Does canned tuna contain preservatives?
The preserving effect of canning is based on 


removing all bacteria and fungi (normally present at least to some extent even in perfectly safe food) by a combination of heat and pressure over a certain time
preventing new bacteria or fungi from reaching the food by sealing the containers
avoiding oxidation by sealing the cans
sometimes supporting this by ingredients that are very inhospital for bacteria and fungi like acids (pickles, anyone?) or sugar (less available water). 


For your tuna, this means the heat/pressure of the canning process is sufficient to prevent spoilage for a long time.  

Special preservatives are typically not necessary. Checking the label should confirm this. If one brand should have choosen to add them nonetheless, you can pick another manufacturer.