Influence of Environment on the Test Shape of emAmphistegina/em

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 μ E/s m ² ; growth rates at 14 and 40 μ E/s m ² 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 μ E/s mz. Saturation light level for A. Zessonii was between 14 and 40 μ E/s m ² ; 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 μ E/s m ² . 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 Amphistegina subjected to water motion were as much as 50% thicker than those of individuals grown without water motion.

Symbionts isolated from A. gibbosa from Puerto Rico were small diatoms of the genus Nitzschia . Although specific identification of the symbiont could not be made because sieve plates were lost, the symbiont is distinct from three other Nitzschia spp. found in Indo-Pacific Amphistegina .