Clams can live up to 240 years off the coast of Maine and up to 500 years in the North Atlantic near Iceland, where waters are cooler.
During that time, they act as sensors, recording important information about the ocean's history.
Every year, clams add another layer to their shells - much like a tree adds rings to its trunk - in a process called annual shell increments.
A team from Iowa State University measured the increments of 29 clam shells in millionths of a metre, looking for fragments of carbon (which denotes colder ocean temperatures) and oxygen isotopes.
With the help of the clams, scientists were able to look 1,357 years into the ocean's past -- the longest continuous record of its kind to date.
The findings were published in the June, 2012 issue of Nature, but researchers say their work is far from over.
By looking into the past, scientists hope to gain a better understanding of current environmental trends.