The name comes from barnacles attaching to a whale. When a query returns big platforms you can't outrank directly, you instead get your business featured on those platforms: a strong profile on a directory, a top-reviewed listing, a well-optimised marketplace page or a ranking YouTube video. You borrow their authority to occupy the results.
It's especially useful for local businesses and e-commerce, and it overlaps with marketplace SEO and Amazon SEO — a well-optimised Amazon or Allegro listing is a barnacle on a giant. It also complements, rather than replaces, building your own backlink profile and domain authority.
The trade-off is control: you're building on rented land. Use barnacle SEO to capture visibility quickly and reach buyers where they already are, while still investing in owned assets so your long-term equity isn't locked inside someone else's platform.