The Giant Crayfish of Lake Mashu

Posted by: mystery_man on October 11th, 2012

Is there an unidentified giant crayfish inhabiting Japan’s lakes? For years, occasional reports of extremely large, unexplained crayfish have surfaced in northern Japan, most notably in Hokkaido’s Lake Mashu.

Known to the Ainu people as “Kamuy-tou,” or “The Lake of the Gods,” Lake Mashu is located in the northeast part of Hokkaido, Japan. It is a caldera lake, formed in the crater of a dormant volcano approximately 11,000 years ago.

Surrounded by high, sheer cliffs, Lake Mashu lies 315 meters (1,033 ft) above sea level, is 20 km in circumference, and 212 meters (695 ft) deep at its deepest point. The lake is notable for having some of the clearest water in the world. Lake Mashu is also famous for the thick fog that veils it for most of the summer months, resulting in it’s nickname “foggy Lake Mashu.” The lake is quite remote, with access to the shoreline limited.

Since at least the 1970s, there have been reports of something strange in the lake. Accounts have surfaced over the years of crayfish far exceeding the size of any known to be in Japan. In 1978 and 1985, trout poachers are reported to have captured extremely large crayfish in the lake that were claimed to measure nearly 2 feet in length. In total, there were three alleged specimens gathered by these poachers, although the claims could not be confirmed due to the illegal nature of the circumstances surrounding the capture of the creatures.

Another account comes from one author of a survey of known crayfish in the lake, entitled “Crayfish in Lake Mashu, Hokkaido,” who reported that he had once captured a male crayfish specimen when he was a young man which had a carapace that measured 47 cm (18.5 inches) in length. It is important to remember that this figure is only for the length of the carapace, and not the total length of the specimen, which would be much longer.

There have been other scattered reports of giant crayfish in the lake as well. In one such report, a fisherman described seeing a crayfish crawling along the bottom in the clear water of the lake that he estimated as being at least 3 feet long.

Japan has only one species of endemic crayfish, the Japanese crayfish (Cambaroides japonicus), that inhabits the cold, clean waters of lakes, ponds, and streams in northern Japan. It is not a particularly large crayfish, reaching sizes of only 4 to 7 cm (2 to 3 inches) in total length, with an average carapace length of 3.5 cm (1.4 inches). Perhaps more importantly, it is not known to inhabit Lake Mashu.

The other two species of crayfish known to live in the wild in Japan are introduced species. One is the the invasive Louisiana crayfish, or red swamp crayfish (Procambarus clarkii) from North America. It is found all over Japan, on all of the major islands. This species is reaches sizes of 5.5 to 12 cm (2.2 to 4.7 inches) in total length. It too is not known to be present in Lake Mashu.

The only species of crayfish known to inhabit Lake Mashu is the introduced signal crayfish of North America (Pacifastacus leniusculus). This species was imported to Lake Mashu from Oregon and the Columbia River system in the 1930s as a potential food source. The signal crayfish reaches sizes of up to 15cm (5.9 inches) in length, with an average carapace length of 3 to 6 cm (1.2 to 2.4 inches), far from the enormous sizes reported for the mystery crayfish of Lake Mashu.

In the summer of 1992, there was a study of crayfish in the lake. Of the 171 males and 517 females caught in tangle nets during this study, no specimen had a carapace length of more than 5.7 cm (2.2 inches). Incidentally, this species has since been introduced to other lakes throughout Hokkaido, where it is notable for posing a serious threat to native Japanese crayfish. The signal crayfish has seriously threatened the native species in many areas through predation, competition, and as a carrier of crayfish plague, a type of water mould that infects crayfish. It is so invasive that it was put on the list of alien species that are banned in Japan under the Invasive Alien Species Law enacted in February, 2006.

So what could be behind the reports of mysterious giant crayfish? The fish in Lake Mashu, such as sockeye salmon and rainbow trout, as well as the sole species of crayfish known to be there, are all introduced from elsewhere. Since Lake Mashu is a Caldera lake, there are no endemic species there. There are also no rivers or significant waterways connected to the lake. The lake is a completely isolated habitat.

Perhaps when trying to ascertain what is going on, the first possibility to consider is other types of introduced, exotic crayfish. The world’s largest species of crayfish, indeed the largest freshwater invertebrate, is the Tasmanian giant freshwater crayfish (Astacopsis gouldi), above and below.

These crayfish attain average sizes of at least 40 cm (15.7 inches) in total length, but even huger sizes of up to 80 cm (31.5 inches) in total length and weighing up to 6 kg (13.2 lbs) have been recorded. The second largest is the Murray crayfish (Euastacus armatus) of the Australian mainland, which reaches overall lengths of 20 to 30 cm (7.9 to 11.8 inches) and weights of up to 2 kg (4.4 lbs). These vulnerable species are both currently protected by Australian law, and they are also banned in Japan by the 2006 Invasive Species Law.

Murray crayfish

Crayfish are popular as pets in Japan, so it seems at least plausible that perhaps some unsuspecting aquarists have bought specimens of one of these Australian species and dumped them when they got much bigger than expected. However, although these Australian species are large, the sizes still don’t seem to add up. Keep in mind that the measured Lake Mashu specimen mentioned earlier was recorded as being 18.5 inches long for its carapace alone (not it’s total length). The total length of such a monster from head to tail would be bigger than even the largest known crayfish species.

Factors in the lake that could be contributing to such large crayfish remain unclear. One possibility is that the known crayfish in the lake are displaying some sort of gigantism, although it is uncertain what would be the driving factor for this in Lake Mashu.

Tasmanian crayfish

Beyond insular gigantism (island gigantism) in their isolated lake environment, there other factors that could contribute to large sizes in the crayfish found there. For instance, these large sizes could come about as an adaptation for competing for scarcer food resources, which could possibly lead to delayed sexual maturity that would result in greater size. The Lake Mashu specimen that was measured was a male that was described as having exceptionally large chelae, or pincers. Since adult male signal crayfish in the lake do not exhibit secondary sexual characteristics in their chelae, this suggests that some degree of neoteny (delayed maturity) could be occurring. This means the specimen could have possibly been still growing.

The only problem with hypotheses on factors for gigantism is the time frame involved. Changes like this typically take place over a long time, and crayfish have only been present in the lake since the 1930s. Crayfish are generally fairly long lived creatures, that take many years to reach sexual maturity. This means there simply will not have been many generations born in the lake during which such changes would occur. The chances of such dramatic size adaptations developing so rapidly, and in so few generations, seem somewhat low.

Whatever they are, the giant crayfish of Lake Mashu make for an interesting mystery.

About mystery_man

13 Responses to “The Giant Crayfish of Lake Mashu”

  1. Ulysses responds:

    Where’s the mystery ? It’s probably from the same radiation that mutated Godzilla , Gamera, etc, and sent Ultra Man over to help. Take care and preserve them as they are unique…… for now .

  2. PhotoExpert responds:

    Mystery_man—I find this story as rather compelling for me. I think this is a definite possibility. In fact, I think it is probable.

    For myself, while fishing for smallmouth bass in the Northeast region of the USA, I have collected my own bait–crayfish. Smallmouth bass love them. I ran out of bait at a local body of water while fishing. I went to the water’s edge to collect a few more crayfish since the fishing was good. Most of the crayfish I collected were in the normal 2″ and under range. But what I saw amazed me. I saw a giant crayfish in deeper water about 3 feet from the shoreline. This thing was immense compared to the normal crayfish I had seen. It looked like a small lobster in size with the coloration of a normal crayfish. That crayfish had to be about 10″ long, a true giant!

    I watched it for several minutes and it came no closer than two feet to the shoreline which was about a foot deep. But there was a huge dropoff about 3 and a half feet out from where I was standing. It eventually retreated back there. But since it did come within two feet of shore, I could see it clearly. The water was crystal clear and is used as a drinking source for the local community.

    I never told anyone about his. In fact, this is the first time I felt compelled to write about it. I guess I felt people would not take me seriously or think I misidentified the crayfish or perhaps that I might be telling a tall tale as a joke. But I assure you, this is no tall tale. And the body of water was manmade. I am not even sure how big normal crayfish get or grow to, in a northeast body of water. But I can tell you, it was not normal.

    If I had seen a 5-6″” crayfish, that would be abnormal although I might be able to explain that as being a crayfish on the giant end of the scale of normalcy. This was double that size. I have no explanation. And since there are rules about no swimming in that body of water, I did not want to risk arrest or a citation by going in after it. I could not use it for bait because I do not know of a smallmouth that would try to tackle that monster.

    The crayfish you are writing about is much bigger than the one I saw. However, if you asked people if it was possible for crayfish to get that big here locally, they would say no or just laugh at the possibility. Which is why I never mentioned it until now. But I saw what I saw. The water did not make it larger than it appeared. I am a great judge of size when it comes to animals in the water. I can judge a fish’s size very accurately. I have seen muskrats and was able to judge their size correctly and have that judgement affirmed when they left the water. When I catch a fish that I thought was 12 inches in the water, it may be 12and 1/2 inches when I catch it, but I am very adept to judging size. So the water did not magnify my sighting. It is what it is.

    I can not explain it. I saw what I saw. And you know what, I believe the reports you have listed here in your article. I think your theory about it being some sort of gigantism is spot on! At least that would kind of explain my encounter.

  3. SOCALcryptid responds:

    PhotoExpert. Take a look at the latitude line running through northern Japan and compare it to your sighting in the northeastern United states. Could these type of invertebrates be experiencing gigantism in the northern part of the worlds hemisphere? This subject brings up more questions than answers. Interesting to say the least.
    Thanks for the post mystery_man

  4. marcodufour responds:

    Another great article as always Brent, do you ever get time to search for any of these or other creatures in Japan ?

  5. corrick responds:

    Think it’s important to point out that all these “sightings” are apparently
    without any specific names that could be factually checked. That makes them no
    different than any anonymous internet post of today, just stories.
    And even if you pursue an outside introduction angle, wouldn’t northern Hokkaido, given it’s location and population, be the LAST place in Japan to expect evidence from exotic pet importation?
    Interesting stories yes, but all I see here are just stories. Love your posts though.

    And PhotoExpert…
    You wrote you had the opportunity to capture a North American crayfish so gigantic as to change all zoology texts about invertebrates. Except, “since there are rules about no swimming in that body of water, I did not want to risk arrest or a citation by going in after it.” And then, “I could not use it for bait because I do not know of a smallmouth that would try to tackle that monster.”
    And I suppose Bigfoot and you watch Monday Night Football together every week. Except you don’t own a camera.
    OK, heavy sarcasm, but PhotoExpert your story makes no sense at all. Zero.

  6. mystery_man responds:

    PhotoExpert- That’s a great story! Thank you for writing of your experience here. I value sightings reports like the one you have described here. Very interesting.

    marcodufour- Thank you for your comment. As a matter of fact, I do get out and search for these sorts of things when I have the time, and I have made several excursions of this sort. For instance, I have been to areas where Japanese wolves are said to still exist, as well as areas said to have the Tsuchinoko (a type of cryptid snake), lakes and rivers in Japan with purported monsters, and others. I have done extensive interviews and research on a little known cryptid known as the Matsudodon, which was seen in the 70s in a river literally right in my backyard in an area called Matsudo, where I live.

    I have never made it to Lake Mashu, sadly, but I would very much like to.

    The problem is that I am quite busy as a teacher and a wildlife researcher of more mainstream animals. I just don’t have as much time and money to search for these things in the field as I’d like, a feeling which I’m sure every cryptozoologist can relate to.

  7. mystery_man responds:

    Corrick- Well, at least one of the accounts can be factually checked. The captured specimen was reported in a survey of the lake, “Crayfish in Lake Mashu,” which was actually an independent scientific survey of the crayfish population of the lake. This survey was apparently done to keep an eye on introduced crayfish, which are indeed present in the lake. The large specimen was treated as an anomaly, and was not the main focus of the report, but it is a verifiable finding.

    The report is long, mostly boring unless you enjoy the intricacies of crayfish populations in the lake, and all in Japanese, but there is that weird specimen and I have tried to boil that down to its essence here.

    As to the other accounts, well, some of them might be just stories, but there are reports that were actually made and in some cases written about, such as with the trout poachers. In conjunction with the actual capture of an abnormally large specimen during a survey of the lake, it seems worth noting that there may be something to the stories. Some of the reports could be just stories, yes, but I suppose the same could be said about many cryptid sighting reports in that case.

    As for introduced species in Hokkaido, I agree that very exotic species such as giant Tasmanian crayfish would be an odd find in such a remote lake. I would not expect to find an exotic pet released there. However, there was heavy importation of the North American signal crayfish as a food source in the lake and they are there in good numbers. Perhaps someone got it into their head to independently try importing something bigger. Who knows?

    It’s happened before, with independent and unauthorized importation of things such as snakehead fish, alligators, and nutria, for the express purpose of farming these species. Could have happened here as well. Farming such species would be illegal activity under the Invasive Species Act, so what better place to do this than a remote, out of the way location like Lake Mashu?

    I’m just the messenger, passing along what I have found. I personally think these giant crayfish reports are intriguing, and would like to make a trip there if I had the chance. Anyway, stories, real, who knows. Here is the information, do with it what you will.

    Anyway, thanks as always for your comments. I appreciate it.

  8. corrick responds:


    Well, if you have actually read the scientific survey report then that is certiaily worthy of serious consideration, especially since as you write it was undertaken to look for introduced crayfish. So there really might be something to it. Just don’t expect some incredible new discovery that rocks science. But an undetected introduced crayfish is a legitimate possibility.

    You give some great examples of the historic American independent and unauthorized importation of animals. If you haven’t already, you should read Kim Todd’s, “Tinkering With Eden.”

    “Caveat Cryptozoology Emptor” My motto. For at least the past 300 years, every animal on Earth needs to be considered as a likelier candidate for any mystery animal sighting before we begin looking for unknown ones.

    btw, bats certainly have had some amazing historical range…think Hawaii, New Zeeland and the Gallapogos so your ideas that stories of giant bats in the southern Japanese islands might come from bats elsewhere is sound science speculation. At least from me anyway…which isn’t worth a lot.

    Will rephrase and ask again. Obviously you are a thorough researcher and a critical thinker. So of all the mystery animals you’ve written about to this group, which three do you think are the most solvable? ie that you’re pretty certain you know the answer, if not a very good educated guess.

    And naturally, what’s the one mystery animal you’ve written about that in your brain and heart, you believe does or recently really did exist?

  9. dogu4 responds:

    Another great report, Mystery Man. Cold dark deep water, rich in oxygen and who knows what other signals that are biologically relevant seems like the kind of environment where processes that go un-noticed on land are more likely to occur. Maybe I failed to catch it, but were the specimen’s ages associated with these great size or is longevity and size less closely correlated than otherwise suggested? Thanks

  10. mystery_man responds:

    Corrick- Good motto. It is never a good idea to explain one unknown with another unknown. I agree completely. Occam’s Razor is a good basis to work from, which is to say that rather than “the simplest answer is best,” it really means something more like, “do not introduce unnecessary pluralities,” or in layman’s terms “when in doubt, don’t make stuff up.”

    I’ll try to answer succinctly, but of the posts I’ve made recently, I would say that the bats are most solvable, followed by the giant worms I posted about recently, followed by these giant crayfish.

    Of all cryptids in Japan which do I believe exists? I can’t say that “believe” is the right word, because for me “belief” doesn’t really come into it. It’s more like the ones that I think to be most plausible based on the natural history and the evidence on hand. In that sense, the ones I currently put most stock in are in this case not the animals completely new to science (your Kappas, Tsuchinokos, or bizarre lake monsters), but rather possible remaining populations of animals presumed extinct. For instance, I think there is good reason to suspect that remnant populations of Japanese wolves exist, as well as Japanese river otters and the Japanese sea lion.

    The other, more mysterious ones? Well, I do not discount them, but as you may have noticed, I do approach them with a critical eye. I like to speculate on the options, but I hope it is in a rational manner. I certainly do, as you mention, try to discount other more mundane explanations first as much as possible. I think I have an open mind, but my brain is still in there!

    Dogu4- With the crayfish specimens from the lake, there is certainly a correlation between age and size. At least in the case of the recovered specimen mentioned here, it exhibited traits of delayed maturity, or neotany, so it is speculated that this allowed it to keep growing for a longer period, allowing it to reach such a huge size.

    However, it is not clear if that is the sole factor that could be conceivably attributed to the stories of these giant crayfish. It could be that neotany is a factor, or it may also be some as yet unidentified factor such as water temperature, nutrients, or something else contributing to some sort of gigantism. It could be all of the above.

    So, age does correlate to the size of crayfish to some extent, but whether that is the main factor here, who knows? Possible contributing factors that could hypothetically be at the root of these giant crayfish reports are certainly an area that more research could be done on with regards to Lake Mashu. We need more info.

  11. PhotoExpert responds:

    SOCALcryptid- Hey! I will do that. That is a great idea. I will definitely take your suggestion.

    Corrick- I am not really sure where your negative attitude comes from, nor do I care. However, I understand where you are coming from, even though you do so with an overly sarcastic tone. Most skeptics are like this. I get it! I feel for you buddy!

    The problem is that I may have been able to answer your questions provided I had more time to write a lengthier post. And also, I wanted to get to the meat of the matter and not ALL of what was going through my head at the time. I left some things out. But for those who are a bit slow on the uptake and for skeptical posters such as you, I will elaborate a bit more. I’ll try to address your questions, although even if I do address them, you will still be the skeptic you are.

    At the time, and to this date, I am not sure if the crayfish I saw was so gigantic, that it would break all record books on invertebrates. I am not a crayfish expert. All I know is that the crayfish I saw was not the typical crayfish I was use to seeing. I have seen some species of dogs that are bigger than normal for the species. That does not necessarily mean that they are so big that they break record books. They are big, but just how big do the biggest ones get of that particular species? Who knows? The crayfish I saw may or may not have broken any record books. And at the time, that never crossed my mind. So what would any normal person do? Dive into freezing cold water, only to have the crayfish analyzed and the answer is that it was pretty big for it’s species but just outside the normal limits of that species of crayfish?

    A normal or rational person, with limited knowledge of crayfish would not risk life or limb to do that. Also, the cherry on top could have been criminal prosecution. Since you seem to like sarcasm, I will give you some in the next sentence. Yes, what you suggest sounds like the exact rational thing to do. I am a bit more intelligent than that. But Einstein, I mean corrick, you seem to know everything. You even were able to break all rules of science and know what my thinking process at the time. You are truly amazing!

    With that being said, the time of year was early spring. The water temperatures were frigid. Taking a swim, after a three mile hike and no change of clothes in sight, would have been the prudent thing to do as far as you are concerned. Have you ever heard of hypothermia? Although you do not think my crayfish did exist, with you skeptical mind, I can assure you hypothermia does exist. Well, as a seasoned outdoorsman, who has been all over the world and even stayed in the Amazon jungle for periods of time, I can tell you, hypothermia will take your life. My life is much more valuable than dying from jumping in a freezing body of water to get a crayfish, that might just be on the upper end of the species. But according to you, who has never seen the crayfish, notes that it was a record breaker for invertebrates. Psst, the photos mystery_man has shown here were much bigger than the crayfish I saw and described. So we both know my crayfish that I witnessed, was no record breaker. So your argument and skeptism has just left the building.

    To answer your other questions, yes, I did not want to chance being arrested or get a citation. Getting arrested may be a regular occurance for you, but for me, it is not. I would not attempt that blemish on my perfect record. Maybe you would, but then again, maybe being arrested is the norm for you or you just don’t care. I have certain standards. And as a pillar of ethics in my community and my involvement in the community, with several national and local charities, any kind of arrest is the last thing I would want. But hey, you are different. You would have just broken the law. More power to you!

    Oh, I do know a thing or two about fish. I know that in that body of water, there was only one o two species of fish that would have been able to tackle that crayfish. A big catfish might try to eat it or a landlocked striper might have been able to tackle it with some difficulty. But I was not fishing for either of those species. As I stated previously, I was fishing for smallmouth bass. And a crayfish of that size, certainly would not have been on the menu! So if I can not use it for bait, why bother risking my life. I did not need to catch a fish to survive. Hence, I did not need to catch that crayfish. Do you understand my thinking process there?

    Shall I continue? Well, you finally got one thing correct in your assumptive posting. I do watch Monday Night Football sometimes, although I do not watch it with Bigfoot as you suggest. So you got one thing partly correct. You are half right on that one point. I am not even sure if BF exists. It has not been proven. How could I sit with an unproven animal? I could have sat with that crayfish if I caught it because it existed. But if you think BF exists and watches Monday Night football with me, then by all means, go on with your fantasy. I am flattered that you think of me.

    Lastly, not only do I own a camera, I own several cameras, all of them professional. However, when this event happened, cellphones were not camera phones. They were just analogue cell phones. And like any avid fisherman who is hiking three miles to their fishing spot with their tackle box, rod and reel in tow, I am not going to carry a professional camera. What is the point? I am there to relax and get away from work. As a professional photographer, the last thing I want is my camera in tow. I was there to fish, not take photos. Get it? Or is that idea beyond your grasp?

    In conclusion, to any normal person reading my post, it makes perfect sense. To someone who is irrational or to someone who is naturally an over the top skeptic, it probably would make no sense at all, as it made no sense to you. So which is it? Are you an over the top skeptic or just an irrational human being? It’s one or the other and now I am curious as to which it is.

    Because of people like you, this is probably why it is the first time I have told my story. But I am still glad I did. Most of the comments were supportive of my story except for one–your comments.

    Hopefully, I have cleared some things up for you. I have a feeling I have not. You will probably want to know what my body temperature was that day or some other irrelevant information, so you can just diss on my story. I promise you, no more information like that will be coming your way.

    Have a great day! And I look forward to hearing more about how you imagine BF and me, sitting on a couch watching Monday Night Football, in your fantasies. Hmmm, after hearing about your fantasies, my crayfish story seems rather bland.

  12. corrick responds:


    Probably only you and mystery-man will ever read this, but it’s still worth writing.

    “Most of the comments were supportive of my story except for one–your comments.”
    Maybe that’s because Cryptomundo is a web site primarily for “believers.”

    You guesstimate you saw a crayfish about 10″ long. On average, American crayfish are 2.5 ” long. And until a recently published article citing a 5″ specimen discovered in 2009 in Tenn, the record length of any American crawfish was 4.5.”

    While 5″ may not seem like a lot, proportionately it’s mind-boogling. It’s like you
    reported encountering a 7’4″ tall great horned owl or a 20′ long black bear.

    Photo-expert. Don’t get me wrong. I’m certain you think your story is true. And that you probably could pass a polygraph. But no one’s eye is a camera and no one’s memory is an accurate recorder of events. Even yours 😉

    So your eyewitness account of a 10″ crayfish without any physical proof is no more
    believable than eyewitness accounts of unicorns, dragons or men with tails.

    Btw, I appreciate the time you took to respond to my post. And I’ve seen my share of crawfish underwater in small streams over the years and until doing a little research I would have sworn that 4′ was about average size.

  13. mystery_man responds:

    Corrick- Well, I don’t respond because I am a “believer.” Far from it. In fact, I am quite critical of these sorts of things. I just appreciate the data and the reports. I can’t be sure if these giant crayfish are real at this point in time, and I don’t “believe” either way.

    What I do see is the existence of actual known very large giant freshwater crayfish and reports of these in this lake, including in a scientific survey. I see these things and I am trying to make sense of them by looking at the reports, the facts, and possible explanations such as introduced animals or possible factors for gigantism.

    Right now, I see no reason to think such few generations could produce any appreciable gigantism in normal signal crayfish. Illegal farming of larger species is a reasonable possibility in this remote location, and in light of the documented cases of just such a thing. I still cannot say with certainty that this is what is happening, but it is possible. These crayfish are a mystery, that’s it. I don’t “believe” or “want to believe.” I want to know the truth.

    As to Photoexpert’s story, again, I just appreciate the report and the data. It is an interesting story and I will also think about these events and consider them, but that hardly makes me a “believer.”

Sorry. Comments have been closed.

|Top | Content|

Connect with Cryptomundo

Cryptomundo FaceBook Cryptomundo Twitter Cryptomundo Instagram Cryptomundo Pinterest


Creatureplica Fouke Monster Sybilla Irwin


|Top | FarBar|

Attention: This is the end of the usable page!
The images below are preloaded standbys only.
This is helpful to those with slower Internet connections.