Family: Tonnidae. Around 20 species exist worldwide. These shells are thin and light, but strong. The snail is actually larger than the shell it lives in. Tun shells are deep tropical water dwelling creatures that feed mainly on fish, crustaceans and sea urchins. Because their free-swimming larval life is quite lengthy they have been able to spread out over great distances. Interesting Facts: The name tun shell means cask shell or wine jar.