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Marine Animal Encyclopedia

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.

Cactus Seaweed habitat mapzoom image
  • Class Bryopsidophyceae
  • Size Up to 10 in (25 cm)
  • Habitat Rock and sand
  • Water_temperature 68–86°F (20–30°C)
  • Distribution Red Sea, Indian Ocean, and western Pacific