Unknown On Live Rock!?!

[peesa]

Hi guys,

I was wondering if someone could help me identify something on my live rock, there is a little cave like area on one part of rock in this is a small 'pistacio shell' like thing which opens and closes throughout the day sort of like a fly eating plant anyone know what this is?

Its very like a pistacio nut shell in apperance when open.

Any comments appreciated

 

[Matthew5664]

sounds like a Bivalve of some sort, without a picture couldn't give any more info

 

[crazyelece]

sounds like a barnacle. I have one in my tank. Everyone I checked with said it wouldn't cause any harm and was a filter feeder so I left it. Do a google search on it and see if it matches.