Scuttling is a wonderfully multilayered word in this poem.  To "scuttle" is to scurry, a sense of the word that accords with the awkwardness and vulnerability of the crab.  On the other other hand, to "scuttle" can also mean to sink a ship by putting a hole in it.  Since the crab's claws are compared to "scuttling works of armament," this meaning is also implied, and emphasizes the strength rather than weakness of the crab.  But since, as we discover, it is the crab who is scuttled--by the gull--rather than doing the scuttling, we are finally drawn again toward the crab's weakness.  See strength and vulnerability.