Halimeda

The genus Halimeda J.V. Lamouroux belongs to the order Bryopsidales under the family Halimedaceae. It has five monophyletic sections—Halimeda J.V. Lamouroux, Micronesicae Hillis-Col, Opuntia J. Agardh ex De Toni, Pseudo-opuntia J. Agardh ex De Toni, and Rhipsalis J. Agardh ex De Toni—which were based on the differences in the fusions of medullary siphons. There are 71 species and 67 infraspecific names listed on Algaebase as of 2015. == Morphology ==

The thalli of Halimeda is distinctly segmented and calcified. Calcium carbonate is deposited as aragonite and calcification begins as early as 36 hours. Their segments are composed of 60–80% aragonite and are separated by nodes which are non-calcified. Halimeda has three types of holdfasts which serve as attachment points to the substrate. The "sprawler" type has a few loose filaments growing at the ends or in between the segments (Fig. 1). In the "rock-grower" type, the matted holdfast is composed of branched filaments which secure the thallus to a rock surface. The last type is the "sand-grower", where the filaments hold on to fine sand particles, forming a root-like structure. Halimeda is coenocytic and siphonous, meaning its cells are not divided by cross walls, and is instead a continuous filament of cells. This differentiates the genus from Acetabularia, which is another genus of green seaweed that is calcified. == Distribution ==

Halimeda is highly abundant in the tropics including the Thai-Malay Peninsula and the Florida Keys. Some species (e.g. H. copiosa, discoidea, gracilis, opuntia, simulans, and tuna) have a global distribution. Halimeda tuna is found solely in the Mediterranean. == Ecology ==

Species of Halimeda with sand-grower type holdfasts grow on sandy or muddy substrates, and are thus common in lagoons and backreefs. Some species such as H. opuntia have been found to produce up to 54.37 g CaCO3 m−1 yr−1. The genus also contributes to reef building, as it is large producer of carbon sediments on reefs, generating a wide range of sediment sizes from coarse particles to silt and clay. Although it was largely assumed that its abundance on reefs is due to it being unpalatable to herbivores, more recent studies have found that Halimeda is in fact subject to grazing by some herbivores such as Scarus rivulatus, Hipposcarus longiceps, and Chlorurus microrhinos. Hard coral cover can actually play a key role in maintaining Halimeda biomass on reefs, as one study found that thalli growing under the canopy of Acropora colonies were larger than those in open areas exposed to herbivory. == Life history ==

Halimeda reproduces both sexually and asexually. Sexual reproduction is rarely observed because it is completed in 36 hours. Some species of Halimeda have been found to reproduce synchronously in mass spawning events similar to that of corals, albeit occurring over several months, with small portions of the population spawning each day. Therefore it is likely that the life span of the genus is limited to a few months to a year. == Chemical composition ==

The genus' photosynthetic pigments are those typical of class Chlorophyta (chlorophyll a and b) and also include siphonoxanthin and siphonein. == Exploitation and cultivation ==

Currently, Halimeda does not appear to be cultivated for aquaculture purposes. == Utilization ==

Methanol and dimethyformamide extracts of Halimeda opuntia have been observed to have antibacterial properties against some species of microorganisms, including Saccharomyces cerevisiae, Escherichia coli, Bacillus subtilis, and most significantly, Staphylococcus aureus. Halimeda opuntia ethanol extract exhibited activity against hepatitis C virus (HCV) due to polymerase inhibitory (HCV-796) binding sites based on molecular docking simulation. Methanolic extracts of Halimeda macroloba have recently been found to exhibit cytotoxicity towards MCF-7 and HT-29 cells, which are derived from human breast cancer cell lines and colon cancer lines, respectively. These results therefore suggest the genus' potential for cultivation as a food source. Halimeda tuna appears to be used as fodder in the Philippines. ==Species==

'' at depth in shallow cave • H. bikinensis • H. borneensis • H. cereidesmis • H. copiosa • H. cryptica • H. cuneata • H. cylindracea • H. discoidea • H. distorta • H. favulosa • H. fragilis • H. gigas • H. goreauii • H. gracilis • H. heteromorpha • H. howensis • H. hummii • H. incrassata • H. kanaloana • H. lacrimosa • H. lacunalis • H. macroloba • H. macrophysa • H. magnidisca • H. melanesica • H. micronesica • H. minima • H. monile • H. opuntia • H. pumila • H. pygmaea • H. renschii • H. scabra • H. simulans • H. stuposa • H. taenicola • H. tuna • H. velasquezii ==References==