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.