clam moving?

[kau_cinta_ku]

why is it that everytime i put my clam (see in the pic) back away from the glass about where the idaho grape cap is, a week later it will have moved up next to the glass and turn sideways?

 

[LTCrunch]

they will use there "foot" to move themselves and turn themselves in a place where light and flow are optimal. they do it very slowly.

 

[orcafood]

My Crocea Clam did the exact same thing. For around two months it constantly moved toward the front glass.

I fixed the problem by burying a rock in the sand that was slightly exposed, placing the clam on the rock and building a small wall of rock between the clam and the glass (against the clam). The clam attached to the rock in a week, and I removed the barricade.