In 1998 experimental biologist Peter Fong found clams so depressing that he fed them Prozac -- apparently, in an effort to determine the creatures' level of cognizance. Within a few hours, the clams were “[spewing] sperm and eggs all over the place.” Despite catalyzing an asexual orgy, Fong found no conclusive evidence that clams processed any awareness or emotion. The study did, however, have a disquieting outcome for clams: it provided clam farmers with a more convenient way to breed and harvest them for consumption. Even antidepressants knocked the poor clam on its proverbial ass.

If you’re still convinced that the clam is the echelon of happiness, consider that millions of other creatures externally exude more convincing signs of joy. We offer you photographic evidence:

So... Are clams happy?

Who knows, but the article is definitely worth reading:

Tracing the precise origin of an idiom is an unrewarding endeavor, often resulting in little more than speculation. “Happy as a clam” is no exception. Who found clams so jovial to begin with is anyone’s guess -- though we do know that the phrase is a shortened version of a saying that was thrown around Northeastern harbors at the turn of the 17th century.

Most clams live and reproduce in shallow ocean waters. At low tide (when the ocean recedes furthest from the shore), clams are exposed and prone to humans and other predators snatching them up. Conversely, at high tide, they are “safe,” and therefore happy. Historians generally agree that sailors and shellfish gatherers began using the idiom “happy as a clam in the mud at high tide” (or “in high water”) by the mid-1600s, to describe feelings of euphoria. 

The shortened, modern version of the idiom (“happy as a clam”) didn’t surface until centuries later, in 1833. On page 571 of Atkinson's Casket, or Gems of Literature, Wit and Sentiment (a collection of philosophy, history, and terrible jokes), it was used to describe the unbridled merriment of a fiddle-playing slave:

A few months later, also in 1833, the phrase fond its way intoThe Harpe's Head: A Legend of Kentucky, a frontier memoir. Here, is was used more appropriately, to relate the joy of a prosperous farmer:

The Harpe's Head: A Legend of Kentucky (James Hall, 1833)

By the mid-1830s, the idiom had been recognized by writers and etymologists as a commonly enlisted descriptor for happiness and pleasure. For instance, an 1834 volume ofHarvardiana, an old Harvard University journal, notes that “the phrase ‘happy as a clam,’ widely used, usually denotes a peculiar degree of satisfaction.” In 1838, New York literary magazine The Knickerbocker elaborated on exactly what it is that makes a clam so happy:

The Knickerbocker (March 1838)

“Happy as a clam in the mud at high tide” is a sweet phrase.

"...as the summer suns warm the roof of his mud-palace" sounds pretty happy to me.

A mud palace is a happy place.