His body is irrelevant. Cthulhu and his kin are not actually made of anything material, as we know it.
"It was curious to note from the pictured battles that both the Cthulhu spawn and the Mi-Go seem to have been composed of matter more widely different from that which we know than was the substance of the Old Ones. They were able to undergo transformations and reintegrations impossible for their adversaries, and seem therefore to have originally come from even remoter gulfs of the cosmic space. The Old Ones, but for their abnormal toughness and peculiar vital properties, were strictly material, and must have had their absolute origin within the known space-time continuum - whereas the first sources of the other beings can only be guessed at with bated breath."
"These Great Old Ones, Castro continued, were not composed altogether of flesh and blood. They had shape - for did not this star-fashioned image prove it? - but that shape was not made of matter. When the stars were right, They could plunge from world to world through the sky; but when the stars were wrong, They could not live. But although They no longer lived, They would never really die. They all lay in stone houses in Their great city of R'lyeh, preserved by the spells of mighty Cthulhu for a glorious surrection when the stars and the earth might once more be ready for Them. But at that time some force from outside must serve to liberate Their bodies. The spells that preserved them intact likewise prevented Them from making an initial move, and They could only lie awake in the dark and think whilst uncounted millions of years rolled by. They knew all that was occurring in the universe, for Their mode of speech was transmitted thought. Even now They talked in Their tombs."
This is not surprising, considering Cthulhu is older than our universe. What he's made of is in pretty much no way linked to matter or the cosmos as we know it.
"The Thing cannot be described—there is no language for such abysms of shrieking and immemorial lunacy, such eldritch contradictions of all matter, force, and cosmic order. A mountain walked or stumbled. God! What wonder that across the earth a great architect went mad, and poor Wilcox raved with fever in that telepathic instant? The Thing of the idols, the green, sticky spawn of the stars, had awaked to claim his own. The stars were right again, and what an age-old cult had failed to do by design, a band of innocent sailors had done by accident. After vigintillions of years great Cthulhu was loose again, and ravening for delight."
