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.
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 nutient-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.   
1. ^ a b Guiry, M.D. & Guiry, G.M. (2007). "Family: Halimedaceae taxonomy browser". AlgaeBase version 4.2 World-wide electronic publication, National University of Ireland, Galway. http://www.algaebase.org/browse/taxonomy/?id=5127. Retrieved 2007-09-24.
Source: Wikipedia, Wikispecies: All text is available under the terms of the GNU Free Documentation License