Most clams answer danger by pulling in and shutting. The geoduck cannot. Its fused siphon alone can extend more than a metre, and the soft body bulges well beyond the shell margins, so the valves gape permanently. The animal's protection is positional rather than mechanical.
A juvenile burrows down over its first few years and, once settled at depth in stable sediment, stops moving. From then on only the siphon tip reaches the surface to draw in water for filter feeding and to expel it. Predators that can reach that depth are few, which is one reason the species is so long-lived: verified ages exceed 160 years, established by counting annual growth increments in the shell hinge.
The geoduck's reputation rests largely on its appearance and on the high prices it fetches in Asian seafood markets. The biologically interesting part is the trade-off it illustrates. Abandoning the ability to close the shell is only viable because burrowing depth substitutes for armour, and the strategy fails immediately if the animal is dug up.