Yesterday I found this creature in a rocky cove in central California, around mid tide (+ 3 feet). It's about 3-4 inches long. At first I thought it was dead, but it closed its mouth when I removed the tiny white pebble, and rotated the operculum rapidly when I poked the bit of exposed yellowish flesh underneath. I think it's some sort of acorn barnacle, but haven't been able to identify a species with the white shell and distinctive red-and-white color and texture differences on the opercular plates. What could it be? Barnacle photo 1

Barnacle photo 2


I think it's a thatched barnacle (Semibalanus cariosus). The iNaturalist app would also be a helpful tool in which to ask this question.

  • 2
    $\begingroup$ Welcome to the site. This is a good answer, but can you tell us why you think it is the thatched barnacle, and perhaps provide some links with information about these animals? It would really improve your answer. $\endgroup$
    – bob1
    Dec 22 '21 at 19:45

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