Several thousand species of red algae have been identified and are classified together in the division Rhodophyta. Most are multicellular marine varieties that predominantly inhabit tropical and subtropical areas, though some are unicellular and prefer different habitats. The color of red algae, which is sometimes blue rather than red, is due to the presence of phycobilin pigments. These accessory pigments aid in photosynthesis and mask the green chlorophyll pigments also present in the algae. The life cycle of red algae is complex, and the majority of rhodophytes reproduce sexually. Both male and female gametes are nonmotile.