Nudibranchs, or sea slugs, are descended from animals with protective shells like those of modern snails. Nudibranchs have lost that shell, leaving them potentially vulnerable: squishy morsels in an ocean full of hungry things. But nudibranchs have some tricks to avoid becoming someone else’s meal: they use their own food to protect themselves.
One trick is to steal the defenses of your prey. Many nudibranchs eat stinging animals like hydroids and anemones. These animals use specialized stinging cells to catch their own prey and to defend themselves.
When the nudibranch eats the stinging animal, its digestive system separates out the stinging cells without triggering them, then stores them in tentacles on its back (called cerata). Anything that attacks the nudibranch then gets stung by those stolen stinging cells.
Other nudibranchs eat toxic sponges and keep the toxins in their body, rendering themselves toxic. Their bright colors mean, “Don’t bother trying to eat me, I taste terrible.”
But not all nudibranchs are so well-armed. Some simply rely on camouflage: if no one can see you, it will be harder for them to eat you. Tritonia wellsi feeds on gorgonians, a type of soft coral. Each gorgonian is a colony of individuals, with a hard, flexible skeleton inhabited by many polyps, which look like tiny anemones. The body of Tritonia wellsi also looks like tiny anemones: the nudibranch seems to be imitating the appearance of the gorgonian in its body shape, with imitation-polyps covering its back.
Just one problem: the color doesn’t match. This nudibranch’s favorite gorgonian comes in red and yellow, but the nudibranch isn’t either of those colors. These guys aren’t well understood, so I don’t have an answer to that puzzle.
Still, even if they are the wrong color, you have to admit that the faux polyps look pretty good.
Doridella steinbergi eats bryozoans. Like gorgonians, bryozoans aren’t toxic and don’t sting, so there are no defenses to steal. Doridella steinbergi has patterns on its back that mimic the grid-like appearance of the bryozoan colony, concealing it as it feeds.
And it has even gotten the colors right.