Why are larger Foraminifera large?

Delayed maturation and growth to large sizes are only advantageous under stable environmental conditions where food resources are limited. Specialization to algal symbiosis is also highly advantageous under those conditions if sunlight is available. The coevolution of these two characteristics has occurred many times in many foraminiferal lineages. These traits are sometimes associated with increased embryon size and suppression of sexual reproduction, which are also characteristics most advantageous under stable environmental conditions. Specialization for these traits, ensuring success in warm, shallow, stable, oligotrophic environments, often dooms the species or lineage to extinction when conditions change.