The longest sword The swordtail can be identified by its special caudal fin. The lower part of the caudal fin resembles a sword. Only the male fish has a caudal fin, and this is how to tell the difference between male and female green swordtails. There are many different kinds of swordtails. Some have a very short 'sword', while with others, the 'sword' is just as long as the body. Mates with the sword Males with long 'swords' use them when it is time to mate. The males surround the females either by quickly attacking or pulling backwards with their bodies bent.