Without claiming any expertise in this field, there are two possible explanations related to the symbol of tortoise in Japan:
In Japanese tales and legends turtles are usually said to live "ten thousand years", thus they represent longevity.
Don't get me wrong, but "turtle head" (kitou) is a strictly medical term referring to male genitalia. It is of course also used in popular slang.[/list=1]
Konnichiwa thomas-san!
Yes, turtle is a symbol of long life.
And a soft shelled turtle is a symbol of persistent man(or unpleasant fellow).
There is a proverb "Tsuki to suppon" in Japan.
"Tsuki" is the moon and is a synonym for beautiful.
"to" means "And".
"Suppon" is a soft shelled turtle. Of course "Suppon" means "dirty".
"Tsuki to suppon" is "Moon and soft shelled turtle" in literal translation but means "It is as different as day and night".