I'm no expert I just play one on TV. I'll relate what I do know from my childhood in Massachusets and fooling with many a barnacle.

- They are actually small crustaceans not molluscs

- They are encrusting obviously

- Seem to be only found in areas of strong currents and many species can survive exposed at low tide for many hours.

- Filter feeders , have a feathery type appendage that comes out with each passing wave.

- They hurt like hell when you scrape yourself on em !

If you want more info google would probably be your best bet.