Halimeda

Common Name: Unknown - Cactus Alga? Coin Green?
Scientific Name: Genus Halimeda - Halimeda discoidea?

Family: Halimedaceae - Halimeda is the only genus
Class: Bryopsidophyceae - macroscopic green alga

Conservation status: Not yet assessed
Sighting: Ocasional

Identifying this species is very difficult for the amateur, as seems common with alga and seaweed. We think this beautiful alga is part of the genus Halimeda, but we can not narrow down to which species it really belongs. Lacking a good alga identification site, we have found pictures of Halimeda discoidea looking very similar, but then, some other H. discoidea pictures looked completely different, showing branches or segments of discs, which do not seem apparent in our pictures. It may well be some of the pictures are misidentified themselves, making the whole search even more challenging. 

In any case, Halimeda, described as a "green macroalgae, with a body (thallus) composed of calcified green segments", and the pictures of some of its species (it has more than 40 species listed, we didn't found examples of all), seem close enough to give us confidence we have the right genus.

Of the various common names we have found for species of this genus, green coin seem to be the most appropriate to this particular specimen. At least that is how we would describe it, they are the size of a US dollar quarter, but thicker, with a characteristic bright green rim. They do look like some Crassulacean plants (cactus without spines, if you want), therefore the "cactus alga" name, which also seem suited to this particular example.

FInally, in this -poor- video you can take a look how the colonies are interspersed within the turtle grass. You can also see a starfish featured in Cushion Starfish post, and the location of the area near Bill Baggs Cape lighthouse where you can find this species.

Look for an update on this page with better video, and hopefully full identification of the species. We will try to carefully analyze the stem structure, to see if it matches the descriptions.