Ricordea. I am not sure if this qualifies 100% as a hitchhiker, but I didn't realize (nor did the LFS) it was on the rock until two days after I purchased it. I was so stoked on life when I realized what it was!
coolest? i have total respect for the worm that killed my 2 clams and countless snails-
Oenone fulgida
here's a pic of one in my refugium....can you see him? look for the one with eyes....
I got a 1 1/2 - 2 lb piece of live rock and it had a Yellow Spiny Oyster on it! I saw it in the rock tank and picked it just for the Oyster. When the LFS guy saw it was alive, he hated to sell it to me for the $5-$7 it cost!
Got 2 red hermit crabs, multiple feather dusters, and a peppermint shrimp. I have been hoping for some of those tiny starfish. They are so cute.
Also got some small sponges. One had a type of mushroom or leather on it, it was in bad shape. Hope it comes back.
This doesn't count because it wasn't my tank... On another site there was someone who had a hitchhiker octopus.
My coolest hitchhiker? 25 feather dusters I guess.
My coolest is a blue hard shell coco worm. It survived my cycle and all the issues I've had learning salt water. I've also had some limpets, turkey clam, barnicals and some kind of white thick legged starfish.
Meadbhb