help me identify PLEASE

[Polscot]

hi, i bought a rock covered in shrooms from a fishstore about 3 months back, (tank is 4m old), well.....i just came back from vacation and the rock isnt a rock at all.....it has a mouth and a hole, im thinking some kind of clam, but i dont know how it has survived 3 months, i only have PC lighting, and i dont dose anything cept calcium......i thought clams needed phyto nd stuff

thanx for your time