THE ANIMAL FILES

Pacific acorn barnacle

Balanus glandula

A fixed shell begins with a drifting larva.

Pacific acorn barnacles attach to intertidal surfaces along western North America. Their plated shells can cover rocks, pilings and other animals, but attachment follows a free-living larval stage. Predators use different methods to get through the adult's hard covering.[1]

Follow the evidence ↓
Pacific acorn barnacle
Image: DOI (Department of the Interior) · Commons · Public domain
A crowded surface
Shells can grow on rocks or other animals.[1]
From drift to attachment
Larvae attach before developing the adult shell.[1]
A shell under attack
Whelks and sea stars use different attacks.[1]

A crowded surface

Upper intertidal rocks, wood and pier pilings are common substrates. Other barnacles, molluscs and crustaceans can also carry attached individuals. A host surface in this sense is a place to settle, not evidence that the barnacle is feeding on that animal's tissues.[1]

From drift to attachment

Free-floating larvae settle on a suitable surface and cement themselves down. Metamorphosis then produces a juvenile with the hard covering. Adult immobility therefore does not describe the animal's entire life or the way new sites are colonised.[1]

A shell under attack

Dog whelks can drill through the shell, while sea stars pull plates apart and extend their stomach into the opening. The calcareous covering resists access, but these predators open it by different means.[1]

A COMMON MIX-UP

Is a barnacle attached to a mussel necessarily parasitising it?

No. Attachment identifies the substrate; it does not establish feeding on the mussel.[1]

TRY A LITTLE REASONING

How can new bare surfaces acquire barnacles if adults stay fixed?

Reveal the explanation

Dispersing larvae reach the surface, settle and metamorphose there.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Pacific acorn barnacle (cited revision)
  2. NCBI Taxonomy — Balanus glandula

Last fact check:

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