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[Administrators: feel free to relocate this post at your discretion; with its zoogeographical focus, it could just as aptly lie within the "Evolution" forum.]
Looking to this map of global crayfish distribution, one is immediately struck by what are seemingly several discrepancies - namely, the complete absence of crayfish from a broad pan-tropical belt encompassing most of South America, continental Africa (but not Madagascar) in its entirety, South/Southeast Asia, and [naturally] most of the Indo-Pacific. Based on fossilized remains, it seems likely that crayfish arose in the far-southern reaches of Permian Pangea:
As the following quotes reveal, said peculiarity has been considered before, but little of substance has come from this basal speculation:
(My italics and bracketed commentary; scientific names are not necessarily contemporarily valid.)
From Chapter Six ("The Distribution and the Ætiology of the Crayfishes") of T.H. Huxley's The Crayfish.
As Huxley wrote, prawns of the genus Macrobrachium [with some 200 species, the largest of the Palaemonidae] are almost ubiquitous (suspiciously so with respect to the global range of crayfish) in the warm freshwaters of the world. Representative species range across a broad swath of the globe encompassing Africa (from Mauritania east to Ethiopia and from the lower reaches of the Nile south to the Great Karoo) - that is, the southern- and western- most precincts of the Eurasian biogeographical zone; the Ethiopian zone; and the Central African zone); the Malagasy realm; southern Italy and the neighboring Balkans (Eurasian zone); the Indian subcontinent, Sri Lanka, and Southeast Asia through the Indochinese Peninsula (Oriental zone); southern and western China across the East China Sea to southern Japan (Eurasian zone); the Philippines, Indonesia on both sides of Wallace's Line, New Guinea (and its respective division), and Australia (Austronesian zone); numerous isles of the southern and sub-equatorial northern Pacific Oceans (in rough correlation to those isles of the Polynesian Triangle having permanent freshwater systems [from Samoa to the Marquesas and northeast to Hawaii]; the West Indies (Caribbean transitional zone); Central/South America (Neotopics); southeastern and extreme southwest North America (marginal Palaearctic).
Its seems to me likely that stockily-chelate Macrobrachium prawns and crayfish together constitute a classic case of parallel evolution, on par with the analogous diversification of placental and Australian marsupial mammals.
Look to the following images for some markedly (both outwardly and in general habits) crayfish-like Macrobrachium spp.:
Macrobrachium sp. "Riesengarnele" ("Giant-shrimp")
Macrobrachium sp. "Panama"
Macrobrachium sp. "Mexiko Papalaopan
[Cumulatively from http://www.wirbellose.de/arten.html.]
Look also to http://www.crusta10.de/index.php?sideid=galerie&showid=M; many of the species shown here appear, at first glance, to be much more than distant relations of crayfish.
Other Potential Competitors
"Krabbenkrebse"/"half cancers" of the genus Aegla (which, despite their appearance, are neither crab nor crayfish but Anomurans affiliated with marine squat lobsters and hermit crabs [see http://www.crusta10.de/index.php?sideid=krabbenkrebse_de&lang=de]) by some accounts forced many South American crayfish into peripheral burrowing roles:
Experiments centered about conspecific competition between crayfish and Potamonid crabs overwhelmingly indicate the "behavioral dominance" of the latter over the former and suggest a "scenario of competitive exclusion" amongst the two:
Italian Freshwater Decapods: Exclusion Between the Crayfish Austropotamobius pallipes (Faxon) and the Crab Potamon fluviatile (Herbst)
1. On the basis of present range and fossil evidence (quite sparse, apparently due to counterintuitively poor preservation of calcaerous chitin), in what timespan of geological time did the genus Macrobrachium arise (alternatively, when did its progenitors colonize freshwater)? In what geographical region (by both historical and present-day continental arrangement) did this transition occur?
2. Is it possible that robust Macrobrachium spp. today comprise an impediment (as per the doctrine of competitive exclusion) to the expansion of crayfish? Are some species of the former grouping "behaviorally dominant" over the latter? Why might this be so? [Macrobrachium spp. certainly possess greater mobility through the water column, typically finer dexterity of the chelipeds, possibly more acutely sensitive chemoreceptive organs, and, in some cases, immediately autonomous benthic larvae.]
3. What past geographical barriers, if any, could have prevented crayfish from having spread to the portions of the continental tropics from which they are absent?
4. Is it plausible that crayfish were once considerably more widespread than at present, and were subsequently displaced by the invasion of freshwater by other decapods?
5. Does anyone know of any formal experiments concerning interspecific competition between crayfish and Macrobrachium to have been conducted?
6. In the regions of Africa where crayfish have been introduced to control disease-harboring aquatic snails or have escaped from aquaculture establishments, have native Macrobrachium (if they exist) proved a hindrance to their advance? Have they, alternatively, lost ground? Is their planktonic larval stage suppressed?
Last edited by Shagreen on Fri Mar 25, 2005 1:06 am, edited 2 times in total.
Some more pertinent information:
(Boldface, italicization, and bracketed text are mine.)
Crayfish-prawn "coexistance" as applies to that regional case, is tenuous at best. However, the text is unclear in one vital regard - while it seems to suggest that crayfish are scarce in streams of the Mississippi/Missouri watersheds of the surveyed region, it gives little mention of the extent of their presence in the river proper (or, conversely, of shrimp colonization of such minor tributaries). If competitive exclusion by riverine prawns was the primary limiting influence, one might expect that the more aquatic species would by today have expanded their range to assume some of the vacated stretches (if dams and pollution would not have impeded such movement).
Note: One must recognize in considering this issue that geographical sympatry is not interchangeable with "side-by-side" coexistance, per se.
A little off the topic but while I went to St louis they didn't have any crayfish to eat. During Mardi Grad there were no crayfish to eat becasue of a storm. Crayfish along with other lake dwllers can be easily wiped out be it a new species added, storms, or ect.
By the way it's healthy. There's the supply of vitamin D and A as well as calcium and potassium, copper and zinc in crayfish.
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