Fish facts

Do you know how they figure this out?

Now, this is interesting...

I know they figure it out by the shell but not the outer shell.
I believe it has something to do with the inner layers of the shell.
They can also figure out things about the water like salinity and more
by studying the shell.

Thankfully I won't have to do this...

The age of the clam was later estimated to be 507 years old.

Clamshells can tell about climate changes and the age of a clam.
The shell of a clam does more than providing a home for mollusks. Clam shells tell about climate changes, seawater salinity and the age of a clam. With the right tools, a layperson can tell the age of a clam by observing the shell it once lived inside.

Know that size doesn’t matter. The oldest known clam was found in 2007 off the coast of Iceland. The 405-year-old clam had a shell less than 6 inches wide.

Open the clam. Unfortunately, this will kill a live clam. The rings on the outside of the clam are not accurate indicators of age. Internal growth patterns are more accurate.

Use a rock-cutting saw to cut the shell, allowing you to view a cross section of the clam shell. You may need a microscope to get an accurate view.

Look for a pattern of one large white band alternating with a thinner dark band. Each occurrence of a white band with a dark band indicates one annual cycle. The darker band was once the outside of the shell.

Count the pattern. Every white band alternating with a dark band indicates one calendar year. The total number of times this pattern appears in the cross section of your clam shell is the age of the clam in years.

http://sciencing.com/tell-age-clam-8726823.html