Found mention of this on reddit and on PhysOrg. Researchers from Bangor University dredged up a clam that appears to be 400 years old, making it the oldest living thing ever discovered.
A team of scientists from Bangor University’s School of Ocean Sciences believe they have found an animal which did just that, a quahog clam, Arctica islandica, which was living and growing on the seabed in the cold waters off the north coast of Iceland for around 400 years.
Of course, they found this out by killing it and counting rings in its shell.
It is very likely that longer lived individuals of the species remain to be found. Although Icelandic waters seem to provide the ideal conditions for extreme longevity, clams with lifetimes well in excess of 200 years have been found both in the Irish Sea and the North Sea.