Halimeda opuntia, sometimes known as the watercress alga,[2] is a species of calcareous green seaweed in the order Bryopsidales.

[3] Halimeda opuntia forms thick, frequently-branched clumps of calcified, leaf-like segments up to 25 cm (10 in) high.

It grows in grooves, depressions and cracks in rocks and between coral heads in moderately protected parts of the reef, at depths down to about 55 m (180 ft).

[5] Researchers found that the larvae of Favia fragum readily settled on Halimeda opuntia, however the alga was ephemeral and therefore unsuitable for long-term survival of the coral.

[5] The secondary metabolites include growth regulators such as auxins, gibberellins and cytokinins, and substances with antibacterial and antifungal properties, but these are not being harvested for commercial use.