Four new alpha-pyrone-containing propionates (5-8) and an unprecedented hydroperoxide 9 have been isolated from the mantle extract of Placida dendritica, a Mediterranean sacoglossan that lives upon the green alga Bryopsis plumosa. The new metabolites co-occur with the related compounds 1-4, which have been described in previous studies of the mollusc. The presence of 9 opens intriguing perspectives on the ecological role of placidenes. This paper reports the isolation and structural elucidation of the new compounds 5-9.