I recently read a article about the primative but cool self defense that a Hagfish uses, slime!  Hagfish are marine craniates of the class Agnatha, also know as Hyperotreti.  Hagfish are known for producing copious amounts of slime and are sometimes refered to as snot-eel or slime-eel.  According to this article scientists from Massey University and Te Papa video taped the hagfish using this self-defense in New Zealand's deepest waters.  In the video you can see several sharks and other fish trying to eat a hagfish that was attracted to a bait bag.  Each time the shark or fish makes contact they immediatly let go of the hagfish or spit it out.  Check out the video:

Also, for fun and as a little example at how bad these fish slime chech out this video: