Of all of the creatures in the water, some of the most beautiful, and mesmerizing, are nudibranchs. Say "nude a branks". They are mollusks that lose their shell when they mature. They are sometimes incorrectly called sea slugs. They are mostly found on the ocean floor, but are found all over the world, in the shallows, in the depths, in warm water, and in cold.
Without shells to protect them nudibranchs may use their colors to blend in and match the surrounding corals, or to serve as a warning for potential predators. Some are toxic, but not all are. Their name means "naked gill".
Nudibranchs are carnivores, mostly consuming sponges (which are living animals), however some will eat other nudibranchs.
|"Glossodoris atromarginata" by Chika Watanabe from Los Altos, USA - Flickr. Licensed under CC BY 2.0 via Wikimedia Commons -|
|Spanish Shawl nudibranch via Wikimedia Commons|
|Ceratosoma tenue from Lembeh Straits, Indonesia (and shrimp) via Wikimedia Commons|
There are roughly 2,500 known species of nudibranchs, and probably hundreds more undiscovered ones.
Nudibranchs are typically slow moving creatures that just travel along the bottom, but when they move through the water they move with a delicate ribbon like motion that is beautiful to watch.