The Case of the Missing Crayfish

      As someone who has a fondness for crayfish, I live in the right part of the world.  North America has more crayfish species than any other continent – about 380 of the world’s 640 or so species.  The heart of crayfish diversity is in the southeastern United States, which is home to more than 300 species of this lobster-like freshwater crustacean, also called crawfish, crawdad, ecravisse, yabby (yabby is also used for a marine lobster), and many other names around the world.

      As ubiquitous as crayfish are in nearly every stream, lake, and pond in eastern North America, Americans might expect them to be similarly common around the world.  But there are large areas of planet earth that are devoid of crayfish … and therein lies a mystery.

Distribution Map

      As this map of the worldwide distribution of crayfish shows, there are no crayfish in Antarctica.  That’s not particularly mysterious.  It’s too cold and there are no real fresh water streams or open water.  There are no crayfish in a large expanse of central and northern Asia.  But the most glaring hole in the distribution of crayfish is Africa.  There are no native African crayfish.   (Some American crayfish, including the Louisiana Red Swamp Crayfish, Procambarus clarki, have been introduced to Africa, but more on that later.) 

      Africa is a huge continent, second in size only to Asia.  But there is not a single species of crayfish across that enormous area.

      When biologists explain the distribution of plants or animals like crayfish they have a menu of choices.  First, it’s possible that the species in question evolved in its current location from a previous ancestor that lived in that location.  Second, it’s possible that the species moved, spreading out across land from some original location, floating or swimming across a body of water, or being carried by another organism (including, of course, people).

      Lastly, it’s possible that the species stayed put, but the location where they are found moved.

      The theory of plate tectonics shows that the continents we know and love today were different in the past.  At some times they were combined into supercontinents (Pangea, Laurasia, Gondwana).  They were in different Pangeaplaces on the globe – sometimes warmer, sometimes colder.  Because the crust of the earth is in constant motion, the continents separate, collide, and slide across the globe.  (For a good explanation of plate tectonics, click here.) 

      Because geologists can date rocks and figure out which parts of which continents were once connected and when, at least roughly, they can create maps of the world hundreds of millions of years in the past.  Paleontologists, meanwhile, find fossils related to existing species like our current crayfish.  Putting the two together can provide compelling explanations for the distribution of animals and plants. 

       For example, there are two distinct lineages of crayfish.  The crayfish of North America, Europe and Asia show clear connections to each other, and the crayfish of South America, Australia, Madagascar, New Zealand and Tasmania likewise share characters that suggest their relatedness.

      The distribution of crayfish in the northern hemisphere holds its own mysteries.  For example, why there are no crayfish in all of central Asia, why the crayfish of southeastern U.S. seem to be more closely related to the crayfish of eastern Asia than the crayfish of the U.S. northwest, which seem to be more closely related to the crayfish of Europe.  But the more well-known mystery regards the crayfish of the southern hemisphere.

      How could the crayfish of South America, Australia, and Madagascar possibly be related?  And what about Africa?

      Although crayfish evolved from a marine ancestor, they can’t survive in salt water.  Given that Australia and South America are separated by large oceans, it’s farfetched to imagine that they evolved in one place, and then spread across the thousands of miles of ocean that separate their modern habitats.

      One possibility is that there were a few marine ancestors that repeatedly colonized fresh water in South America and Australia and the other large islands of Madagascar, New Zealand, and Tasmania.  This might explain why all the crayfish of these land masses are more like each other than they are like the crayfish of North America, Europe, and Asia, which might have been colonized by a slightly different marine ancestor.

      Another possibility is that if the landmasses were once connected, a single colonization by a marine ancestor that then dispersed and evolved on all the connected southern landmasses that later spread across the ocean would also give the same pattern, where southern crayfish are more closely related to each other than to northern crayfish, because the southern continents stayed connected as Gondwana for tens of millions of years after the northern continents moved away.

      How can we tell which is more likely to have happened?

      It turns out that biologists can gain clues by looking at the differences in DNA from one species to the next.  (For a simple analogy, using words, of how this works, click here.)  When they do that analysis with southern hemisphere crayfish, it very strongly supports the hypothesis of a single colonization by a marine ancestor, then subsequent spread and diversification across the southern landmasses. 

      The earliest fossil crayfish suggest that crayfish first colonized land roughly 200 million years ago.  At that time, Australia, Madagascar, Antarctica, Australia, New Zealand, and Tasmania were all connected in one giant continent called Gondwana.  So if crayfish evolved and spread across that supercontinent for the tens of millions of years that it stayed together, that would explain why the crayfish of these now distant landmasses are related.

      Mystery solved, right?

      Except for one problem.  Also included in that giant continent of Gondwana, in fact, right smack in the middle of that giant continent of Gondwana until about 110 million years ago, was Africa.  If crayfish spread across Gondwana before it split up and the pieces moved across the globe, why are there no crayfish in Africa?pangea

      There are several possible explanations.

      First, it’s possible that even though it was connected to all those other parts of Gondwana, the part that became Africa was different, climactically or ecologically, from the other parts, so that the conditions needed for crayfish to thrive weren’t present.  For example, what if there were no rivers and streams.  What if ancient Africa were one giant desert?  That might explain why there are no crayfish.  Studies of fossil animals and plants of Africa from that time period rule out that possibility, however.

      Another possibility is that there were crayfish in Africa when it was connected to South America and Australia, but something happened to Africa after the breakup of Gondwana that didn’t happen to the other landmasses.  That’s the hunch of Dr. Keith Crandall, who studies crayfish evolution, among many other subjects, as a professor of biology at Brigham Young University.  Dr. Crandall notes that Africa moved closer to the equator and got warmer and warmer as it did so.  He points out that modern crayfish are almost exclusively found in temperate areas.  Though there are crayfish on both the east and west coastal regions of South America, for example, there are none in the Amazon region, or the other truly tropical areas of South America.

      This hypothesis also gains some credence from the fact that aside from frigid Antarctica and Africa, there is one other Gondwanan landmass that has no crayfish:  India.  Though India is today part of Asia and in the northern hemisphere, it was once part of the southern supercontinent of Gondwana.  When Gondwana split up, India went skittering across the globe, right across the equator, until it was stopped by its collision with Asia.  It’s possible that as it traversed the equator, it was so warm that whatever crayfish it might have had as part of Gondwana couldn’t take the heat.

      So one idea is that Africa and India cooked their crayfish as they moved north from the Gondwana break-up.

      If this were true, however, we’d expect that there would be fossils of crayfish in the rocks of Africa and India from before they took their journey to the equator.  So far, no one has found such fossils. 

      Dr. Loren Babcock, professor of earth sciences at The Ohio State University, who also studies the evolution of crayfish, cautions, however, that it’s very hard to find crayfish fossils.  “Terrestrial arthropods like crayfish don’t fossilize well.  They often get eaten before they get a chance to become fossils.  Their exoskeletons break down faster than bone.  Marine arthropods can get quickly covered by sediment, but this is far less likely to happen to fossils in fresh water streams and lakes.  We have only about a dozen species of fossil crayfish from 200 million years of crayfish evolution in the entire southern hemisphere!” 

      Like Keith Crandall, Dr. Babcock thinks there probably were crayfish in Africa when it was part of Gondwana and we just haven’t found the fossils yet.  He suspects crayfish became extinct in Africa as the new continent moved off on its own.

      But one problem with the warm climate hypothesis is that it ignores the entire southern third of Africa, which has never  been that close to the equator.  In fact, parts of South Africa reach the 35th latitude, which in the northern hemisphere corresponds to the southeastern United States, where crayfish thrive.  Certainly South Africa as a whole is more arid than the southeastern U.S., but parts of South Africa get as much rain as parts of the range of crayfish in the United States, and there are rivers and streams.

      There is also the issue of the crayfish of Madagascar.  It too, moved to a warmer climate after the breakup of Gondwana, although it never reached the equator.  But Madagascar has crayfish.  Dr. Crandall suggests that those Madagascar crayfish may just be a clue, however.  He points out that the six species of crayfish in Madagascar are all found at high elevations in relatively cold water streams.  They don’t break the rule against hot climate crayfish.

      There are crayfish in Australia that do, however.

      Some parts of Australia have a very warm climate, yet they have crayfish.  So the rule against warm climate crayfish seems strong, but not absolute.  And Procambarus clarkiremember that I said that some crayfish (Procambarus clarki) from the U.S. have been introduced to Africa, where they have done so well that they are considered an invasive species that threatens native animals and plants. 

      Another possibility for why there are no crayfish in Africa might be exclusionary competition with other similar organisms.

      It is difficult for two organisms that occupy a similar ecological niche to survive in the same place in the long term.  If two organisms use the same resources of food, shelter, etc., over time one is likely to be slightly better than the other at surviving and multiplying in that location.  Eventually, one will outnumber and outcompete the other and only one species will continue in that location.

      Crayfish and freshwater crabs are that kind of competitors.  There are over 1,400 species of freshwater crabs that, like crayfish, are generalist feeders who will eat almost anything.  They also, like crayfish, shelter under rocks.  And they are of similar size to crayfish.  Crayfish versus freshwater crabs offers textbook quality potential for competitive exclusion.    

      However, while crayfish are primarily animals of the temperate zones, freshwater crabs are primarily animals of the tropics.  There are, for example, over 300 species in South America.  Over 800 in the tropical Orient, including India and Southeast Asia.  And 149 species in Africa.  By comparison, North America has only 19 species of freshwater crabs, mostly from Central America and the Caribbean islands.

      If I had grown up in Africa instead of North America, I’d probably be a big fan of freshwater crabs instead of crayfish.

      Fossil evidence suggests that freshwater crabs evolved from multiple colonizations by separate marine ancestors beginning about 65 million years ago, much more recently than crayfish, and after the southern supercontinent of Gondwana had split up.  It’s possible that as freshwater crabs diversified and spread, they outcompeted crayfish in the warmer parts of the world, leaving the distribution we see today, with no crayfish in tropical South America, and none in Africa or India.

      It may be just as likely, however, that exclusion goes the other way.  Maybe the reason we don’t see many freshwater crabs in North America is because crayfish were already there and they outcompete the crabs.  And maybe we see so many crabs in tropical South America, Africa, and tropical Asia because there weren’t any crayfish there to compete with.  It’s interesting that in places where crayfish have been introduced to Africa, they don’t seem to have trouble competing with freshwater crabs, and in fact, may threaten local crabs with extinction.

      So why would crayfish go extinct in Africa, when they didn’t in other parts of Gondwana?  Maybe for a combination of the reasons I’ve touched on:  Africa wasn’t as suitable for crayfish as other parts of Gondwana to begin with, large parts of Africa are arid and or tropical, and Africa is home to a large number of species of freshwater crabs.  Still, it seems unlikely that if crayfish had diversified in Africa before it split off from South America and the rest of Gondwana, that not a single one of those species would have survived or adapted to the changes the continent went through. 

      So as it stands now, Dr. Crandall, Dr. Babcock, and others who have studied this question will readily admit that we just don’t know the answer to the question of why there are no crayfish native to Africa.    

      I’m just glad I was lucky enough to be born in a part of the world where crayfish reign.

(Ecological message:  Dozens of species of American crayfish are at risk because of the spread of non-native species.  To avoid contributing to this problem, don’t move crayfish from their original bodies of water.  Don’t release crayfish bait, pets, or science projects.)




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