The first sea slug I saw at the ocean's edge was an opalescent nudibranch, Hermissenda crassicornis. I have since spotted many nudibranch species, but the opalescent is still my favorite. This is one gorgeous slug. And not only does this dazzling mollusk spread color through the tidepools, it also adds intrigue.
Lacking the shell of their snail relatives, sea slugs have developed clever ways to defend themselves. When the opalescent nudibranch consumes poisonous prey like hydroids, it steals their stinging cells packed with venomous darts. The stinging cells migrate to the colorful fringe (cerata) on the back of the slug. The cerata glows bright orange to warn potential predators of the pilfered poison.
This story is one of the best I've learned during my journey through the tidelands because it highlights why the sea is so compelling: The human imagination could not conjure a creature more gorgeous and more bizarre than a nudibranch.