Shiripuno Wildlife Series: Freshwater Sting Ray

There are many different theories as to the origins of South America's freshwater stingrays. We do know that they are most closely related to the genus Himantura (family Dasyatidae), which has representatives in both the Caribbean and the Pacific, so it is difficult to determine exactly when or where the progenitors of the Potamotrygonids entered what would become the Amazon basin.

The evolutionary history of the Amazon's fishes is as complex as the numerous geological events that forged the South American continent. During the early part of the Cretaceous period, the river flowed into the Pacific Ocean rather than the Atlantic, and like many present-day marine fish, ancient stingrays most likely made frequent trips into freshwater and then back to the ocean. However, the formation of the Andes mountain system around 90 mya eventually cut off the route to the Pacific and caused the river to flow in the other direction, towards the east. Over the years, some evolutionary biologists have postulated that the stingrays inhabiting the river became trapped and either died or adapted and moved further upstream, where they evolved independently of their saltwater relatives.

However, new evidence derived from molecular techniques including mitochondrial DNA analysis suggests that the Potamotrygonids diverged from their saltwater ancestors much later than was previously believed, sometime during the Miocene between 12 and 22 mya. In addition, the incursion appears to have come from the eastern coast of the continent, which contradicts earlier speculations (Lovejoy, Bermingham & Martin, 1998).

Taxonomy

The majority of stingrays in the family Potamotrygonidae belong to the genus Potamotrygon, which consists of approximately 20 known species, and many more that are still either undiscovered or unclassified. The other two genera, Paratrygon and Plesiotrygon (antenna rays), are both represented by only one species.

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[jmack633]

I'm glad I didn't step on one of those when I visted the Shiripuno river!