Most likely will not keep for long in your tank. The waters off of California are pretty cold, compared to the same lattitude on the East Coast. The waters there are what, in the 50s and 60s? Not at all tropical!
I work in a grocery store and we sell manila and cherrystone clams. Customers with tanks come in all the time to get them for their fish, but report that the clams only last about a week or so, but long enough for eels and such to eat live food.
Additionally, you have to be careful about where you collect - lots of California coasts are protected wildlife sanctuaries, and you don't want to pay hefty fines. You're better off buying one at your LFS or getting one online. When the urchin you collect dies, it could end up costing you more than the price of a tropical one.
