If this picture says anything it says the Flabellina Nudibranch are out in force, getting jiggy on the reef. If you dive on the East side of a dive site here called Koh Doc Mai you will be hard pressed not to run into a few of these beautiful little purple monsters hanging onto Hydroids that grow there. Currently though they are out in force, I counted maybe 30 of them in the first 20 minutes of my last dive yesterday, must be mating season!
They definitely have a strong tenancy to appear on the Greyish/Green Hydroid’s that you will see all over the wall and reef floor on Doc Mai. Hydroid’s look like little ferns, just the aquatic mini versions, the Flabellina feeds on the Hydroid, making it a nice safe place to hang out in the daytime thanks to the Hydroid’s Stinging nature, yep they hurt if you brush up against one.
Seeing the Nudi’s mating on the Hydroid, like in the picture above isn’t uncommon either, sometimes you will get lucky, pardon the pun, and possibly see three or more on a single Hydroid, they are the more liberal of Nudibranch. Flabellina are relatively easy subjects to photograph as the don’t move around much, but the Hydroid won’t help you any, it will surely move around if there is any current or water movement, so you do have to time your shots correctly in most cases.
The tentacle like appendages that come off the Flabellina are named Cerata, they serve a dual purpose for this type of Nudibranch, one purpose is that they will store some of the stinging cells from the Hydroid in the tips of their cerata, making them a fairly unpalatable meal for fish and other predators. It’s also theorized that the Nudi can actually store other cells from the Hydroid also, like the ones that produce energy when hit by sunlight, making these types of Nudibranch basically solar powered. Enjoy!