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.

Turtle Grass

Common Name: Turtle Grass
Scientific Name: Thalassia testudinum

Family: Hydrocharitaceae - aquatic plants
Class: Angiospermae - flowering plants

Conservation status: Least concern, population stable
Sighting: Very frequent, all over Biscayne Key

Turtle grass is the first thing you'll find when you snorkel in Key Biscayne. It is specially abundant in Bill Baggs Cape Park, starting a few feet from the shore. Also in Crandon park, as soon as the sand starts to get covered by green, that is mainly Turtle Grass.
After a strong storm, you will also find large amounts of turtle grass leaves in the seashore, creating a brown dam, in same cases up to a couple feet high.  

As you can read in the links, turtle grass is actually a flowering plan, growing from a rhizome. The name comes form the Green turtle (Chelonia mydas), that feeds on the grass, although is a natural habitat for many animals, and feeds multiple herbivorous fishes of the florida waters, some of which eat the algae that grows in their leaves, rather than the seaweed itself. And of course, manatees graze turtle grass!

Although it looks uneventful, almost boring, if you look carefully, you can discover a variety of creatures hiding in the beds of turtle grass. Starting with other seaweeds and algae, sponges, crabs, conchs, and many different solitary swimming fishes, like pufferfishes. 

In the Crandon park area, turtle grass grows large, specially in the shallow waters on the left of the main beach, by the natural park protected areas.

The interface between the higher beds of turtle grass and the clear sand , like the one in the picture at the right, are also great areas to see all sorts of fish, hiding in the dark shadows created by the turtle grass. If you are patient and quiet, once your the shock for your presence is over, small and larger fishes will start swimming out of the shadows, making for a great beginner snorkeling experience.