On the Origins of Freshwater Amazonian Stingrays

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ewurm

Aimara
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Jan 27, 2006
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The Amazonian stingrays are a favorite of mine. Their patterns vary greatly, even within individual species. A Motoro ray from one area of the Amazon can have a background color and pattern that is strikingly different. This is due to genetic drift. It's the same reason that we find equatorial human populations with a darker skin tone than in temperate regions. It's an evolutionary advantage. The same can be said of Motoro rays with varying patterns. While they are still the same species, they have adapted to their environment. The coloration of the markings as well as the background color is a direct product of the coloration of their environment. Their color pattern allows for better camouflage within the region of the Amazon in which they live.

Even more interesting is how stingrays came to live in the Amazon. There are two competing hypotheses. The first of these is freshwater invasion, which occurs when marine species venture into fresh water, probably because there is an abundance of food at the mouth of a river. An individual that is more tolerant of freshwater is more likely to survive and reproduce because they have more options available to them for food sources. Eventually, over a great period of time, the marine population becomes more tolerant of fresh water (euryhaline) and can spend a greater amount of time, if not all of their time, in fresh water. In the latter case, the species may revert to intolerance of differing salinity and become stenohaline, just as freshwater Amazonian stingrays happen to be.

The second hypothesis is the entrapment of marine rays by the uplift of the Andes mountains. As time passed bodies of water on the western coast of South America slowly became cut off from the ocean. This rise reversed the flow of the Amazon river (it used to travel east to west). As the saltwater flowed out of the bodies of water in which the rays were trapped, freshwater flowed in through run-off. This occurred at a pace that was slow enough (over many generations) to allow the marine rays to adapt to their changing environment.

Either circumstance presents us with an amazing answer to a bewildering question. Such is the beauty of life.

amazon_basin-avry.jpg

amazon_basin-avry.jpg
 
There is an abundance of scientific literature on this topic, and yes you are correct about the two most likely origins. I believe that the incursion of marine water from the northern Atlantic/Caribbean during the Miocene epoch is the more accepted origin, as this author noted many flaws with the original hypothesis (rise of the Andes). With much sophisticated molecular analysis and Amazon paleographic data, it is hypothesized that the ancestor of freshwater rays originally came from the Caribbean Sea, not the Pacific as previously thought.


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Interesting...Just curious about the 3rd option....

No chance there was always freshwater rays??

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If you are proposing that stingrays have always been on the planet, then all one would need to do is go and dig up a stingray fossil in Devonian strata. Pack your Gatorade, you'll be digging for a long time.
 
There is an abundance of scientific literature on this topic, and yes you are correct about the two most likely origins. I believe that the incursion of marine water from the northern Atlantic/Caribbean during the Miocene epoch is the more accepted origin, as this author noted many flaws with the original hypothesis (rise of the Andes). With much sophisticated molecular analysis and Amazon paleographic data, it is hypothesized that the ancestor of freshwater rays originally came from the Caribbean Sea, not the Pacific as previously thought.


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It's interesting to think that the population of rays in the St. Johns river in Florida may in the distant future turn into a separate species. It seems that these animals are living proof of the freshwater invasion theory.
 
They are amazing creatures. Nice to see you ewurm I haven't seen you around for a while. Maybe I am looking in the wrong spots though :)
 
Thanks for the warm re-welcome. I've been entirely too busy what with work and school to remember to pop in from time to time. Still love the fish though.
 
It's interesting to think that the population of rays in the St. Johns river in Florida may in the distant future turn into a separate species. It seems that these animals are living proof of the freshwater invasion theory.

Dasyatis sabina in the St. Johns is a very cool case, and I agree that eventually they will become their own species. I haven't read much about their osmoregulatory adaptions, but as far as their electrosensory system, they still possess the exact same ampullary organs as their marine counterparts. It probably takes a lonnnnng time for all these adaptions to occur!


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Dasyatis sabina in the St. Johns is a very cool case, and I agree that eventually they will become their own species. I haven't read much about their osmoregulatory adaptions, but as far as their electrosensory system, they still possess the exact same ampullary organs as their marine counterparts. It probably takes a lonnnnng time for all these adaptions to occur!


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It certainly takes a long time. Many generations in fact. But it's interesting to think that these rays may eventually undergo the same transformation as their Amazonian counterparts.
 
To me it would be even more interesting if they found a different method to deal with freshwater life, hopefully one that doesn't involve large amounts of urea
 
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