Halimeda

Halimeda is a genus of green macroalgae. The algal body (thallus) is composed of calcified green segments. Calcium carbonate is deposited in its tissues, making it inedible to most herbivores. However one species, Halimeda tuna, was described as pleasant to eat with oil, vinegar, and salt.

As in other members of the order Bryopsidales, individual organisms are made up of single multi-nucleate cells. Whole meadows may consist of a single individual alga connected by fine threads running through the substrate.

Halimeda is responsible for distinctive circular deposits in various parts of the Great Barrier Reef on the north-east coast of Queensland, Australia. Halimeda beds form in the western or lee side of outer shield reefs where flow of nutrient-rich water from the open sea allows them to flourish, and are the most extensive, actively accumulating Halimeda beds in the world.

The genus is one of the best studied examples of cryptic species pairs due to morphological convergence within the marine macroalgae.

Some species grow so vigorously in tropical lagoons that the sediment is composed solely of the remains of their tissues, forming a calcareous "Halimeda sand". In fact some tropical reef systems, such as atolls, consist largely of Halimeda sand accumulated over the aeons. Overall, Halimeda represents the most common green algea large grains in the sediment of the lower latitudes.