In situ growth rate experiments on the symbiont-bearing foraminifera Amphistegina lobifera and Amphisorus hemprichii.

Both species show a distinct growth rate maximum of 3-6% day at 25-30m, attributable to optimal light conditions for symbiont photosynthesis, and possibly to optimal food availability. Growth rates in the dark are roughly an order of magnitude lower than in illuminated experiments; feeding in the dark increases growth rate by a factor of 3. The mid-water growth optimum of A. lobifera coincides with its maximum abundance which suggests that when this species grows in optimal conditions, a larger proportion of the population may reach maturity and successfully reproduce.-from Authors