Halimeda

Halimeda is a genus of green macroalgae that forms calcified, segmented thalli and contributes significantly to carbonate sediments on tropical and subtropical reefs. Species are widespread in warm shallow seas and are commonly found on coral reefs, in lagoons, and among seagrass beds.

The thallus of Halimeda is coenocytic and siphonous, lacking true cross-walls, yet its tissue is organized into calcified segments or plates. These segments accumulate to create a durable skeleton that readily fragments, producing sand-sized particles composed of calcite. The light-green to pale white calcareous segments give Halimeda its characteristic appearance and distinguish its skeleton from the surrounding fleshy tissue.

Ecology and life history: Halimeda species prefer bright light and warm temperatures and are commonly found in shallow, clear coastal waters. They can reproduce sexually and asexually, with fragmentation serving as a common means of dispersal and new growth. The calcareous skeleton they produce contributes to the reef carbonate budget and influences sedimentation patterns in their habitats. Grazers such as parrotfishes and herbivorous invertebrates interact with Halimeda as both a food source and a contributor to reef sediment dynamics.

Human relevance and environmental considerations: Halimeda is sometimes kept in marine aquaria as a source of calcium carbonate and a habitat for small organisms. Its calcified skeleton makes it sensitive to changes in ocean chemistry, particularly ocean acidification, which can reduce calcification rates. In paleoenvironmental studies, Halimeda remains are used as indicators of past reef conditions and carbonate sedimentation.