From: http://www.bbc.co.uk/worldservice/learningenglish/language/2014/04/140401_todays_phrase_clam_up.shtml
A giant clam on the seabed near Zanzibar
Today's Phrase
To clam up means to become silent because you are nervous, embarrassed or don't want to talk about something.
Examples:
Adam was very naughty and ate all the cookies. When his mother asked where they were he clammed up and couldn't tell her.
When I was young I used to clam up when a stranger asked me a question.
Take note
Another phrase which comes from a shellfish is the world is your oyster. When you say this to someone you mean they have the ability to go and to do whatever they like.
Example:
Now that you have your college degree, the world is your oyster!
Interesting fact
A
clam found off the coast of Iceland in 2006 is thought to have been the
longest-lived mollusc ever discovered. Scientists, who named it Ming,
believe it was over 500 years old when taken from the seabed.