The large, edible, and near-cosmopolitan brown mussel Perna perna (Linné, 1758) has a history of few and infrequent occurrences in Portugal since the Atlantic “climatic optimum” of the Holocene. Moreover, it is likely that some previous citations of Mytilus spp. from other Holocene deposits, including kitchen middens, may represent true Perna specimens. The species was recently found by Lourenço et al. (2012) in the south Portuguese coastal localities of Ilha do Farol and Vila Moura. Here, we summarize these and other previously published occurrences of P. perna in the areas of Armação de Pêra, Lagos and Aljezur, discussing morphologic, ecologic and biogeographic aspects of this species in the Portuguese fauna. The brown mussel is also known from archaeological records from Padrão I (Sagres – Vila do Bispo, Ancient Neolithic) and Arrifana (Aljezur, XII century). These chronologies are contemporaneous with the climatic warming intervals of the Atlantic Period (circa 8000-5000 BP) and “Medieval Warm Period” (circa 1000-1200 AD). During these and other related intervals of warmer coastal surface waters, it is very likely that this subtropical “warm guest” colonized open marine rocky areas of the South and Southwest Portuguese coast, with settlement of stable and permanent populations.
