The Adventures of Torso Boy

OK, so for those who don't know, the PNW Tree Octopus was an internet hoax created in 1998 consisting of a website detailing the animal's life history and conservation efforts. It's completely fake - saying that up front. This animal never existed.

But if you look at this from a speculative biology standpoint? It's genius.

There is one, and only one, thing preventing Octopus from colonizing and being hugely successful in terrestrial environments in the PNW, and that's the fact that no cephalopod has ever been able to overcome the osmotic stress of inhabiting freshwater. We don't know why this is; other mollusks evolved freshwater forms just fine. But if you hand-wave away that one, single limiting factor, the PNW is just primed for a terrestrial octopus invasion.

The Pacific coast of North America is an active tectonic boundary, meaning the coast transitions pretty much immediately into the Cascade and Coastal mountain ranges (contrast with the east coast and its broad Atlantic plain). It's also a lush temperate rainforest, with very high precipitation. This means lots and lots of high-gradient mountain streams with lots of waterfalls and rapids and cold, highly oxygenated water, and not as many large, meandering rivers.

This has important consequences on the freshwater fauna. For one, there are not many freshwater fish in the Pacific Northwest - the rapids and waterfalls are extremely hard to traverse, so many mountain streams are fish-free. There also just isn't much fish diversity in the first place - there's sturgeon in the big rivers, salmonids, a few sculpin and cyprinids and... that's pretty much it. These cold northern rivers are positively impoverished compared to the thriving fish communities of the Mississippi or Rio Grande.

Few fish means few predators, and depending on the size of the first freshwater octopus, salmon and trout just wouldn't be much of a threat. And while these rivers don't have much in the way of fish diversity, there's lots of prey available - crayfish, leeches, mosquito larvae, frogs and tadpoles, water striders, and other aquatic insects, just to name a few. So the first Octopus pioneers to invade the rivers would be entering what essentially amounts to a predator-free environment with lots and lots of food and no competition. Great for colonization.

These ideal conditions get even better once you get up past the rapids and waterfalls, since there's no fish whatsoever in those streams. Octopus, with their sucker-lined arms, are perfectly equipped to navigate fast-moving, rocky-bedded streams and climb up cliffs. They'd also be well able to traverse short stretches of dry ground to access even more isolated pools and ponds. In fact, once Octopus overcome the osmoregulation problem there's nothing at all preventing them from colonizing land in earnest, since the PNW rainforests are so wet; there's no danger of drying out.

Finally there's the question of reproduction. Octopus are famously attentive mothers, because they need to keep the water around their eggs moving and well-oxygenated. In a mountain stream, this wouldn't be an issue, because the cold, turbulent water holds lots and lots of oxygen. Breeding in high mountain streams would be ideal, and the mothers might not even need to attend to their eggs, freeing them up to evolve away from semelparity and allowing them to reproduce more than once in their lives; their populations would thus increase rapidly and dramatically.

I think, if octopus managed to invade freshwater ecosystems in the PNW, it would dramatically change the ecology much like an invasive species. They'd be unstoppable predators of frogs, bugs, slugs, maybe even larger animals like snakes, birds, and small mammals. Nothing would eat them except maybe herons, and things like bears and raccoons would give them a wide berth due to their venom. They would rule that landscape.