A type of sea slug, with exposed gills, hence the name nudi (naked) branch (gill). There are many thousands of species, ranging from a few mm to half a metre long, most of them are very brightly coloured. Look them up they are quite stunning, typically much prettier than their terrestrial cousins!
They are amazing. Many are poisonous because they eat poisonous creatures, like Man O War. That's why you see such amazing colours (it's a warning). However some nudibranches and other animals (like baby frogfish) mimic them for protection. If you want to go down a rabbit hole, google nudibranches!
Kept trying crawdad, crawfish, crayfish, etc. for lobster before finally getting it. Then I got to the shrimp and thought, "Ah, there's the crawdad." Then I didn't see any claws, and it dawned on me these were sea creatures, not things I find in my Missouri ditches.
Also, Gastropod should be accepted for the Conch. Since a conch isn't an animal but rather the shell left behind...