Now this is my kind of artwork: A little bit creepy, not at all "usual" and what a venue to showcase them at.
Great shots and what a lovely day to get out there and capture these images.
Your information about sea urchins is definitely the truth. I was a dive instructor for a few years and it seemed like almost every trip an inexperienced diver ended up with those spines in their leg. Very painful and there really isn't much that you can do about it either. We used to roll a battery on them and crush the spines so that your body could consume them faster.... it still takes many days! Pretty agonizing.