Halimeda is a tropical green seaweed. This species tends to inhabit the clear, warm waters of tropical areas. There are 30 species worldwid. Within these pages we’ll look at some of the interesting features of Halimeda, where it comes from? What is it? What effects it has on the surrounding environment? How does it combat predators? and more.

What is it?

CaulerpaHalimeda is a calcareous green algae of siphonous construction forming a coencytic 3D structure consisting of interwoven filaments. Put simply Halimeda is a giant web of strings of cells that are joined together to form a large semi-rigid structure. The taxonomic classification of Halimeda places it under the same class and order as the prolific Caulerpa (an invasive species that is causing havoc in tropical areas), but divides into the family Halimediaceae.

There are around 25-30 species of Halimeda, this figure varies by the indices used for identification and the species definition used. Most species are exclusively marine, with one single freshwater species (see quote).

Taxonomic tree for HalimedaHalimeda copiosa | Halimeda cryptica | Halimeda cylindracea | Halimeda discoidea | Halimeda gigas | Halimeda goreauii | Halimeda gracilis | Halimeda hummii | Halimeda incrassata | Halimeda incrassata | Halimeda lacrimosa | Halimeda macroloba | Halimeda macrophysa | Halimeda magnidisca | Halimeda melanesica | Halimeda monile | Halimeda opuntia | Halimeda scabra | Halimeda simulans | Halimeda taenicola | Halimeda tuna | Halimeda velasquezii

Members of the Halimeda genus are very distinct. Commonly called the money plant as it is reminiscent of numerous small coins joined end to end. Scientifically the plant is recognisable by the entire plants being comprised of numerous flat segments ranging between 0.5 cm to 5 cm in diameter. The structure is semi-rigid with each segment being calcified in the form of calcium carbonate aragonite crystals and connected together via uncalcified flexible nodes (aka geniculae).

PsammophyteThe holdfast system of Halimeda is psammophytic (see picture below). The root like holdfast can attach in almost any benthic substrate such as mud / loose sand or rock, this gives Halimeda a tremendous advantage over other species of algae and is the main factor behind the colonisation of bare areas by Halimeda species.

The presence of hard calcium carbonate within the tissues make the plant inedible to most herbivores. Although calcification is a useful adaptation it does have problems, for instance if the plant was entirely calcified, wave action and collisions with water borne particles or pelagic organisms could break parts of the plant off with very little energy. Also ocean acidification could pose a threat in the near future. The adaptation of having a partially calcified structure is not specific to Halimeda, other algae such as Corallina offiincialis has also evolved this adaptation.