Well, yeah, it’s legit. You’ll get no argument from me that it’s not a word in English. I still think it sounds dumb.
As bibliophage’s article indicates, the form is still based on an error. In my book, using pseudo-Latin makes one come across as pseudo-learned. It’s an attempt to appear learned to the ignorant, which instead ends up making one appear ignorant to the learned. 
Since a well-established regular English plural exists, the preference should be to use octopuses. (That’s a rule I personally follow: to use the English plural whenever one is available.)
Related question: copper is present in seawater in very low concentrations-whereas iron is much more common. Would a copper-based blood animal have a hard time making enough blood in a freshwater environment
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Yes, Legit due to usage. Mind you, usage is a “legit” way of determining what is legit, but “Octopi” is bad on several levels. It’s not the proper Latin or Greek plural nor does it follow the general rule of English plurals.
Bit by bit we’re getting rid of the irregular plurals, and that’s a Good Thing. Finally we can say “mouses” and “gooses”. I hope we get rid of them all, but I know “man/men” will remain long past my lifetime.
The irregular plurals, as well as all the rest of that crap slow down reading for kids and learning ESL. They are bad and we need to get rid of them. We got rid of brethren in most uses, I hope we drop “fungi” soon, etc.
I give full credit for that to Dr Seuss. " And then he did the same thing to the other Whos’ houses, leaving crumbs much too small for the other Whos’ mouses.".
Wait, “mouses” is okay now? I’ve only used it in that sense for the plural of computer “mouse,” but that war seems to have been lost with “mice” taking over as the plural. But now “mouses” is okay is all usages? I’m surprised, and I can’t find any source that indicates this is normal usage. In my experience, it’s not. Everybody seems to still say “mice.” I don’t mind moving towards “mouses,” but that doesn’t reflect to me the actual usage of anyone I know.
The plural of the device is “mouses” and thus “mouses” for the rodent is gaining acceptance.
There’s also “flat-foots”(as in a number of police) and “low-lifes” not “Low-lives”.
This is interesting, as I’ve seen it going the other way. The plural, as I remember, for the computer mouse had been “mouses” (well, there was always some discussion about this) – and I’ve long supported that plural – but almost everyone I know nowadays says “mice.” So I’ve felt like I’m losing that battle. Computer “mouses” are becoming “mice,” not the other way around. It’s interesting that you see otherwise, as I have literally never heard anyone refer to multiple instances of the creature as “mouses.” In my experience, the irregular plural of the creature is being applied to the computer device. In fact, I can’t think of the last time I’ve heard anyone refer to “mouses” in the computer sense. I remember it being fairly common in the 90s, though.
And “Maple Leafs” like the team from Toronto. Those make sense to me. And “still lifes,” too, as the phrase has become it’s own syntactical unit and subject to regular pluralization rules.
Okay someone has to … don’t forget all the Proudfeet.
Must be pretty slow, as I’ve also never seen it.
That’s a somewhat different issue, since that involves a compound being treated as a single word and receiving a standard plural.
Back to the OP (remember that?), there are fresh-water forms of a number of normally salt-water organisms in Lake Nicaragua that are found no where else on Earth. The reason is due to the unusual history of that lake. Originally it was just an arm of the sea, but it was gradually enclosed by lava flows and eventually turned to fresh water. But it was such a gradual change that many species were able to evolve fast enough to adapt. I thought octopodiums[sup]1[/sup] were among these, but apparently I was mistaken.
[sup]1[/sup] If y’all can’t make up your minds about the correct plural, I’ll just make up my own.
An’ the Octopodiums 'll git you
Ef you
Don’t
Watch
Out!”
They’ll get rid of the incoming asian carp. Then when there’s too many octopuses we can just import some sperm whales to eat them.
Do freshwater clams use copper based blood?
Well, octopuses and squid have very different lifestyles than snails and clams.
Clams just sit there, happy as a, well, you know. Snails move, but only at the pace of a, well, you know. They’re both happy to eat whatever filterable food particles drift by, or little patches of algae they come across. They’re also both relatively small and don’t need a whole lot of food.
In contrast, squid are (generally) high-speed predators, who cover big distances looking for fish or other prey. Octopuses are on-the-bottom predators, roaming over big patches of seafloor looking for prey ('m not sure but I think they generally prefer rocky areas).
So this means that first, there are a lot more population-sized freshwater habitats for snails and clams than for cephalopods – I’m not sure how many freshwater lakes could even sustain a squid population. And second, there are even fewer transition paths. There are plenty of brackish water spots where a whole population of clams could happily live, gradually adapting freshwater mutations and slowly expanding into fresher water and eventually splitting off subpopulations in the fresher water that can continue the process – nearly any estuary or shoreline is going to have some spots. But I’m not sure there are really any areas of brackish water suitable for a strong-swimming predator.
IANAO (I am not an octopodiumologist), but cephalopods seem to have enjoyed significant success and diversity through many periods of earth’s history. I find it hard to believe that lifestyle issues alone would be enough to keep this these guys from eventually invading rivers and lakes if everyone else could do it.
BTW, did anyone see the TV special “Aliens of the Deep”. It reports that octopodiumuses off Italy seem to be developing “culture”. These are normally self-raised creatures (mothers die after eggs hatch, fathers probably after mating, I think), but juveniles seem to bebuddying up with wiser elders in these waters.
They are purported to be intelligent in a way that other cold blooded beings are not, and that is very cool evidence of it.
Sorry, the title is “Aliens of the Deep Sea”
Directed by Jérôme Julienne & John Jackson, and produced by Idéacom International and MC4.