adaptations for land-based octopuses

Question

This is a follow up question to: How realistic is the Swampus?

What adaptations would current octopus have to evolve, to become land based, meaning that they spend at least 1/3 of their time on land?

How long would the development of such features take, and can you come up with a scenario, that makes land based cephalopods plausible?

Helpful traits I came up with:

1. leathery skin, so they don't dry out in the sun
2. ability to breath air, similar to lung fish?
3. the redevelopment of their internal shell to support land based movement
4. development of a joint based internal shell in their arms, to support movement
5. improved camouflage abilities to avoid faster predators

Answer 1

A method of controling the osmolarity of their bodies

One of the big challenges in moving on land is dealing with osmotic balance (how salty the internal environment of the cells in the body is). Most organisms have internal cell contents that are quite salty, but maintaining a consistent salinity level is necessary for survival and proper metabolism.

A lot of terrestrial organisms are thought to have moved into fresh or brackish water before moving onto land, and even if you do go straight from marine waters to land if you want to go inland you have to deal with every reliable source of water potentially being too dangerous to drink. Saltwater animals retain freshwater and excrete high amounts of salts. Freshwater animals absorb salt through their gills and excrete a lot of water. It is possible to do both, but you have to be able to regulate your internal salinity and it is hard (as evidenced by the rarity of salt-and-freshwater tolerant fish rather than just salt or just freshwater).

Most cephalopod species are osmoconformers, and are stenohaline, which means they mostly keep their body salinity at the same level as the surrounding saltwater and really don't like it when the salinity changes. Kind of like sharks to some degree. A giant squid or a Humboldt squid could get large enough to simply ignore salinity changes for short periods of time due to osmological inertia, but large cephalopods generally don't live close to estuaries. A Pacific giant octopus might be able to handle it, and there are apparently reports of this happening, but given that is self-reported on the internet I would take it with a grain of salt.

The first step to putting a cephalopod on land would be to give them a system of osmoregulation they can use to tolerate the more osmologically varied environements of tide pools, estuaries, and rivers. This isn't impossible probably, fishes have done it and at least two other groups of mollusks have colonized freshwatr (clams and gastropods), and notably gastropods then went on to colonize land (though I'm not sure how well they osmoregulate given the whole salt thing). Notably, there are no freshwater cephalopods and as far as we know there never have been in the fossil record, though a couple of species tolerate brackish water.