|Look on and despair. (Image source)|
Why? Well, what it basically comes down to is that I don't worry about beetles or sea slugs taking over the world. For cephalopods I have no such peace of mind.
First of all, we'll never see them coming. Many of them have an incredible capacity to blend in to the environment. Some might call them "the chameleons of the sea," but frankly that's insulting because compared to cephalopods, chameleons are amateurs. Unlike what you might see in cartoons (or faked YouTube videos), chameleons cannot perfectly mimic whatever color or pattern is near them. While some species can indeed change color and while it is useful for camouflage (mainly in the ranges of green and brown), they certainly can't replicate complex patterns and the primary use of the ability is to convey emotional state. (Many of the videos you'll find online involve holding one up to a mirror so that it's fooled into thinking there's a rival in its presence, triggering the response.)
The good (or terrifying) news is that there actually are creatures who almost live up to that fictional prowess of chameleons, and which actually surpass it in some ways. Cephalopods such as cuttlefish and octopuses can indeed change their coloring to mimic their surroundings, and do a decent job of replicating complicated and artificial patterns:
As you can see, they go above-and-beyond chameleons both in the accuracy and subtlety of their colors and in the fact that they can actually change the shape and texture of their skin to blend in more completely. Here's a short clip of an octopus blending in amazingly well before being spooked.
Some go even further, and imitate the shapes and behaviors of other living things:
Octopuses are also quite intelligent, and certainly the smartest of all invertebrates. They're notorious for escaping from aquarium tanks (even undoing latches in some cases) to steal fish from other tanks, for example. They are fantastic escape artists because they combine their smarts with their incredibly manipulative arms and the fact that they can slip through any opening that can fit their beak.
Now you may see the fact that their bodies are so... squishy... and assume that that means they'd be easy to defeat
I love seeing another example of tool use—a behavior we've traditionally reserved for "higher" animals such as ourselves—among invertebrates (I previously talked about another example here). And while I'd hardly call it "sophisticated" as the video's description on YouTube does, it is still an incredible sight. Wait, I was being hyperbolic, wasn't I? WE'RE DOOMED.
Oh, and don't go thinking you're safe just because you're on land...
Hopefully it's now as obvious to you as it is to me that we're just keeping this planet warm for the cephalopods until they decide the time is right to strike. If you ask me, any resistance would be in vain, so I'll simply wish our future octopus overlords luck, and hope that they can do a better job with this world than we have. :P