Finely grate rind from limes to measure 1 tablespoon; juice limes to measure 1/4 cup.
Place rind and juice in a large bowl. (use a glass or ceramic bowl, aluminum or copper bowl will react with the citrus juice and give it a metallic taste).
Add tomato, avocado, cilantro, salt, pepper, garlic and shrimp. If you want, you can add some chopped onion and fresh jalapenos. Toss well to combine.
Cover and chill for 15 minutes, stirring occasionally.
Spoon about 1/2 cup of shrimp mixture down the center of tortilla; fold and eat.
You can serve it plain and/or eat with crackers or you can use hard tacos shells, tostadas, tortilla chips or whatever you like. My dad eats it with saltine crackers. He adds extra onion and jalapenos, personal preference!