The saying "happy as a clam" cropped up in America during the early 19th century, especially in the northeastern states. Clams don't have smiling faces – the simile is derived from the longer expression "happy as a clam at high tide." Didn't grow up ocean-side? Here's what it means: Clams are bivalves -- invertebrates sporting a shell divided into two separate sections. They also have a strong foot that burrows into sand.
People head out to go clamming at low tide, when it's much easier to find the buried bivalves. Thus, clams are supposedly happy when it's high tide and they don't have to worry about people trying to pop them out of the sand and turn them into entrees [source: Know Your Phrase].