Why do adult barnacles stay glued in one place for life?

Answer: Staying fixed lets them filter drifting food from moving water

Staying fixed lets them hide from roaming fish and crabs — Wrong. Barnacles do have hard shells that protect them, so defense is part of the picture. But permanent attachment is chiefly about staying in moving water to filter plankton, not about hiding from mobile predators. A glued-down barnacle is not concealed; it is parked where food drifts past.

Staying fixed lets them save energy for making eggs — Wrong. Staying put can save energy, and making eggs does take reserves, but that is a side benefit rather than the main reason barnacles glue themselves down. Their permanent holdfast keeps them in current-rich spots where they can extend feathery legs and catch plankton. Energy saving does not explain why the glue forms for life.

Staying fixed lets them filter drifting food from moving water ✓ — Correct! Adult barnacles secrete a very strong natural glue and stay fixed to rocks, ship hulls, and whales. In that fixed spot they extend feathery legs called cirri to filter plankton from passing water, and current-rich places bring the most food. The permanent attachment is mainly a feeding strategy, not just energy saving or hiding.