Zooxanthellae are a brownish color unicellular dinoflagellate that lives within Cnidaria Anthozoa, such as corals, to maintain a symbiotic relationship with. They have a spherical shape and typical range around the size of 6 um to 12.5 um
https://www.youtube.com/watch?v=x7cWzYqnnp8
There are many different types of zooxanthellae that vary depending on the type of coral it is living within. Each type has its own genomes as well as different sizes. They tend to vary from the host they are in; however, their genomes typically range from 3,000 to 215,000 Mbps (base pairs).
https://algaeresearchsupply.com/pages/how-do-coral-get-their-zooxanthellae
They contain two flagella; one is known to trail in the water while the other wraps around the cell. These flagella fall off the zooxanthellae when it enters a host.
http://www.aquasymbio.fr/en/symbiodinium-microadriaticum-cassiopea-xamachana-1-0
The zooxanthellae have photosynthetic chloroplast bounded by three membranes to produce nutrient for the host. They also include a nucleus, nuclear membrane, vacuole, lysosomes and thylakoid bodies.
https://www.advancedaquarist.com/2016/2/aafeature
It is interesting to learn about the zooxanthellae, as they are essential to the lives of corals. Without them, it would be impossible to have the wonder various corals that exist today.
Via Giphy
Wow, 3,000 to over 200,000 base pairs, that is quite a range in genetic complexity! Given their relatively minuscule size (only about the diameter of a red blood cell), I would be interested to learn just how many of these microorganisms a single coral is able to host. It’s interesting that the flagella simply fall off after entering the host, rather than adapting in to some sort of anchoring structure. I can only assume that either some other structure forms or the coral provides enough protection that no anchoring structure is needed.