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Abstract. Factors were examined that affect survival and growth of two common species of large foraminifera from the Red Sea, Amphisorus hemprichii Ehrenberg and Amphistegina lobifera Larsen, 1976. The former is host for dinoflagellate and the latter for diatom zooxanthellae. Experimental conditions were modeled on conditions at 25 m during spring at Wadi Taba, Gulf of Elat, Israel, the season and site where the experimental organisms were collected between 1983 and 1988. The two species responded quite differently in nutritional experiments.A. hemprichii grew, on average, 0.270 mm in diameter in 3 mo on a diet of Nitzschia subcommunis Hustedt, Chlorella sp. (clone AT) or Cylindrotheca closterium Rabenhorst isolated from their native habitat. Unfed controls did not grow. In contrast, unfed populations of A. lobifera grew as well or better than those that were fed unialgal diets. Growth of both species was enhanced on particular mixed algal diets. Both species required photosynthetically active symbionts. Even when fed weekly and supplied with nutrients, neither species survived in the dark. All individuals of A. hemprichii died after 8 wk incubation in the dark; A. lobifera survived longer, but all were dead by 13 wk. The highest growth rate of A. hemprichii (0.037 mm wk-1) was obtained when they were fed, the medium was enriched, and the medium was changed weekly. All other conditions being the same, growth rate dropped to 0.009 mmwk -1 when the medium was changed every 3 wk. In contrast, A. lobifera grew fastest when the medium was changed every 3 wk. Food or enrichment with nitrate or phosphate did not stimulate growth (0.03 mm wk-1) over that of the controls. Specimens of Marginopora kudakajimensis Gudmundsson from Japan, another dinoflagellate-bearing species, were also tested. They grew best (0.02 mm wk-t) when cultured in light, in media enriched with nitrate and phosphate changed weekly, and fed. All three species withdrew nitrate and phosphate from the medium in chemostat experiments.

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