The symbols in Great Blue are related to the ocean. So we have a turtle, a shark, some kind of flat fish (hey, we are not marine biologists!), starfish and seahorses (yes... don't count on it). There is also a whale, oyster shell and playing card icon from 10 to A.
Oyster shells are game dispersions that can easily appear anywhere on the shaft. If you happen to encounter three or more, you will activate the Great Blue bonus game. You will automatically start 8 free spins, using a 2x multiplier, so you need to choose two of the five oyster shells to win more free spins and/or multipliers. At best, you can win 33 free spins and/or a multiplier of up to x15.