Is it "normal" (ha!) to find tiny little empty clam shells strewn about on my sandbed? They are mostly about pea-sized, largest one might have been dime-size, all open and empty. I know they can be in your live rock undetected, but who is cracking them open and eating them?

Tank inhabitants:
purple tang
yellow tang
flame hawk
long-nose hawk
coral beauty angel
lawnmower blenny
pair of mandarins
pair of maroon clowns
pair of yellow tail damsels
yellow watchman goby
pistol shrimp
sand sifting star
basket star
serpent star
misc. snails and hermits

I only started noticing them in the past few weeks and I have been seeing them regularly.

-Kristy