Barnacles are a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. Typical acorn barnacles develop six hard calcareous plates to surround and protect their bodies. For the rest of their lives, they are cemented to the substrate, using their feathery legs (cirri) to capture plankton. They have four active swimming larval stages.
Free-living barnacles are attached to the substratum by cement glands that form the base of the first pair of antennae; in effect, the animal is fixed upside down by means of its forehead.
Since the intertidal zone periodically desiccates, barnacles are well adapted against water loss. Their calcite shells are impermeable, and they possess two plates which they can slide across their apertures when not feeding.
How could this crustacean be made into a large sea-creature? Given the limited food supply how large could it get?
I like my creatures grounded as it adds that sweet flavour of realism. Details about the necessary adaptations for gigantism and lifestyle are what I'm asking (for those who haven't caught on). They don't have to be crustaceans per se but I am attached to the overall barnacle form. Ideally they'd live near the shoreline to inspire myths and legends.