. Isolated chloroplasts occur in 2 Nonionidae and 6 Elphidiidae species. Light quality influences depth distribution of symbiont-bearers: hosts bearing chlorophyceans dwell in shallow waters, those with dinophyceans or rhydophyceans between 0 and 70 m, and diatom bearers between 0 and 130 m. Symbionts are concentrated immediately below lateral shell walls. This position offers optimal conditions with regard to light, nutrient supply and exchange of ions and molecules with the environment. The adaptive significance of shell architecture, internal shell structure and wall permeability to symbiosis are discussed, also in terms of foraminiferal taxonomy and evolution.