Petshrimp.com Discussion Forum

[More of a biology-oriented question; I apologize beforehand for the length of this post.]

From the following map of global crayfish distribution...



... one is immediately struct by several apparant discrepancies - namely, the complete absence of crayfish from a broad pan-tropical belt encompassing most of South America, continental Africa in its entirety(Madagascar, however, has indigenous crayfish genera), South/Southeast Asia, and much of the Indo-Pacific (not clear from the image). Now, based on fossilized remains, it has been established that crayfish originated in the Permian Antarctic:

Recent discovery of an Early Permian Antarctic claw has “extended the fossil record of crayfish by approximately 65 million years”, demonstrating that decapod crustaceans had radiated into freshwater habitats by the late Paleozoic; burrows in Lower Triassic strata of Antarctica are among the oldest construed to be constructed by crayfish (their morphology is similar to that of modern crayfish tunnels, demonstrating that burrowing behavior was established early in the evolution of this group. The new discoveries show that the earliest Permian crayfish were distributed in high paleolatitudes of southernmost Pangea, where they lived in freshwater lakes fed by glacial meltwater. Modern crayfish habitat, used as a guide to crayfish temperature tolerance, indicates that summer temperatures of streams and lakes near the South Pole that supported the crayfish probably reached 10–20 °C during Permian-Triassic interglacial intervals...

Fossils discovered on an NSF-funded expedition in Antarctica last December show new evidence that freshwater crayfish evolved at least 65 million years earlier than previously thought. Researchers in the Shackleton Glacier area discovered crayfish burrows in 240-million-year-old deposits of the Triassic Period, and identified a fossil claw of the Late Carboniferous-Early Permian Age (285-million-years-old).

The newly found crayfish claw is the oldest known evidence of decapod crustaceans from freshwater deposits anywhere on earth. Crayfish are important components of present freshwater ecosystems in their role as large and abundant omnivores. Their presence in these ancient deposits suggests that freshwater ecosystems resembling those of today developed much earlier than was previously thought. The breakage pattern on the claw, which appears to have been caused by a predator or scavenger, supports this theory in suggesting the presence of a community of species.

There is thus no inherent barrier to crayfish presence across what was previously Gondwanaland; at the time, river systems were established across what was to become southern Africa, then practically contiguous with Antarctica.

...Which brings up several questions:

- As it seems likely that competitive exclusion from "krabbenkrebse"/half cancers of the genus Aegla (despite appearances, neither crab nor crayfish but anomurans affiliated with marine squat lobsters and hermit crabs; see http://www.crusta10.de/index.php?sideid ... de&lang=de) effectively emplaced an ecological barrier to South American crayfish expansion, what is the possibility that a similar situation played out with freshwater prawns (Macrobrachium) displacing or precluding crayfish dispersal in the regions from which the former are absent?

The restricted range of Aegla is very similar to the endemic South American parastacid freshwater crayfish genera, Parastacus, Samastacus, and Virilastacus, and suggests a similar route of colonization. In fact, their ranges overlap so extensively that Riek (1971) suggested that competitive exclusion by aeglids forced crayfish out of streams and rivers and into burrowing lifestyles along river banks and fields.

The overlapping fronts between crayfish and Macrobrachium prawns (Southern North America, Irian Jaya, Japan, New Zealand) tend to result in the ecological marginalization of both parties.

As the following quote reveals, this issue has been considered before, but, our understanding, to my knowledge, has not expanded beyond what I have stated:

In warm climates, however, not only the large prawns which have been mentioned, but Atyæ and fluviatile crabs (Thelphusa- possibly an obsolete denomination) compete for the possession of the freshwaters; and it is not improbable that under some circumstances, they may be more than a match for crayfishes; so that the latter might either be driven out of territory they already occupied, as Astacus leptodactylus (possibly archaic/obsolete heading) is driving out A. nobilis (possibly archaic/obsolete heading) in the Russian rivers; or might be prevented from entering rivers already tenanted by their rivals.

In connection with this speculation, it is worthy of remark that the area occupied by the fluviatile crabs is very nearly the same as that zone of the earth's surface from which crayfish are excluded, or in which they are scanty. That is to say, they are found in the hotter parts of the eastern side of the two Americas, the West Indies, Africa, Madagascar, Southern Italy, Turkey and Greece, Hindostan [Indian subcontinent], Burmah [Myanmar], China, Japan, and the Sandwich Islands [Hawai'i]. The large-clawed fluviatile prawns are found in the same regions of America, on both east and west coasts, in Africa, Southern Asia, the Moluccas, and the Philippine Islands; while the Atyidæ not only cover the same area, but reach Japan, extend over Polynesia, to the Sandwich Islands, on the north, and New Zealand, on the south, and are found on both shores of the Mediterranean; a blind form (Troglocaris Schmidtii), in the Adelsberg caves, representing [substitute "analogous to"] the blind Cambarus of the caves of Kentucky.

In the regions of Africa where crayfish have been introduced to control disease-harboring aquatic snails or escaped from aquaculture establishments, have native Macrobrachium proved a hindrance to their spread, or have they lost ground as a result of environmental disruption?

Finally, with more relevance to shrimp in aquaria, how does the behavior of large Macrobrachium sp. compare with that of similarly-sized tropical crayfish?

If a crayfish and a Macrobrachium prawn were to be placed together in the artificial environment of a suitably large aquarium, which would most likely prove more efficient in utilizing resources (cover, range of potential foodstuffs, effectiveness and speed of food item discovery/hoarding) and enduring environmental duress (water quality preferences; disease susceptibility)?

Has a well-documented undertaking of cohabitation taken place?

Has a well-documented undertaking of cohabitation [between crayfish and Macrobrachium] taken place?

I now remember having read of such a case (in which a pair of large M. rosenbergii were placed with a "huge blue crayfish" (probably Cherax sp.). As one might have expected, the results were not pleasant. The prawns were "so aggressive" that one immediately began attacking the crayfish (easily several times its size), only to be dispatched with ease; a similar fate befell the second prawn - both were "torn apart" literally moments after introduction into the aquarium.

Would such a result be typical of an encounter [crayfish-prawn] if both specimens were to approximate each other in size?

... one is immediately struct by several apparant discrepancies - namely, the complete absence of crayfish from a broad pan-tropical belt encompassing most of South America, continental Africa in its entirety(Madagascar, however, has indigenous crayfish genera), South/Southeast Asia, and much of the Indo-Pacific (not clear from the image).

--Yes, that is very striking.

Now, based on fossilized remains, it has been established that crayfish originated in the Permian Antarctic:

--I would not say "it has been established", there is just more prove that crayfish existed longer than thought before. It makes it likelier that Antarctica might have been their origin, but it does not give conclusive proof.

There is thus no inherent barrier to crayfish presence across what was previously Gondwanaland; at the time, river systems were established across what was to become southern Africa, then practically contiguous with Antarctica.

--Here is a map that will help visualize things:

http://www.scotese.com/newpage5.htm

The continents were connected up to a large degree back then, but we don't really know about natural barriers such as mountain ranges and deserts, which could have prevented the spread of the crayfish. Plus, you have to remember the largest extinction of species that ever happened on this planet at the end of the Permian period. This could have wiped out most of the crayfish species and left just a few species to carry on. I could also imagine that there was a northern and a southern population of crayfish that survived the extinction. Then, later the norther population spread as far south as they could (as the natural barriers allowed them) and made up the majority of the crayfish species seen today. The souther population did not flourish and kept its spotty distribution. Then it becomes clear why the rest of South America and all of Africa do not have
crayfish species: Natural Barriers. North America and South America were not connected for a very long time and the middle east has no rivers that connect into Africa in addition to being mostly desert.
This is all my theory anyway...

-

As it seems likely that competitive exclusion from "krabbenkrebse"/half cancers of the genus Aegla (despite appearances, neither crab nor crayfish but anomurans affiliated with marine squat lobsters and hermit crabs; see http://www.crusta10.de/index.php?sideid ... de&lang=de) effectively emplaced an ecological barrier to South American crayfish expansion,

--I doubt that the Aegla are the reason why crayfish did not spread in South America. If you look at the distribution of the crayfish in South America you will notice that most of them are behind the Andes and in temperate zones. So, natural barriers such as mountain ranges and temperature are most likely culprits.

what is the possibility that a similar situation played out with freshwater prawns (Macrobrachium) displacing or precluding crayfish dispersal in the regions from which the former are absent?

--Again, unlikely. Macrorachium and crayfish have been living happily side by side in the Mississipi, Madagascar, New Guinea and Australia.

The overlapping fronts between crayfish and Macrobrachium prawns (Southern North America, Irian Jaya, Japan, New Zealand) tend to result in the ecological marginalization of both parties.

--I can't seem to make sense of this sentence. How would *both* groups be marginalized? You would think that one group would outcompete the other if they are occupying the same ecological niche. In any case...where are you getting the information from that they are even competing in the areas where they occur together? As I told you obove they have been living side by side in some areas for a long time and they don't seem to affect each other all that much.

As the following quote reveals, this issue has been considered before, but, our understanding, to my knowledge, has not expanded beyond what I have stated:

--Where are all these quotes from? A citation/source would help.

In the regions of Africa where crayfish have been introduced to control disease-harboring aquatic snails or escaped from aquaculture establishments, have native Macrobrachium proved a hindrance to their spread, or have they lost ground as a result of environmental disruption?

--No idea. I am not aware of any studies about this. It's also puzzling why they would introduce crayfish for snail control since Macrobrachium shrimp eat shrimp just fine.

Finally, with more relevance to shrimp in aquaria, how does the behavior of large Macrobrachium sp. compare with that of similarly-sized tropical crayfish?

--No idea since I have never kept them together. But I can imagine that the crayfish would be stronger than the macrobrachium in a one on one confrontation, but the Macrobrachium would probably outcompete the crayfish when it comes to locating food (i.e. pellets etc). I can also imagine that the Macrobrachium would eat all of the crayfish babies, whereas large crayfish don't bother or aren't able to catch small shrimp. I am almost convinced that they are occupying different ecological niches in nature so that they are not directly competing with each other.

Has a well-documented undertaking of cohabitation taken place?

--Not that I am aware of.

Mustafa

--I doubt that the Aegla are the reason why crayfish did not spread in South America. If you look at the distribution of the crayfish in South America you will notice that most of them are behind the Andes and in temperate zones. So, natural barriers such as mountain ranges and temperature are most likely culprits.

Sorry about my wording – I was more focused on past interspecific competition between crayfish and Aegla sp., as outlined in the “Introduction” of http://www.museum.vic.gov.au/memoirs/do ... _Tudge.pdf. Do you accept that these generally temperate freshwater anomurans may have played a role in forcing crayfish (along the "fault line" of historical overlap) out of their traditional aquatic benthic roles?

Even on the basis of gross external morphology, the Aegla evince distinct covergency (even considering homology) with crayfish, and are often confused with such.

--Again, unlikely. Macrorachium and crayfish have been living happily side by side in the Mississipi, Madagascar, New Guinea and Australia.

The Big River Region includes the Missouri and Mississippi rivers. Many kinds of fish are characteristic of the channels of these streams, but crayfish occur only as small local populations or stray individuals [Incision: there is presumably no outlier of salinity]. Four-inch shrimp (Macrobrachium ohione) was formerly abundant in the Mississippi River, but disappeared about 30 years ago [Incision: this was due to unsustainable harvesting and artificial channelization of the river; they have, since then, been "rediscovered" in stretches of their former range]. Several crayfish species characteristic of the other regions are common in sloughs and marshes on the river floodplains [Incision: atypical habitat for a fluviate prawn]. These include the White River crayfish (P. acutus) and dwarf crayfish (C. shufeldtii) along the Mississippi River, and the papershell crayfish (O. immunis) along the Missouri River. The devil crayfish (C. diogenes) is the common burrowing crayfish on floodplains of both rivers.

(From http://www.conservation.state.mo.us/nat ... istrib.htm)

Crayfish-prawn "coexistance" as applies to that regional case, is tenuous at best. However, the text is unclear in several regards; while it seems to suggest that crayfish are scarce in streams of the Mississippi/Missouri watersheds of the surveyed region, it gives little indication for or against their presence in the main river body. If competitive exclusion by riverine prawns was a major limiting influence, one might expect that crayfish have today expanded to assume the now-largely vacated stretches (if dam construction and pollution have not impeded such movement).

It must furthermore be noted that geographical adjacency is not interchangeable with "side-by-side" sympatry, per se.

The overlapping fronts between crayfish and Macrobrachium prawns (Southern North America, Irian Jaya, Japan, New Zealand) tend to result in the ecological marginalization of both parties.

--I can't seem to make sense of this sentence. How would *both* groups be marginalized? You would think that one group would outcompete the other if they are occupying the same ecological niche. In any case...where are you getting the information from that they are even competing in the areas where they occur together? As I told you obove they have been living side by side in some areas for a long time and they don't seem to affect each other all that much.

When species meet, competition reduces population densities in the region of overlap, which, in turn, intensifies the asymmetry in gene flow from center to margin. This reduces the ability of each species to adapt to local physical conditions at their range limits. - http://www.ncbi.nlm.nih.gov/entrez/quer ... t=Abstract

Interspecific competition tends to narrow the use of resources (encourages specialization as a way to reduce amount of competition between species)
A) exploitative – One species indirectly reduces the abundance of another species by reducing the shared limiting resource
B) interference - One species directly or aggressively interferes with another species access to a resource.
http://www.britannica.com/eb/print?tocI ... icle=false

Apparant "coexistance" of prawns and crayfish comes at the margins of both groups' range; overlap rarely occurs in zones of peak diversification (in which established crayfish fauna has firmly occupied benthic crustacean roles). One might put forth the case of Australia, the epicenter of crayfish diversity, and make note of the freshwater Macrobrachium dwelling there, but, for a variety of reasons (forthcoming), this is not inconsistent with occupation of similar ecological niche.

Evidence/consequences of competition:

1) Adjacent, but non-overlapping ranges (i.e., parapatry)
2) Differences in resource use when two species are allopatric
3) Less than random co-occurrence pattern
4) Change in abundance of natives, with the introduction of nonnatives:

a) Replacement hypothesis – environmental degradation eliminated sensitive native fish; tolerant fish assumed ecological role
b) Displacement hypothesis – indigenous species are displaced by invading nonnative

[Parapatry is a type of speciation in the concept of natural selection. It occurs when the zones of two species with a similar ecological niche abut but do not overlap. The two species may have contact at the common boundary of their zones. The concept of parapatry is necessary to satisfy the postulate of natural selection that two species cannot co-exist in the same location over the longer term with the same ecological niche. It suggests that there may be some factor apart from distance or ecological role which keeps the two species apart.

It contrasts with allopatry where different species are geographically isolated from each other, and sympatry, where different species evolve in the same area by finding different ecological roles.]

See http://aentry.nrifs.affrc.go.jp/~dbmngr ... _1_001.jpg

An understanding of crayfish-prawn interaction must take into account a broad range of biological conceptions:

- Ecological release. In ecological release, a species occupies a broader niche or geographical area in the absence of a closely related competitor. An example comes from the distribution of two species of planaria in streams. When found alone in a stream (allopatric distribution) each species occupies a wide range of stream temperatures. When both species are found in the same stream (sympatric distribution), however, the distribution of both species is restricted. P. montenegrina is found from 5 to about 13.5 C, whereas P. gonocephala occupies the warmer portions of the stream from 13.5 to approximately 23 C (Beauchamp and Ullyott 1932).

- Contiguous allopatry. In this phenomenon, two species occupy distinctly different geographical areas directly adjacent (contiguous) to one another. Another study on chipmunks on the east slope of the Sierra Nevada Mountains in California provides an example of both contiguous allopatry and ecological release. Heller and Gates (1971) found four species of chipmunks living at different altitudes. The least chipmunk (Eutamias minimus) is found at the lowest elevations in the sagebrush range. When all other chipmunks are absent, the least chipmunk can occupy all altitudes up to the alpine. However, the yellow pine chipmunk (E. amoenus) through aggressive behavior restricts the least chipmunk to the hot, dry sagebrush areas. If the least chipmunk is absent, the yellow pine chipmunk does not invade the hot, dry habitats. Evidently this dry area is not part of the fundamental niche of the yellow pine chipmunk. The lodgepole pine chipmunk (E. speciosus) is the most aggressive of the four species, but it is most vulnerable to heat stress and is restricted to shady, cool forests. It apparently limits both the upper distribution of the yellow pine chipmunk and the lower distribution of the alpine chipmunk (E. alpinus). Therefore, these four species, when all are present, show contiguous allopatry by altitude. When one or another is absent, the least chipmunk, and perhaps others, shows ecological release.

- Niche partitioning. In niche partitioning, two or more species coexist while sharing one or more resources in such a way that the niche overlap apparently violates the competitive exclusion principle. Upon closer investigation, the resources, though shared, are used with different frequencies or are used in different ways so as to allow coexistence. For example, the root systems of coexisting annual plants can be shown to partition the soil by depth, thereby avoiding direct resource competition (Wieland and Bazzaz 1975). In a classic study, MacArthur (1958) showed that five species of Dendroica warblers coexisted by foraging in different portions of trees in a coniferous forest. Although there was overlap, each species spent the majority of its foraging time in a unique portion of the trees. Other examples come from Diamond (1978). In one case he describes the coexistence of four species of fruit pigeons on islands of the Bismarck Archipelago. The pigeons range in size from 91g to 722g. The fruit sizes they consume range from 3 mm in diameter to 50 mm in diameter. Again, while there is a great deal of overlap among the pigeons in terms of fruit sizes consumed, the mean usage rates are clearly different. The smallest pigeon consumes fruits that are on average, 8 mm in size, the 135g pigeon consumes fruits that average 11 mm, the 470g pigeon, 18 mm and the 722g pigeon, 25 mm. In a similar analysis, Diamond (1978) shows that eight species of fruit pigeons in New Guinea coexist not only by specializing on different sizes of fruit, but also by the position of the fruit on the branches.

Niche partitioning can also occur within a species. Male red-eyed vireos forage for insects primarily in the upper canopy of the forest, while females concentrate their foraging in the lower canopy and near the ground (Williamson 1971).

- Character displacement. Character displacement describes a situation in which two species, when living in separate geographical ranges (allopatric distributions) have nearly identical physical characteristics (i.e. beak sizes in birds, overall body sizes in lizards and snails, canine sizes in the cat family). When sympatric, however, these physical or morphological characteristics diverge in one or both species. This divergence minimizes competition for food and allows the two species to coexist. Brown and Wilson (1956) appear to have introduced this idea. When examining the overall size and beak lengths of specimens of the eastern (Sitta tephronota) and western rock nuthatches (S. neumayer), they found that the allopatric populations were almost identical in both average size and in the range of sizes. However, these two species become sympatric in Iran. In sympatry, the eastern rock nuthatch increases in size while the western species becomes smaller. In this sympatric zone their beak and body sizes are completely non-overlapping. This allows them to feed on different sized prey items and therefore coexist.

In the above examples only pairs of species were examined. However, size displacements may occur among several coexisting species. Strong et al. (1979) called this “community-wide character displacement.” A pattern of regular differences in some size-ranked sequence has been accepted as evidence that past competition helps to mold niche differences. In fact G.E. Hutchinson (1959) proposed that a ratio of only 1.1 to1.3 between closely related species in this size-ranked sequences was necessary for coexistence. This is known in the literature as Hutchinson’s ratio or rule. There are many who have questioned the meaning or significance of such a rule (Simberloff and Boecklen 1981, Strong and Simberloff 1981). However, an interesting paper by Dayan et al. (1990) has shown that among wild felines in Israel, character displacement of canine diameters is remarkably constant at 1.10-1.14. These size ratios include three species of Felis, but also include male and female dimorphism in canine sizes. A great deal more information is needed on: 1) the size distribution of potential prey; 2) whether prey populations are limiting to predator populations; 3) whether larger canines correlate with the capture of larger prey; and 4) habitat use by the cats. Nevertheless, the data suggest that competition has played an important role in the evolution of the feline community.

Taper and Case (1992) have cautioned that any study purporting to demonstrate character displacement should meet the following criteria:

1. Morphological differences between a pair of sympatric species must be statistically greater than the differences between allopatric populations.
2. The observed differences between sympatric and allopatric populations must have a genetic basis.
3. Differences between sympatric and allopatric populations must have evolved on the site and not be due to different founder populations.
4. Variation in the character must have a known, ecologically important function.
5. Competition must be known to occur for a resource in short supply and the character must play an important role in that competition.
6. Differences in the character cannot be explained by differences in resources available to the sympatric and allopatric populations.

- Historical replacement. Finally, over human history there are documented cases of one species invading and eliminating another species from its original range. For example, Diamond (1978) has described both historical replacement and niche segregation in two species tits (Parus). The blue tit (P. caeruleus) is found in Europe west of the Ural Mountains where it occupies a wide variety of habitats from forest to riparian thickets. The azure tit (P. cyanus) was formerly found in Asia east of the Urals. But in the late 19th Century it spread 1000 miles west across Russia to the Baltic Sea. Over the next 10 years, however, the azure tit retreated several hundred miles eastward from the Baltic. At present, the two species overlap on eastern edge of the geographical range of the blue tit. However, in the overlap zone the two species segregate by habitat. The azure tit is found in riparian thickets, while the blue tit is in upland forest.

There are many known examples where introduced species have outcompeted the native flora or fauna. Starlings and house sparrows are infamous for evicting native cavity nesting birds such as bluebirds and flickers from nest sites. The purple loosestrife (Lythrum salicaria), a native of Europe, has invaded wetlands in temperate North America is crowding out native wetland plants. Finally, Australia and Hawaii are living laboratories demonstrating the effects of introduced competitors (as well as predators and herbivores) on native flora and fauna.

http://mason.gmu.edu/~lrockwoo/sp02%20I ... 0Niche.htm

--No idea. I am not aware of any studies about this. It's also puzzling why they would introduce crayfish for snail control since Macrobrachium shrimp eat shrimp just fine.

I share your confusion. It may be that the regions of introduction do not possess native Macrobrachium or that they have suffered from environmental degradation. Even in such a situation, it may have been a more conscientious to favor the introduction (with respect to water systems with no linkage to estuarine or marine bodies) of Macrobrachium over crayfish, as the former option restricts potential for unwanted population recruitment.

Macrobrachium & Snails: http://www.ncbi.nlm.nih.gov/entrez/quer ... t=Abstract

Crayfish & Snail Fever: http://www.unm.edu/~quantum/quantum_sp1 ... fever.html

My question wasn't restricted to simple "competition" between prawns and crayfish; I also have questions relating to convergency.

As related by the image of a Macrobrachium sp. (Americas)...



...Some "freshwater prawns" have taken an evolutionary path remarkably similar (if only in outward physiology) to that of crayfish; how similar are they in terms of ecological habit (particularly as applies to chelate, "stubbier" prawns, as that depicted above)?[/img]

Mustafa wrote:

Veneer wrote:what is the possibility that a similar situation played out with freshwater prawns (Macrobrachium) displacing or precluding crayfish dispersal in the regions from which the former are absent?

--Again, unlikely. Macrorachium and crayfish have been living happily side by side in the Mississipi, Madagascar, New Guinea and Australia.

After some extensive experiences keeping crayfish and Macrobrachium shrimp together in one tank I have to revise my statements above. I now think that Macrobrachium shrimp are probably the primary reason why crayfish are limited in their distribution to certain areas of the planet.

Crayfish are *very* vulnerable right after molting. They are *literally* soft as a cotton ball and can't even stand on their own feed. Macrobrachium shrimp smell the newly molted crayfish (the crayfish must smell a certain way signaling that it has molted) and search the whole tank to find the crayfish. Once found they chase the poor crayfish, catch it and finally proceed to eat it. This way, even smaller Macrobrachium species can hunt down and kill larger crayfish. The crayfish can't do anything about its fate since it takes about a day or so for the crayfish's shell to harden sufficiently and about as long for it to be able to use it's pincers for defense. Macrobrachium shrimp do not display this type of behavior towards conspecifics. They only get into a "feeding frenzy" when they smell a newly molted crayfish. Neither do display such behavior towards other crayfish unless two males are crowded together (especially with Procambarus clarkii) and one of them ends up getting killed right after the molt. Macrobrachium shrimp can end up losing limbs right after molting (or even before molting for that matter) but I have never seen them hunted down and eaten by other macrobrachiums post-molt.

So, this kind of behavior in Macrobrachium shrimp developed as a response to competition with crayfish, that could explain why crayfish don't usually live in places where there are a lot of Macrobrachium shrimp. The fact that crayfish occur both in the northern and southern hemisphere and on so many different continents indicates that they used to have a widespread global distribution, most likely occuring on every continent. Macrobrachium shrimp most likely developed later from tropical/subtropical marine ancestors and once they started colonizing freshwater environments they outcompeted crayfish in most tropical/subtropical places. This would also explain why most crayfish nowadays occur in temperate zones. In areas where crayfish and Macrobrachium do occur together, the crayfish most likely occupy ecological niches not covered by Macrobrachium shrimp. Crayfish in these areas most likely avoid bodies of water already occupied by Macrobrachium.

Hence, the question should not be "why could the crayfish not disperse further than they did" but more "how much of their former range did crayfish lose due to competition with Macrobrachium shrimp?"

Anyway....just a theory backed by my observations. I'm sure this theory will develop over time.

Mustafa