Answer:
Tall when seated, short when standing.
Edible beach crawler served at seafood restaurants, similar to a lobster, but quite different.
From that which comes within itself, It builds its table on my shelf.
He's big, old and fluffy, And looking rather scruffy. He always needs a shave. It's best to just avoid him, And leave him in his cave
It can be white or blue or black, Humped or have a horn. And when it talks it sings its menuets, as it's travelling the depths.
Lighter than what I am made of, more of me is hidden than is seen. What am I?