a big ?

what is this creature that has made his home in my 55 gl saltwater tank?
i just found him a month ago and noone knows what he is. he's been there for a year but i just found him when i moved my rocks to clean my tank. he looks like an accorn shell but opens up and i can see the animal inside. i thought it was just living in this shell but the shell is part of him because the shell itself opens and closes. he attaches firm to the rock when you get near him but is loose when open. he never moves from his rock. I will post pic of him open and closed.

this is my little creature open.

Difficult to see - the pics aren't too clear. Could it be a barnacle?

BTW - your tank looks really lovely

sorry the pic are not clearer it's hard to get a close up with my dig. camera the tank distortes the pic for some reasion what exactly is a barnacle ?

A barnacle is a small salt water crustatean that attaches itself to rocks and other things (even large fish like whales) It comes in several forms, one of which is an acorn barnacle. I've posted a link here that gives some info on them. Hope this helps

great link thank you i learn something new every day.