Test shape in Amphistegina spp. grown in culture is influenced by light availability and water motion. Saturation light level for A. gibbosu was between 6 and 14 pE/s m2; growth rates at 14 and 40 pE/s mz were 5.9%/d and thickness-to-diameter (T/D) ratios were 0.36. At reduced light levels, growth rates and T/D ratios were reduced down to 3.9%/d and 0.31 at 3.9 pE/s mz. Saturation light level for A. Zessonii was between 14 and 40 pE/s m2; growth rate at the higher level was 7.8%/d and T/D ratios were 0.41. Likewise, growth rates and T/D ratios were reduced at reduced light levels, down to 1.3%/d and 0.36 at 1.6 pE/s m2. The thickest tests, with T/D ratios of up to 0.5, were found in both species when grown at saturation light levels with gentle water motion. Sectioned tests revealed that differences in T/D ratios are a direct result of differences in secondary lamellar thicknesses; secondary lamellae of Amphisteginu subjected to water motion were as much as 50% thicker than those of individuals grown without water motion.Symbionts isolated from A. gibbosu from Puerto Rico were small diatoms of the genus Nitzschiu. Although specific identification of the symbiont could not be made because sieve plates were lost, the symbiont is distinct from three other Nitzschiu spp. found in Indo-Pacific A mphistegina.
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