I just learned what these are today! I was in awe of their beautiful color and when I saw the picture of the one above, I thought for sure the internet was fucking with me. But according to Wikipedia this is a real creature. So, what is it? It is a nudibranch, from the Latin nudus, naked, and the Greek brankhia for gills. They evolved from a hard-shelled mollusk that shed it’s shell after the initial larval stage.
Nudibranchs are often called ‘sea slugs’ but they are actually from taxonomic groups (nerd alert).
I have never seen one of these in the wild, although it is stated you can find them around the world, I will definitely be on the lookout the next time I’m in the Keys.
Nudibranch body shapes and colors vary greatly and unlike other types of gastropods they are bilaterally symmetrical externally and internally. These little guys can grow anywhere from teeny tiny (.79in) to a probably totally amazing at 2 feet in length. They are hermaphrodites, like other awesome shelled and sluggish creatures. And they are also carnivores, and some are even cannibals. In order to defend themselves from predators, nudibranchs have evolved different tactics like camouflage, bright contrasting colorations that warn off predators that they are distasteful or poisonous, or they eat other various poisonous creatures like man-o-wars, and store the toxins in their body, thus making them repellent to their predators, cool huh?
It turns out nudibranch are not recommended for personal aquariums as they are highly specialized predators and require very specific feeding rituals. Which is a bummer, because IF I had an aquarium, I would totally want a bunch of these rad dudes hanging out.