Nudibranch (Indonesia) - Also sometimes called "sea slugs," nudibranchs - whose name means (roughly) "naked lung" - have external gills that are generally located at the back (in this case, the feathery cluster on the left). They come in a wide array of colors, but are often brightly colored as a warning to would-be predators that they are poisonous to eat (similar to the poison frogs of the rainforest).
Nudibranch (Indonesia)
Colorful nudibranch (Indonesia). This nudibranch actually eats stinging/poisonous corals, and rather than digesting them, is able to transfer the stinging cells into the tips of its "spines" so that anything that tries to eat/nibble on it gets a sharp dose of stingers!
