Many years ago, I saw a Japanese bronze box in the local art&craft museum. The lid was decorated with a turtle that carried a train of seaweeds. The turtle is thought to be 1000 years old. This illustrates a Buddhist metaphore: It is as rare to be reborn as a human being as the chance of a blind turtle that dives up to the sea surface once in 1000 years and by chance sticks his head through a wooden yoke that is swimming on the sea. Or you can say: It is important to use the precious situation of being human to be good.
This is the deeper meaning. On a more common level, it is a symbol for long life. This is today's pattern.