The acorn barnacle Perforatus perforatus is a common European species that is distributed from southwest England to the Mediterranean and the northwest coast of Africa. Here, we present the first record of invasion of P. perforatus in Japan. This species was found in September, 2012, at a high density on a buoy in Toga Bay in Akita Prefecture, northwest Honshu. It was identified based on the morphological characteristics of the shell and opercular valves, and by molecular analysis using a 300 bp nucleotide sequence of the mitochondrial 12S rRNA. Additionally, a detection and quantification method for P. perforatus using species-specific primer sets for quantitative real-time polymerase chain reaction was developed. Using this method, it was possible to identify and quantify the planktonic nauplius larvae of this alien species.
