Nudibranchs eat using a radula, which is “kind of like a sharp, hard tongue,” says Goodheart. Watching a nudibranch eat isn’t, at first glance, all that different from watching a slug on a leaf.

You’ve probably walked right by these nudibranchs without realizing it. They blend in almost perfectly with the seaweed, appearing as tiny, yellowish blobs on the sand. But look closer ...

It seemed like a lot of effort for a tiny reward. Then I encountered my first nudibranch, a type of vibrant sea slug. I was ...