First of all, take a look at the help & options, it's really useful.
Take a pygmy and make him go left/right/left/right over and over at the bottom of the screen. You'll see a shark coming out of the water, that will count as 1. At first, i did a little more than 100 and it didn't unlock. It was the same with more than 200. Then i tryied more than 1000 and it unlocked so i'm guessing that you have to get at least a 1000 combo of shark snaps.
The achievement will unlock when the combo will stop.