You can substitute any kind of protein you'd like, actually! In Thailand we make this dish only with crab and shrimp, but you could certainly use tofu, fish, or even chicken. Pre-cook your protein of choice first though, and then follow the recipe as is without any modifications. Keep in mind however that if your protein is bland, like chicken or tofu, you might want to marinate it a bit with some soy sauce or fish sauce before cooking.