The unique habitats of marine environments including some organically rich, but physically variable, benthic environments, and the rather uniform, regionally stable, open ocean environments have both produced species of biomineralizing protozoa of remarkable size ranging from several hundreds of microns to several millimetres (for solitary forms of foraminifera and radiolaria for example). Some colonial radiolaria are several centimetres in diameter and may reach lengths of over a metre for filiform species. These larger, free-living protozoa aften harbour algal symbionts and exhibit unusual adaptations to enhance their survival in their unique habitat and to stabilize, if not enhance, their association with their algal symbionts through structural and physiological modifications to complement the unique requirements imposed by animal–algal associations.
