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Freshwater stingray history

freshwater stingray history

Freshwater stingray history

All stingrays both fresh and saltwater belong to the class Chondrichthyes. This includes all 850 species of sharks, rays, skates and chimaeras. Based on fossil remains it is known that this class of fish have been around since the Palaeozoic period (250 million years ago approx.), however it is believed that the adaption of the flattered (ray like) body only occurred in the Jurassic period some 100 million years ago. Freshwater stingray history is certainly an interesting topic for the fish keeper of today.

The development of the body shape and the loss of a swim bladder enabled these early rays to hide and lay practically invisible to a predator and to its pray. Rays have developed powerful jaws designed for crushing shells and molluscs which is a primary food source for these animals. Fossil remains prove that little has changed in the anatomy of stingrays over the last 50 million years.


Chondrichthyes (Cartilaginous Fish)


Elasmobranchii (Sharks, rays and skates)


Rajiformes (rays and skates)


Dasyatidae (stingrays)


Potamotrygonidae (river rays)



Freshwater stingray history – How did Stingrays adapt to Freshwater?

There are many theories on how the south american stingray adapted to freshwater to create a new family Potamotrygonidae (river rays).  A longstanding hypothesis suggests that before the Andes mountains were created about 80 million years ago, marine rays from the Pacific lived in and around the coastal areas near the Amazon and became tolerant to brackish water. Over time the Amazon reversed its flow as the Andes were formed.  The reversal of the river meant that stingrays became trapped in freshwater.

Another proposal put forwarded based upon DNA sequencing suggests that stingrays evolved far more recently. The theory suggests that marine rays from the Caribbean infiltrated into rivers draining into the Atlantic Ocean. If this theory is correct, then it suggests that the adaptation of the freshwater stingray could have been as recent as 20 million years ago.

It is not possible to know which theory of freshwater evolution is correct.  However it is extremely likely that the did develop from marine rays. Ray fossils prove that freshwater and marine rays have changed little in the last 50 million years.

Freshwater stingray history – The adaptation to freshwater

Sharks and rays are inherently marine animals simply based on their physiology. Seawater is usually saltier than the blood of most fishes, but instead of actively pumping ions and other solutes out of their bodies like marine bony fishes, elasmobranchs simply match their internal osmotic concentrations to that of their external environment. They do this by maintaining concentrations of organic solutes (namely urea and an enzyme called trimethylamine oxide or TMAO) within their bodies. Although urea is toxic to fish, the TMAO counteracts the protein-de-stabilizing effects of urea. Excess monovalent ions that they ingest, namely sodium and chloride, are eliminated from the body via specialized rectal glands.

The Amazon River

Since Amazonian rays live in freshwater, they have exactly the opposite problem of their marine cousins: instead of losing water to their external environment, they have to worry about gaining it, since their internal osmotic concentrations are higher than that of the water in which they live. One result of this situation is that freshwater stingrays no longer have any need for rectal glands, and these structures are now vestigial (greatly reduced in size and no longer capable of secreting salt).

They have also lost the ability to retain urea, allowing them to sever their ties with the ocean and evolve into exclusively freshwater organisms.

Freshwater stingray history – Taxonomy

The family of Potamotrygonidae belonging to the genus Potamtrygon consist of about 20 species of rays. In 1986 Ricardo Rosa re-assessed the taxonomy of freshwater stingrays.  This can be tricky. Most rays are highly polymorphic. This means that variations is patterns, physical structure and tail length are highly common.

The word Morph describes different looking specimens in the same species. In only a few cases the ray has a constant shape and pattern. A good example of a non-polymorphic species is the Tiger Ray which has a consistent colour, pattern and shape between specimens. On the other hand a great example of a highly polymorphic species is the Motoro stingray. This ray has many pattern variants and shapes within its species.