Life in the Amazon River: Freshwater Stingray

Making the change from ocean to lake should be relatively easy, right?

Life in the Amazon River: Freshwater Stingray
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You’d think the raging high seas would be a much tougher place to survive than the calm waters of a freshwater lake or river. Making the change from ocean to lake should be relatively easy, right?

Well, that’s not necessarily the case, let’s dive deeper into the biology and chemistry to uncover the answer!

Outstanding Osmosis

The bodies of ocean animals are packed with a lot of salt. When you move them to a body of freshwater with a much lower concentration of salt relative to their bodies, osmosis kicks in.

Osmosis involves the bidirectional movement of water in and out of an animal’s body, from an area of high concentration to an area of low concentration. This delicate balance between salt and water within an animal’s body is known as osmoregulation.

Just like when we breathe, osmoregulation doesn’t require the animal’s active thought but is vital for their survival. This means if you were to put an ocean animal into a freshwater system it would be fatal as water would come rushing into it’s cells causing it to swell and burst.

How Do They Survive Today?

If you were to ask a general audience to describe the rivers of the Amazon, it is likely that a few people would mistake it for an ocean. This isn’t much of a surprise as the Amazon Rainforest ecosystem is teeming with animals typically associated with the ocean, including pink dolphins, needlefish, and stingrays.

amazon rainforest

So how are these creatures thriving today if marine life can’t survive in freshwater systems?

The answer is in the perfect conditions that must have existed 10 million years ago to make way for this new wave of life. Melting sea ice caused sea levels to rise, effectively drenching the entire upper Amazon with saltwater and life from our oceans.

Pebas Mega Wetland

This area is known as the Pebas Mega Wetland and covers what we know today as Brazil, Peru, Ecuador, and Columbia. This brackish (a mix of salt and freshwater) region once home to 11.8 metre or 39-foot crocodiles and behemoth sized-turtles becoming the perfect grounds for evolution.

The formation of the Andes Mountain range within the Pebas Mega Wetland was the main tectonic event that would change biology forever.

The emergence of this physical barrier, cut off freshwater rays from their ocean cousins and overtime, the saltwater lakes slowly turned to freshwater. This slow change in salinity means avoiding the osmosis problem and allows for the animals to adapt and evolve as necessary.

Extraordinary Evolution

Evolution doesn’t occur overnight but rather over the course of millions of years where individuals that are more adapted to their environment have a higher likelihood to survive and pass on their genes that led to their success. Overtime, these adaptations show themselves very clearly as we can see today in the appearance of freshwater rays found within South America.

white blotched river stingray
White-blotched river stingray

Whether it is the ocellate river stingray ( Potamotrygon motoro ) or white blotched river stingray ( Potamotrygon leopoldi ), these freshwater stingrays are now a whole different genus from their saltwater counterparts.

How Habitats Play a Role

Freshwater habitats differ drastically from the open ocean which would explain the stark contrast in their appearance. The murkier waters and darker coloured substrate (branches, logs, leaves) found along the bottom here would also account for the array of appearances and body types we see in this region.

Tiny spots, marbled swirls, and different jaw shapes had all emerged by the end of the Miocene 5 million years ago within these isolated populations. There was even a study conducted where the team analyzed the jaws of different stingrays in the region. Not only did they find a difference in physical appearance in these rays, but a difference in jaw shape and size. This was a result of rays specializing and adapting to different kinds of prey that they now had available.

Some rays have evolved mouths with fewer teeth and more jaw gaps to allow them to suck up fish quickly. Other rays had evolved much longer mouth bones to crush the hard shells of crustaceans. Another group of rays evolved kinetic jaws which allowed them to chew the tough exoskeletons around insects.

Succesful Adaptation

Each one of these respective specializations, allowed for the rays to dominate their environment. Interestingly, the ocellate river stingrays are actually a generalist, their ancestors didn’t hone in on one type of prey but still managed to be quite successful and that is why they can be found all over South America today.

Sadly, the rays in South America are facing many issues today that they have not had to before. If we don’t do our part to keep our habitats clean, these lovely creatures will likely become extinct.                                                           

Sensing You Want More?

Swim on over to Ripley's Aquarium of Canada and see our rays up close like never before!

About The Author

Sampras Lee, Ripley’s Aquarium of Canada Educator

Sampras Lee, Ripley’s Aquarium of Canada Educator

My name is Sampras, and I'm a full-time Educator here at Ripley's Aquarium of Canada. If you ever se…

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