Why is it "as happy as a clam"? [closed]

The earliest in the OED is:

1834 Harvardiana I. 121 That peculiar degree of satisfaction, usually denoted by the phrase ‘as happy as a clam’.

I found a couple of slight antedatings. First from Atkinson's Casket, or Gems of Literature, Wit and Sentiment (Page 571, No. 12, Philadelphia, Decemeber 1833):

On entering, he found the negro in the only dry spot in the house — the chimney corner — as happy as a clam, fiddling most merrily.

On entering, he found the negro in the only dry spot in the house — the chimney corner — as happy as a clam, fiddling most merrily.

Second from a book, The Harpe's Head: A Legend of Kentucky (Page 46, 1833) by James Hall:

He was as happy as a clam. His horses thrived, and his corn yielded famously ; and when his neighbors indignantly repeated their long catalogue of grievances, he quietly responded that King George had never him any harm.

He was as happy as a clam. His horses thrived, and his corn yielded famously ; and when his neighbors indignantly repeated their long catalogue of grievances, he quietly responded that King George had never him any harm.

Finally, not an antedating, but an extract of why clams are happy from a six-page discussion of "Clams!" in The Knickerbocker (Volume 11, March 1838):

only hope Reader have you a sympathy for clams 1 Happy as a clam is an old adage It is not without meaning Your clam enjoys the true otium cum dignitale Ensconced in his mail of proof for defence purely his disposition being no ways bellicose he snugly nestleth in his mucid bed revels in quiescent luxury in the unctuous loam that surroundetb him or with slow and dignified motion worketh nearer the surface as the summer suns warm the roof of his mud palace or sinketh deeper within from the nipping frosts of winter