Cactus Seaweed Halimeda opuntia
The heavily calcified skeletons of species of Halimeda contribute much of the calcareous sediment in the tropics. Cactus seaweed consists of strings of flattened, kidney-shaped, calcified segments, linked by uncalcified, flexible joints. By day, its chloroplasts are in the outer parts of the frond; at night they withdraw deep into the plant’s skeleton. This, along with sharp crystals of aragonite, and the presence of toxic substances in the frond, protects the cactus seaweed from nocturnal grazing.