The thresher shark (Alopiidae) is most notable because of its long upper caudal (tail) fin which may account for half the shark's total length. The thresher shark eats small fish, and will sometimes tap the water with its tail to herd schools of fish into tight spots, making them easier to eat. They can also use their tails to stun their prey with a good smack. Thresher sharks are usually 10-15 feet long, and can grow up to 20 feet long -but remember, about half of that length is tail fin