The most colorful gastropods are the nudibranchs, also called sea slugs because they have no shell. This lack of a shell led to the order’s name, Nudibranchia—a name that means “naked lungs”—because without the shell, their gills rise up off their back in the shape of a heavily branched bush.
Without the benefit of a defensive shell nudibranch species have to employ other means of defense. Some manufacture compounds that are toxic to predators. Others taste foul, causing would-be predators to spit them out.
Still others have the amazing ability to create armor by feedin on sea anemones and its kin, the arms of which are covered in stinging cells called nematocysts. These nudibranchs eat the flesh of the anemone, but through a process as yet poorly understood, rather than the nematocysts being digested, they somehow migrate out into the skin of the gastropod where they are kept as a stinging defense against predators.
Nudibranchs are the only known animals with this ability to move whole cells that are ingested through tissue and into the skin layer. As with many other poisonous and dangerous animals, their color is thought to be a warning sign to animals that might want to eat them.
There are several species in northern Iceland, but they have been poorly researched, so exactly how many species there are and the distribution of each are not known.