Common whelk or channeled whelk?

[Maswired]

I recently discovered a whelk in my daughters saltwater tank while I was transferring her sandbed to another tank. It appears to be either a common whelk or maybe a channeled whelk. Is there a way to tell these two species apart? The tank I moved the sandbed to was empty except for its original sand bed and live rock. No other inhabitants. The whelk can live there in isolation. I figure dropping in a cube of mysis twice a week will keep the cycle and the whelk alive. My main curiosity is how to tell the whelks apart.