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.
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.
