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This short communication represents the first published record of Megabalanus coccopoma from North America. Species identification was confirmed by discovery of opercular plates in the original collection. Tergum is particularly important here, as it identified the Louisiana population as originating on the west coast of Panama, around the Canal. A geographic variant of M. coccopoma in that area has a tergum with its spur placed twice its own width from the scutal margin, and in its closed spur furrow.
It is always a good idea to collect and study opercular plates when identifying balanomorph barnacles. Shell morphlogy follows function, and anatomical characters will identify subfamily and genus groups. Operculars are necessary for positive species determination. Megabalaninae shells are remarkably similar in structure, and in crowded conditions, can be confusing. Coloration can be helpful, as it is in M. coccopoma, but is of secondary importance in identification.