These group is commonly referred to as the red algae. They have only one type of chlorophyll, which is masked by the red accessory pigment, phycoerythrin. A related blue pigment, phycocyanin also occurs in the red algae. The red algae are predominantly marine with only few exceptions that live in freshwater. They are often found attached to a solid substrate, like rocks or sea walls, but can also be found (although rarely) on shells, seagrass or other algae. Most are restricted to growing in shallower waters where there is much light for photosynthesis, however, this would depend on the clarity of waters (they can grow deeper in places with clearer waters). Conversely, red algae have been found at greater depths than any other photosynthetic organisms.