Sweet Methuselah! Oldest Animals on the Planet

This week, a clam was found to be 500 years old, making it the longest-living animal known. The clam, an Ocean Quahog, was 100 years older than previously thought. Researchers at Bangor University dated the mollusk to a ripe old 507. If you've ever had clam chowder, you've likely eaten flesh from this species -- and that tasty bite may have been several hundred years old.

The scientists studying the clam aren't sure how it lives so long. It appears to have evolved an extremely effective defense against normal damage due to aging. And the study of its tissue could provide a better understanding of the aging process in other animals, including humans.

Ready to see some other extremely old critters? Read on for more long-living species.

Some Bowhead whales, the oldest living mammals, have been found with 19th century ivory and stone harpoons stuck in their flesh, suggesting they survived hunts that occured more than 100 years ago. Scientists examine their eye nucleus and DNA to determine their age, which can extend beyond 200 years.

The whales' secret? Scientists point to a low metabolism rate that helps them stay warm in Arctic waters. Their only predators are humans and Orcas.

The Rougheye Rockfish gives the Bowhead Whale a run for its money. The rougheye is likely the oldest fish on the planet, living as long as 205 years. It's common for the fish to have 2-10 spines along the lower rim of their eyes -- hence the name -- but some don't have these spines.

Researchers are studying the genetic code of the rockfish, but it's unclear for now why the rougheye, which matures and produces late in life, lives so long.

The Red Sea Urchin is another animal that can hit the double century mark. They frequently live longer than 30 years and some have been clocked at 200 years. Scientists have verified their long lifespan using A-bomb radiocarbon testing.

The urchins are found in the Pacific Ocean and are served up in sushi and soups.

They grow very slowly after a growth spurt in its first two years. And they appear to grow -- and mate -- throughout their lives. They show little sign of aging up until the time that something makes a meal out of them.

Galapagos tortoises are known for their massive size. Males can grow up to 6 feet long from head to tail and weigh more than 500 pounds. It takes between 20 and 25 years for the species to reach full size and sexual maturity. Adult tortoises have been known to live more than 150 years, according to the San Diego Zoo.

The tortoises -- they're also the largest in the world -- have a slow metabolism and store large amounts of water. They can go a year without eating or drinking.