My brain screams
NO! but to be honest, I am not sure anyone has tried it. Going strictly by what I know of the two animals, I would say the lion would find the octo a tasty dinner and is protected by its spines from being an octo snack. Size plays an important roll in the predator/prey game but the poisonous spines of the lion may negate size reversals. One of the primary reasons
REEF is so focused on the spread of lions in the Atlantic is it's varocious appetite and lack of controlling predators. Octopuses, from birth, are pretty much a marine menu item for most predators (humans included) so trying to put one in a predator tank would logically be preposterous. The infamous story about the Seattle aquarium's experiment with an environment that should provide enough space and hiding room to allow small sharks and a GPO to live together has been a
popular video and more or less shows that inhibiting nature's mortal enemies
is not simply a matter of keeping everyone well fed (they have learned to keep and release the Great Whites
after the first one started eating its tank mates).
You will have to make some decisions about what you expect from your aquarium. If you are new to marine tanks, then learn how to keep them traditionally and traditionally (a very short tradition in a very new field), octopuses belong in a species only tank. Once you have some experience with both octopuses and marine environments, you may wish to experiment but until you understand how to add, subtract, multiply and divide inventing a new math concept is out of the question.
We recommend staying away from anything that packs a strong sting or can damage the octopuses skin. In the wild, octopuses will learn to avoid these but in a small environment, the octo has navagational issues and the stinging cells stay in the water column much longer. Any skin damage is an infection concern. I don't know of any in situ studies regarding infections and healing (perhaps the animals simply become food) but topical treatments are almost impossible to administer, water antibiotics will destory your natural filtration and QT tanks for an octopus are difficult to create. We have seen limited success with oral antibiotics but often an animal with an infection will stop eating before you can administer a treatment in its food (which will still put some of the antibiotic into the environment).
Sooo, what CAN you safely put in a tank with an octopus? Most sofies (leather corals, kenya, some of the low stinging polyps) won't hurt the animal but need to be well fastened to the substrate (live rock is a substrate) and hardy enough to be "walked on". Brittle stars (greens being a suggested exception because of their aggression once they reach a about 10" in diameter), serpent stars, common starfish (there are concerns with the bahama star because of its style of eating), often peppermint shrimp will avoid capture but are an expensive meal if caught, snails (also sometimes eaten but harmless to the octo), hermit crabs (same as snails), sponges (hard to keep, avoid the red ball sponge), gorgonians (again care with placement is recommended) and pencil urchins (possibly pin cushion urchins. I have not kept the pin cusion urchins with mine but others seem to have success). Brittles and serpents are particularly enjoyable as they can often be hand fed. My basic "go by" is, "if it swims, avoid it unless you expect it to be food".
Be sure to wear gloves while cleaning the tank. I have had the unfortunate experience of being aggressively attacked by an animal I raised from very young. The pain is worse than what you may have already heard and lasts a very long time. I happened to be stung in a finger joint and it was a full year before I had normal use of the finger. I was less than a minute away from the only antidote, scalding hot water.
