An improbable opportunistic predator: the functional morphology ofPinna nobilis(Bivalvia: Pterioida: Pinnidae)

Abstract

The functional morphology ofPinna nobilisis described, with special reference to the uniquely pinnid pallial organ, the similarly unique buccal (formerly pallial) gland, the stomach and its contents. The pallial gland produces sulphuric acid which as well as functioning as a shell cleaning swab may be involved in prey capture. The buccal gland discharging into the oesophagus has proteolytic digestive functions while the stomach is adapted for the reception and digestion of captured, mucous-bound, mesozooplanktonic and epi- and endo-benthic, prey items.Pinna nobilisis thus not simply either an accidental or incidental predator of such species but is opportunistic. The buccal glands and stomachs of other Pinnidae are not so specialized as inP. nobilis,possibly indicating that in the particular, oligotrophic, environment of the Mediterranean and Adriatic Seas, and in which it is endemic,P. nobilishas, in addition to being a typical ctenidial suspension feeder, become an opportunistic predator. Although the deep sea representatives of the Septibranchia (Anomalodesmata) and Propeamussidae are obligate predators, this is the first record of any bivalve functioning as an opportunistic predator with unique morphological adaptations to facilitate this.