When you think of ocean animals that totally own their competition, great white sharks and killer whales probably come to mind first. But the real badasses of the underwater world aren't always so obvious. Whatever you do, don't underestimate these 10 tough aquatic animals.
1. Blue Dragon Sea Slug
At only 1.2 inches in length, this formidable sea slug can take down a Portuguese man o' war -- as in the Portuguese man o' war that sends sensible swimmers racing for dry land. The blue dragon intentionally ingests the venomous stingers of its prey and stores them within its own tissues to use against future predators.
2. Peacock Mantis Shrimp
This stomatopod has both beauty and brawn. Its club-like appendages can strike prey with the force of a .22-caliber bullet -- so fast that the surrounding water boils. Who you callin' a shrimp?
3. Vampire Squid
When threatened, the vampire squid expels a cloud of bioluminescent mucus that glows for up to 10 minutes, allowing it to escape to safer waters. Few creatures can make such a fabulous exit.
Octopuses have plenty to brag about. They're 90 percent muscle, can fit through an opening the size of a quarter and can change both the color and texture of their skin at will. Did we mention limb regeneration?
5. Immortal Jellyfish
The immortal jellyfish can reverse its own aging to become a polyp, Benjamin Button-style, all over again. But while it may elude death, it can't outrun predators or diseases forever.
6. Sea Cucumber
Don't let its dull disposition fool you: The sea cucumber is far from boring. When in danger, this echinoderm violently expels its toxic internal organs out its anus to distract would-be predators. Who's hungry?
7. Greenland Shark
The Greenland shark's cells contain a natural antifreeze -- an ammonia-like compound called trimethylamine oxide -- that allows it to survive temperatures as low as 28 degrees Fahrenheit.
8. Longhorn Cowfish
This fish may seem cute and cuddly, but it can release a toxin when stressed. Plus, its spikes and poisonous flesh make it a dangerous snack.
9. Bobbit Worm
This photo needs a banana for scale -- the bobbit worm can grow up to 10 feet long. It sits on the seafloor and waits patiently for prey, which it then chomps down on with enough speed and strength to split a fish in two.
10. Japanese Spider Crab
Experts say this guy -- whose leg span is a startling 12 feet from claw to claw -- is relatively harmless. But let's be real: If that giant forest spider in “Harry Potter" had an aquatic cousin, this would be it.