The population of cephalopods, whose membership includes octopuses, squids, and cuttlefish, has been blowing up since the 1950s. This is mainly due to overfishing, which has thinned out the fish population to such an extent that cephalopods are starting to fill in the gaps. What these sentient plastic bags have over all other aquatic species is sheer survivability. They tend to thrive when other species falter. This adaptability prompted marine biologist Gretta Pecl to refer to them as the "weeds of the sea," as they're nigh impossible to stamp out.
We thought seaweeds were the weeds of the sea but, hey, we're not the marine biologist here.
Take the Humboldt "jumbo" squid, which lives in the Eastern Pacific waters, a body currently plagued by extreme and rapid climate change. This has caused the water's temperature to keep swinging wildly, wreaking havoc on the local marine wildlife. But not ol' jumbo, which not only managed to use the cooled water to slow its maturation and double its lifespan, but also managed to keep growing, reaching 10 times its normal size. It's almost as if squids are being written by hack sci-fi writers trying to find a reason why their movie monster can't be killed.
One of their only known weaknesses are fireballs shot by Italian-American plumbers.