The famous giant clam, the world’s largest species of mollusk, can be found up and down the Great Barrier Reef. Like their smaller cousins, giant clams are composed of two hard shells housing the animal's soft muscle tissue inside. Unlike the smaller variety, however, giant clams can grow up to four feet long and weigh more than 500 pounds. The hefty creatures also live to be more than 100 years old. While giant clams are rumored to trap unsuspecting divers in their quickly closing shells, there have never been any substantiated reports of human deaths from these animals.