Why are there no freshwater cephalopods?

by Matthew Cobb

Not in a lake near you: a blue ringed octopus, courtesy of http://www.ucmp.berkeley.edu

Last year we discussed one of the questions I routinely ask my students – why are there no insects in the sea? Today’s poser is a question a student, Xaali, recently asked me: Why are there no freshwater cephalopods?

Now I don’t know much about molluscs (arthropods are more my bag), but the main reasons might be:

• Salinity (but other molluscs seem to cope quite well, and what about low salinity areas in estuaries? Is it all to do with the cephalopod ‘kidney’?)

• Oxygen levels (but is there really a difference between freshwater and marine O2 levels?)

• Food availability or some other ecological factor (but there are some pretty big bodies of freshwater out there, so this seems unlikely)

Whatever factor is involved (and there may be more than one), it would presumably have to be pretty substantial. Although the idea of cephalopods migrating up rivers to lakes might seem unlikely, at least some freshwater lakes will have been formed from inlets where cephalopods were presumably trapped, and then died out as the water turned less saline.

So, WEIT readers, what are your hypotheses? And above all how could you test those hypotheses?

For those who’d like an introduction to the issue, there is this thread over at tonmo.com which has stretched over a couple of years… Try not to repeat those folk!

Oh, and I expect PZ to weigh in here!


  1. chriskg
    Posted January 16, 2012 at 10:34 am | Permalink

    The bigger question is did cephalopods survive the Flood with all that fresh water or was is it salt water rain?

    • JBlilie
      Posted January 16, 2012 at 12:06 pm | Permalink

      Heh, heh, heh …

    • PB
      Posted January 16, 2012 at 9:21 pm | Permalink

      Was cephalopods represented in Noah’s Ark? I don’t think so. This “kind” is of no use to Noah, calamari rings had not been invented.

  2. Posted January 16, 2012 at 10:42 am | Permalink

    I’m not sure, but glad that means I won’t be attacked by one coming out of Lake Erie.

    • Posted January 16, 2012 at 9:13 pm | Permalink


      (I grew up on the shores of Lake Erie in the Cuyahoga-river-fire era, when you couldn’t eat anything you caught in the river or the lake)

    • Dominic
      Posted January 17, 2012 at 3:38 am | Permalink

      They rely on water to maintain shape and buoyancy, and out of water find movement difficult, so fear not by any shore!

  3. RFW
    Posted January 16, 2012 at 10:53 am | Permalink

    There are sporadic examples of cephalopods (specifically octopodes) going into fresh water. Within the last year, perhaps on this very site, there was a video of an Alaskan octopus in a fresh-water creek.

    I’ve seen myself a dead octopus perhaps 200′ upstream from the mouth of Fulford Creek on Saltspring Island. Possibly it was pushed there by the tide after dying but it’s impossible to tell now.

    It seems that aside from exceptional examples like these, cephalopods stay in salt water.

    Note that in both cases, it would have been a giant Pacific octopus. The one in Fulford Creek was a big sucker!

    • Dominic
      Posted January 17, 2012 at 3:35 am | Permalink

      We could test all this – if it were ethical – by placing cephalopods both mature and larval in tanks with different concentrations of salinity.

      • Aidan Karley
        Posted January 17, 2012 at 9:54 am | Permalink

        You could probably test it without straining ones ethics muscles too badly by constructing (in the sea if necessary) a salt-water / fresh-water gradient, then seeing what level of salinity your test cephalopod positions itself at against some other force – such as a food supply.
        I’m envisioning something like a selected island (pier) with a predictable tidal race, and diverting a flow of fresh water into it (stream ; trough ; construction work ; make it float because you only need to stop it mixing with the sea until you get to your experiment). Train the cephs or tempt them towards the light, and measure the salinity at the level to which they choose to go (conductivity sensor?). Video monitoring and graduate slave labour to process the video into data. Vary salinity by varying the flow of tidal sea-water and fresh water in your trough.

  4. Ray Moscow
    Posted January 16, 2012 at 10:57 am | Permalink

    Other molluscs have made the trip to fresh water — and even to dry land — and so it does seem like a puzzle.

  5. Posted January 16, 2012 at 10:58 am | Permalink

    Is it perhaps not so much the salinity/lack of salinity itself, but the stability of the levels of various salts in something the size of a sea.

    In fresh water there can be all kinds of salts and minerals from rocks/rain water etc… and the amount of rainwater that falls as a percentage of the body of water could have an effect.

    Perhaps cephalopods need a more stable salinity.

  6. troy
    Posted January 16, 2012 at 11:02 am | Permalink

    No freshwater cephalopods? Have you never heard of Fons volatilis?

    • Marella
      Posted January 16, 2012 at 2:42 pm | Permalink

      Which is apparently descended from the Brazilian freshwater squid the Fons brasiliensis.

      So it turns out that the reason there are no fresh water cephalopods is that we don’t know much about cephalopods.

    • Matthew Cobb
      Posted January 16, 2012 at 3:26 pm | Permalink

      I think you’ll find that’s a joke.

  7. Posted January 16, 2012 at 11:23 am | Permalink

    My first thought would be to look at the ecological niche that cephalopods inhabit, and see if there is just too much competition already in the corresponding freshwater niche.

    • David Leech
      Posted January 16, 2012 at 4:35 pm | Permalink

      This might be nearer the mark as cephalopods are osmoconformers which would allow them to live in deep water/high pressure environments.

  8. gillt
    Posted January 16, 2012 at 11:31 am | Permalink

    I would throw money at the problem.

    Sub-sample a wide representation of specimens of estuarine cephalopods and oceanic or pelagic or whatever cephalopods. Then through the awesome data-generating powers of next-gen sequencing I would do genome-wide alignments and look for variation across loci. That would hopefully provide enough data for a more a informed investigation. Of course, things would be a lot easier if there was a consensus genome already out there.

  9. Gregory Kusnick
    Posted January 16, 2012 at 11:32 am | Permalink

    How sure are we that cephalopods have never invaded freshwater habitats? Is this something the fossil record can tell us, given that soft-bodied cephs probably don’t fossilize well? Suppose there had been multiple such invasions over evolutionary time, but we just happen to be living in one of the in-between times when there aren’t any extant freshwater ceph species. Would we be able to tell?

    • Ryan
      Posted January 16, 2012 at 11:52 am | Permalink

      Wait, is it really the case that we can tell the difference between fresh- and saltwater fossils? How exactly does that work?

      • Achrachno
        Posted January 16, 2012 at 12:18 pm | Permalink

        Nature of the surrounding sediment, including associated fossils and extent of beds. One can tell lake sediments from marine sediments in a number of ways, and river is even easier.

      • Gregory Kusnick
        Posted January 16, 2012 at 12:19 pm | Permalink

        I assume you can look at other fossils in the same stratum to see if they’re known marine or freshwater species.

      • morkindie
        Posted January 16, 2012 at 12:32 pm | Permalink

        If the fossil was laid down in a creek bed, it might be freshwater. If it is in ocean sediment, it is probably saltwater.

        • Posted January 16, 2012 at 5:12 pm | Permalink

          You can also look at oxygen isotopes (or something like that) in the actual fossil itself, at least in the case of bones. I read about this when Indohyus was discovered.

  10. ChasCPeterson
    Posted January 16, 2012 at 11:39 am | Permalink

    As far as i know, freshwater gastropod and bivalve mollusks have adapted primarily by tolerating extremely dilute body fluids (extra- and intracellular). My guess is that such dilution makes it much more difficult to maintain the transmembrane ion concentration gradients necessary for rapid and sustained nerve and muscle function. The more active the mollusk lifestyle, the less likely it will work physiologically in freshwater.

    • Achrachno
      Posted January 16, 2012 at 12:19 pm | Permalink

      I think this is the best approach. Several testable things there.

    • Posted January 16, 2012 at 3:34 pm | Permalink

      Not just active, but complex, involving the coordination of eight independent tentacles and their suckers, and requiring considerable intelligence – which they display.

    • Posted January 16, 2012 at 11:23 pm | Permalink

      That’s interesting actually, maybe it does have something to do with nerve conduction then.

      Squid at least do have higher potassium ion concentrations in their extracellular fluid compared to other molluscs, which I saw suggested could have something to do with nerve conduction

      I guess if these ECF k+ concentrations were necessary for their active and complex lifestyle, there might not be too much lee-way for adaptability to a more diluted environment like you said

  11. gsenski
    Posted January 16, 2012 at 12:00 pm | Permalink

    OMG!!! What is a Cephalopod post doing on Ceiling Cats web page???

    • Matthew Cobb
      Posted January 16, 2012 at 12:46 pm | Permalink

      The lunatics have taken over the asylum!

    • Aidan Karley
      Posted January 17, 2012 at 9:59 am | Permalink

      Waving a tentacle at the ceiling. Clearly this cephalopod does not believe in Ceiling Cat.
      It will learn, when the Paw of Destiny hooks it out onto the Carpet of Doom.

  12. Another Matt
    Posted January 16, 2012 at 12:06 pm | Permalink

    Also, there needs to be more study of the land-dwelling cephalopods:


  13. Basr
    Posted January 16, 2012 at 12:17 pm | Permalink

    Perhaps the reason is not directly related to cephalopods themselves, but simply that they lost to other species.

    For instance I have a hard time envisioning an octopus doing well crawling up (or down) a shallow rushing stream. Wouldn’t a species with that ability have an advantage?

    Also, do we even know that there were never any freshwater cephalopods or cephalopod-like things? Maybe they were there long ago but got pushed out.

    • PB
      Posted January 16, 2012 at 9:25 pm | Permalink

      Because they could not stand the desert climate near Noah’s ark? (Case of Under-Representation of Certain Species in Noah’s Ark)

  14. Posted January 16, 2012 at 12:21 pm | Permalink

    Pardon my ignorance…but I thought that crustaceans, such as lobsters, shrip, and crabs, are insects? I mean, at the least, they’re certainly arthropods, as are insects and arachnids.

    It seems to me that the question might be no more significant than asking why large flying animals are predominantly avians with a small sampling of mammals. Dinosaurs first captured that niche (the sky), and the mammalian newcomers (mammals) haven’t had much of a chance to displace them yet. But both birds and mammals are warm-blooded vertebrate quadrupeds, so it’s not all that much of a difference between them, just as it’s not all that much of a difference between lobsters and scorpions, or crabs and spiders.

    Might as well wonder why there aren’t any (fully) aquatic apes, alpine penguins, or arboreal tubeworms.



    • ChasCPeterson
      Posted January 16, 2012 at 12:26 pm | Permalink

      Insects are (a subset of) crustaceans.

    • Gregory Kusnick
      Posted January 16, 2012 at 12:47 pm | Permalink

      The distance between crabs and spiders is much greater than the distance between lobsters and scorpions. (Look at the chart Chas linked.) In fact you can’t get any farther apart than crabs and spiders and still stay within the Arthropoda. Which makes them about as closely related as horses and seahorses.

      • Tim
        Posted January 16, 2012 at 1:29 pm | Permalink

        I’m not a biologist, but according to my Timetree app, the ‘distance’ between the members of both pairs is the same: 580 million years (give or take).

        • Gregory Kusnick
          Posted January 16, 2012 at 2:03 pm | Permalink

          You’re right; scorpions are arachnids. My mistake.

          However I still take issue with Ben’s claim that there’s “not all that much of a difference” between arachnids and crustaceans, which (according to TimeTree) are in fact more distantly related than horses and seahorses.

    • Aidan Karley
      Posted January 17, 2012 at 10:15 am | Permalink

      Pardon my ignorance…but I thought that crustaceans, such as lobsters, shrip, and crabs, are insects? I mean, at the least, they’re certainly arthropods, as are insects and arachnids.

      All arthropods, as you say, but my very faulty memory tells me that a major point in the subdivision of the Arthropoda is the pattern of fusion of segments to form the head and other divisions of the body, “tagmosis”, IIRC.Since this acts pretty early in development (before, for example, the development of appendages on each segment) it’s an effective distinction between the phyla, and it’s relatively deep, evolutionarily. Then, depending on the local environment, similar chemical gradients (under similar HOX genes) lead to formation of as antennae on the fore-head, mandibles on the mid-head, legs on the three thorax segments, and something on the abdomens. If you’re an insect. But if you’re a cheliciate (spelling? spider / scorpion) you get, for the same HOX genes, pedipalps, several sets of mandibles, 4 pairs of legs and spinnerets. (On scorpions? I’m floundering. And as for the Eurypterids?)
      given this fundamental set of similarities between the arthropods, then … how similar are their excretory organs (for maintaining electrolyte balance)? Given that they’ve all got chitin, along with the fungi. (I think!)

  15. eric
    Posted January 16, 2012 at 1:16 pm | Permalink

    Is ‘historical coincidence’ reasonable? Less than 0.1% of the world’s water is in lakes and rivers. 800 species x 0.001 = 0.8 species expected to be freshwater. 🙂

    Also, a brief web search pulled up that Lolliguncula brevis may be encroaching up the Chesapeake bay.

    • Posted January 16, 2012 at 11:27 pm | Permalink

      yeah some can tolerate brackish water, but none live permanently in freshwater

      but i don’t think “historical coincidence” is that reasonable – there may be 800ish species of cephalopod alive today, but that’s certainly not all there has ever been!

  16. NewEnglandBob
    Posted January 16, 2012 at 1:37 pm | Permalink

    I have no answer but subscribing to see if others do.

  17. Another Matt
    Posted January 16, 2012 at 2:06 pm | Permalink

    Why are “ecological factors” so summarily dismissed in the OP? Cephalopods’ peculiar anatomy and behavior is shaped in part by food supply and predation, yes? One would at least want to find whether a freshwater niche is available for something like a cephalopod, in addition to looking for other proposed physiological factors like whether a complex mollusk nervous system requires salinity.

  18. Posted January 16, 2012 at 2:27 pm | Permalink

    A freshwater giant squid would be totally awesome. I have visions of somebody walking their dog by the river, when suddenly a giant tentacle rose from the surface, and dragged the dog to its doom.

  19. Thanny
    Posted January 16, 2012 at 2:39 pm | Permalink

    Regarding the insects in the ocean question, it seems less puzzling if you rephrase it. Given that insects are crustaceans, the question is, why haven’t any crustaceans, after having invaded the land (becoming insects) returned to the ocean? Because there are still other crustaceans there, presumably occupying available crustacean niches too efficiently for another land-based crustacean to invade.

    Regarding cephalopods, my best guess is that there’s no advantage to be had in brackish water anywhere, so the path to freshwater tolerance just won’t be travelled.

    • Posted January 16, 2012 at 2:56 pm | Permalink

      If insects evolved from crustaceans, why are there still crustaceans?


      • Torbjörn Larsson, OM
        Posted January 16, 2012 at 4:45 pm | Permalink

        Beause they are too crusty to die?

    • ChasCPeterson
      Posted January 16, 2012 at 8:49 pm | Permalink

      Here my guess would be more respiratory–insects are pretty much obligate air-breathers (except for some larvae) with tracheal systems that wouldn’t work in water. They’d have to re-evolve gills.

  20. TheBrummell
    Posted January 16, 2012 at 3:01 pm | Permalink

    To use a probably poor analogy: teleost fishes and mammals are both groups of vertebrates, yet we see no instances of mammals extracting oxygen from water, and only a very few instances of teleost fishes extracing oxygen from air.

    While we see gastropods in freshwater (and on land, using lungs – you can drown a terrestrial snail in clean water, they’re obligate air-breathers), we don’t see cephalopods in freshwater, and these two groups may share a common ancestor about as remote as the common ancestor of mammals and teleosts. A physiological restriction, as suggested, regarding the cephalopod “kidney” (or “nephron”, or whatever its called) could be something that’s just not likely to evolve very often as an exaptation, and the common ancestor of all extant cephalopods, a few generations after the common ancestor with (at least some) gastropods didn’t have the ability to handle effectively zero salt levels. Related factors could have to do with specific ions or compounds present in ocean water but usually absent or at much lower concentrations in freshwater, such as calcium (yes, I know the gastropods got around that one, it’s an example!).

    To reiterate: there ARE insects in the sea, living their entire lives in marine environments – waterstriders of the genus Halobates. Disproving a strong negative statement is always so ridiculously easy – simply find ONE example that goes against the supposed rule (and none of that misinterpretation of the old cliche about “the exception proves the rule” – the use of the word “prove” there is a synonym for “test”, not “demonstrate true”).

    • ChasCPeterson
      Posted January 16, 2012 at 8:55 pm | Permalink


      eh, they live on, not in, the sea. (I don’t know anything about their osmoregulation.)

    • Chris Booth
      Posted January 18, 2012 at 1:52 pm | Permalink

      As a child, I threw a rock into a pool in a stream in which there were lovely waterstriders. To my dismay, those for whom the water tension was disrupted and they became submerged, drowned. Even when I tried to quickly scoop them out of the water, they died very quickly. Water striders are obligate air-breathers, too. Alas.

  21. Kari McKern
    Posted January 16, 2012 at 3:17 pm | Permalink

    OMG! You mean tree octopus are a hoax. Nooooo……

  22. Jim Thomerson
    Posted January 16, 2012 at 5:22 pm | Permalink

    In fresh water, squid are rapidly eaten by bull sharks, and thus seldom seen.;-)

  23. Posted January 16, 2012 at 5:41 pm | Permalink

    This reminds me of a similar question about jellyfish. It turns out there is one freshwater jellyfish (and it flies!) This suggests it can be very hard to distinguish fundamental reasons from historical reasons to explain why a group is restricted to one habitat or the other. If not for this one crazy freshwater jellyfish, we would be asking this same question about jellyfish.

    • Posted January 16, 2012 at 8:02 pm | Permalink

      The scientific name of this, my favorite jellyfish, is Craspedacusta sowerbii. Lots of videos of it on the web.

  24. Greg G
    Posted January 16, 2012 at 5:54 pm | Permalink

    I know! I know! Because they prefer the taste of saltwater fish.

  25. Diane G.
    Posted January 16, 2012 at 7:32 pm | Permalink


  26. Posted January 16, 2012 at 8:23 pm | Permalink

    Kink says he’s definitely a freshwater Kat.

    He said saltwater is for sissies and sissypods.

  27. Jim Thomerson
    Posted January 16, 2012 at 8:45 pm | Permalink

    Here is a review of “Marine Insects” 1976, edited by Lanna Cheng.


    I heard an anecdote of someone catching a squid with a flyrod, some 80 miles inland.

  28. Achrachno
    Posted January 16, 2012 at 8:50 pm | Permalink

    There seem to be 3 jellyfish species in freshwater, according to one website.

    • Posted January 17, 2012 at 4:46 am | Permalink

      Are they all Craspedacusta, or are there genera I did not know? The species delimitations in Craspedacusta are controversial, some biologists lump them all into a single species and others split them into several.

  29. Daniel Schealler
    Posted January 17, 2012 at 2:03 am | Permalink

    God only made them to live in the ocean. He never wanted them to live in fresh water, so they don’t.

    Such big sillies!


    (yes, that was a poe)

  30. Dominic
    Posted January 17, 2012 at 2:50 am | Permalink

    Without reading all the other comments, I would say that they perhaps evolved into a physiological form where there was no going back. And perhaps the ecological niches were not there in fresh water, or were occupied?

  31. Posted January 17, 2012 at 3:43 am | Permalink

    I think it’s fairly obvious. Cats drink freshwater and not salt water. You don’t try to move on kitteh’s water supply without gettin’ cut. It’s nature’s roshambo: cat claw beats sucking tentacles.

    • Aidan Karley
      Posted January 17, 2012 at 10:20 am | Permalink

      It’s nature’s roshambo: cat claw beats sucking tentacles.

      So … how does the Paw of Destiny (op.cit.) fare against the various “Giant Squid” genera (whose names I forget) which use hook-bearing tentacles instead of suckers?

      • Posted January 17, 2012 at 6:20 pm | Permalink

        Clearly a heretical question. Cats are not much interested in the sea, but were they to change their minds, fear not that squid, giant or not, would quickly meet the same fate as they did in fresh water.

  32. Posted January 17, 2012 at 4:18 am | Permalink

    Matthew Cobb, I went back to read the post you wrote here in June of last year where you linked to the non-open access paper of yours found here: http://www.sciencedirect.com/science/article/pii/S0960982210006020

    It appears to be open access now inasmuch as I’m comfortably reading it from the confines of Teh Cave.

  33. Kharamatha
    Posted January 17, 2012 at 7:25 am | Permalink

    What now? Why is PZ not here yet? Has there been an accident?

  34. RFW
    Posted January 17, 2012 at 12:49 pm | Permalink

    Another data point:

    Supposedly, some tattooists like to use sepia (cuttlefish ink) in their work. To have a supply of fresh sepia always on hand, they keep a cuttlefish in the toilet tank.

    That’s a cephalopod living in fresh water; involuntarily, admittedly.

    Note carefully that “supposedly”. I’ve heard or read this somewhere, but it may be purely urban legend. I’d think that there would be sterility problems, at the very least. But the matter is worth looking into more closely by anyone desperately interested.

  35. Chris Booth
    Posted January 18, 2012 at 1:18 pm | Permalink

    The answer is simple:


  36. Posted January 18, 2012 at 4:43 pm | Permalink

    I’m going to have to agree with Chas’s hypothesis. Cephalopods are so dependent on a finely tuned nervous system that the kinds of changes to ion concentrations that would be required for their methods of maintaining osmotic balance would be tricky to navigate.

    I’d also point to the jury-rigged solutions cephalopods use to get around the limitations of axonal propagation speed in the absence of myelin. Those weird squid achieve fast conduction by generating very large diameter axons, up to 1mm; it may also make them more sensitive to variation in salt concentration.

    So any squid that tried to invade freshwater environments would have an additional imposition on their efforts to adapt, the need to modify an osmoconforming ionic regulating mechanism (and cephalopods are all stenohaline as well, with only a few species able to cope with brackish water, and of those, none able to survive for long at less than 20% salinity) to an osmoregulator to avoid disrupting nervous system function.

  37. Forrest
    Posted May 31, 2012 at 8:53 pm | Permalink

    I was curious if it could be a possibility to selectively breed various types of cephalopods to focus on various systems that could possibly have an impact on the ability to survive in freshwater. Taking out factors such as food supply and predation to simply check the anatomical differences seems like an effective start.

    You could even test the ability of various cephalopods to survive in various levels of freshwater and see what traits make them unique, allowing for some kind of good base to start setting goals for the selective breeding. (Yes, I am aware this would be a very time consuming process and would probably end up being someone’s life work)

  38. Rick Dickson
    Posted July 25, 2012 at 2:27 pm | Permalink

    The answer is ammonia. Cephalopods have substantial concentrations of ammonia in their blood and muscle tissue. They use ammonia for critical buoyancy, and is one reason they can turn on a dime. Ammonia would be much more difficult to produce in freshwater than saltwater due to different chemistries.

    Do I win a prize for solving this problem?

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