They are definately some type of whelk. They may appear to be directly harmless, but most whelks are carnivorous. Many feed on microphauna. Keep a close eye on them. They may be harmless in small numbers, but may be capable of doing some damage in the masses.
They probably aren't much to worry about, but I would try to get an ID from Dr. Ron anyways..
I recently discovered some similar whelk like creatures, but I was unable to ID them. They looked like nausarius snails a wee bit. here's a pic:
