The ancestors of this saltwater snail changed oceans twice.

California horn snails (Cerithideopsis californica) are native to the Pacific Ocean from the California coast to Baja California Sur in Mexico, but a DNA study published in 2011 found they contain traces of a closely related Atlantic snail, Cerithideopsis pliculosa, and vice versa.  The DNA mixing went like this:

• 3 million years ago North and South America joined at Panama, blocking sea travel between the two oceans.
  
• 750,000 years ago, Pacific Ocean snails somehow traveled east to invade the Atlantic.
  
• 72,000 years ago, Atlantic Ocean snails came back to invade the Pacific.

How did the snails cross Central America from one ocean to another?  Twice?

On Throw Back Thursday, find out in this vintage article: Flying Snails