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Freshwater or saltwater? The complex evolution of marine animals

The evolution of marine animals is an odd one.  All mammals once lived on land. To be more precise, mammals evolved to live on land, but before that, like all animals, their deep origins are in the ocean.  

It’s a little odd to think of an animal evolving for so long outside the water only to return to the ocean once again.  It doesn’t happen overnight. Some mammals like otters are young to the ocean, maintaining a lot of adaptations such as paws with claws that they had on land.  Otters in particular can move about pretty well on land. According to Otter World, modern otters may only have evolved about 7 million years ago.  From otters it seems a small step to get to seals and sea lions, mammals that can still walk around on land but are less graceful.  Otters look like they belong on land as well as at sea, or in some cases freshwater.

Pinnipeds, which are comprised of walrus, seals and sea lions, likely evolved from a common ancestor adept at living both on land and at sea, diverging from their closest relatives the bears and mustelids about 50 million years ago.  Whales and dolphins similarly evolved from land mammals in the distant past.

What’s even more interesting about the evolution of whole groups of animals is not only their evolutionary path from water to land and back to water, but also, the evolution from marine environments to freshwater environments, places where the water is unsalted but the animals seem decidedly marine.  

The Amazon River ecosystem is a perfect example of something that resembles a giant inland, freshwater ocean.  There are dolphins, manatees, stingrays and 200 pound turtles that lay eggs on the river beaches.

The stingrays of Amazonia are hard to believe when you see them in photographs, as they appear to be fish designed by butterflies.  The rays are patterned with delicate spots, sometimes vivid, sometimes subtle, but nearly always beautiful. The freshwater stingrays of Latin America are all in the family Potamotrygonidae.  Freshwater stingrays found in Africa, Asia and Australia are found in the family Dasyatidae. The Amazon stingrays as well as the (Dasyatidae family) clearly evolved from marine stingrays. It’s unknown whether the Potamotrygonids evolved from a species from the Pacific or the Atlantic as there is some evidence in favor of each, however they’ve done well in freshwater, evolving into at least 22 species throughout Latin America.  

River dolphins are the next obvious example of ocean animals that have taken to the Amazon River system.  Amazon River dolphins are either gray or a pretty color of pink and have a unique, strangely boxy shape (for a dolphin).  The boto, as river dolphins are known in Brazil, has a place in local lore. Botos are said to turn into attractive men at night who seduce village women on the river banks before resuming their dolphin form and slipping back into the river.  Native peoples sometimes play games of tag with river dolphins with paddles, from their canoes. Another myth says that the river dolphin is the protector of the Amazon River Manatee. Throughout the world there are five species of living river dolphins and two subspecies.  Some of these river dolphins also live in brackish water, such as river deltas where ocean and freshwater mix. All of these dolphins evolved from ancestors that lived entirely at sea.

The Amazon River Manatee lives entirely in freshwater.  The Florida Manatee shifts from freshwater rivers to the ocean and back again.  African Manatees also live in separate populations both on the coast and inland.  It’s believed that manatees share a common ancestor with elephants.

The Amazonian Manatee is the smallest undisputed species of manatee and seems to be the only living entirely in freshwater, in this way it matches the Amazon River Dolphin.  We don’t have any clear population numbers on the Amazonian Manatee but it appears their numbers are declining.

The Amazon also includes lobos de rio, that is Giant River Otters.  The Giant Otter is the largest member of its family growing to as long as 5.6 feet.  The otter eats mostly fish but will also prey on turtles and even small caimans (a crocodilian).  The name lobo de rio in Spanish means ‘wolf of the river’ and the name seems apt for a smart, social predator like the giant river otter.  

It may seem like a small feat to change from a life in salt water to one in freshwater or visa versa, but it includes a host of adaptations.  Salt water can kill if an animal has no way to purge the excess mineral. Some sea birds weep extra salt from pores on their beaks. Bony fish constantly pump salt from their bodies through their kidneys as well as through special gill cells.  These are all adaptations that are costly and are usually lost when an organism turns to life in freshwater. The evolution of any organism is incredibly complex, full of twists and turns. It’s helpful to remember we all share one common ancestor way back in the distant past. There may be species that evolved more recently than others, but we all started trying to find our way on this planet at the same time.           

By Zach Fitzner, Contributing Writer

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