Balanus trigonus, the triangle barnacle, is a species of barnacle in the family Balanidae. They are steep-sided, conical creatures, have six shell plates and are dark pink in colour. Originally found only in the Indo-Pacific and along the eastern Pacific coast, they were likely introduced to the Atlantic Ocean in the 19th century through human activity and now has a global distribution. Usually living on subtidal rocks and shells, they are also foulers of ships and dock infrastructure, and, in areas where they are invasive, may compete with native species for living room. The species was first described in the genus Balanus by Charles Darwin. It has since had its mitochondrial genome sequenced twice, with slightly differing results, leaving its phylogenetic position unsolved.

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