Halimeda is a family of moderately fast growing calcareous algae found in tropical oceans around the world in shallow to deep waters. It has a holdfast (root) which may be attached to a sandy substrate or to live rock. The body of the algae is comprised of a string of green flattened segments or plates jointed together which are composed of calcium carbonate as is found in the skeleton of stony corals or in coralline algae. The jointed segments resemble some the flattened prickly pear cactus thus it is sometimes referred to as the Cactus algae.
There are 3 Halimeda species that are most common in the hobby. H. tuna and H. discoidea have larger segments with fewer segments per chain. These two are hard to tell apart, but H. discoidea tends to have more disc shape segments while H. tuna has more kidney shaped segments. H. opuntia has smaller somewhat heart-shaped segments, with more segments per chain and frequently forms fairly dense mounds of the algae. H opuntia is also usually a lighter green color than H. tuna or H. discoidea in reef aquaria.
Good or Bad?
Halimeda is arguably the best macro algae to keep in a reef tank. Unlike Caulerpa, it is slower growing and grows from a holdfast rather than from runners and therefore does not spread and takeover a reef tank as some algae can and is more easily controlled.. It also does not go sexual and spontaneously combust as readily as Caulerpa is prone to do. Occasional light pruning seems to help minimize the risk of this further. The calcium carbonate in its tissues prevent most fish from considering it as a food source. it is also known to create some toxic compounds in its tissues to further dissuade any potential foragers.
Being calcareous, Halimeda requires fairly high calcium and alkalinity levels similar to stony corals to survive. In this regard, they can be a good barometer of tank conditions, but they also compete for the available calcium and carbonates in the system.
Segments of the algae will occasionally die and turn white. The white color is from the calcium carbonate contained in its tissues. In the wild, Halimeda skeletons can contribute a great deal to the formation of sand in many beach areas and are often a bigger contributor to the beach formation than stony corals.
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