Well, the mention of mermaids eating crabs reminded me of something... I made a collage with a crab once.
Collage/Found Objects, 14"x 13"
That, my friends, is a real crab shell that I had to sanitize the crap out of for a couple of days so that I could work it into this piece. After a mermaid probably ate it. It seems to have held up pretty well, all things considered... and it now lives with my sister in Boston.