Help to ID this on my hermit crab.

[havoc17]

Tonight I noticed on my largest hermit crab's shell, 3 round barnacle looking items. When the hermit crab walks around the tank, the items on its shell extend a feather looking hand and seem to filter feed. I have no idea where this shell came from. All of the hermit crabs that I have bought have been small and I just assumed this one grew faster than the rest.
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[tbub1221]

That is indeed a common barnacle , they can spread usually in to darker areas of the tank under rock etc .
I'd bet a crab just recently found that shell and it was more than likely vacant a wile before .