Ingestion of mesozooplanktonby three species of bivalve; Mytilusedulis,Cerastodermaedule and Aequipectenopercularis Clare Lehane* andJohn Davenport DepartmentofZoologyandAnimalEcology&EnvironmentResearchInstitute,UniversityCollegeCork,LeeMaltings, ProspectRow,Co.Cork,Ireland.*E-mail:clarelehane@hotmail.com Mytilusedulis, Cerastodermaedule and Aequipectenopercularis werefoundtoingestzooplanktonwhensusp- ended in mesh cages in the water column in the Firth of Clyde. Zooplankters were also found in the stomachs of bivalves that had been taken directly from their natural habitat.The bivalves consumed a wide range of zooplankton species, but selectively consumed smaller categories of zooplankton present. Condition of zooplankton in the stomachs of the bivalves varied with zooplankton species. A degree of larviphagywasevident,particularlyin Mytilusedulis. INTRODUCTION Bivalves are generally described as herbivorous and if present in su/cient numbers e.g. in mussel beds, may controltheabundancesofprimaryproducers(Norenetal., 1999). However, since 1882 researchers have reported ¢nding ‘minute animals’ (Savage, 1925) either in the stomachcontents or the excreta of bivalves. Ryder (1882) examined the stomach contents of American oysters ( Crassostrea virginica) and found them to contain‘diatoms, rhizopods, infusoria, monads, spores of algae, pollen grains, oyster larvae, worms, crustacean nauplii, minute crustacea, larvae of sponges, hydroids, hydrozoa, worms and molluscs’ (reported in Savage, 1925). In his own dietary studies, Savage (1925) found nauplius larvae of copepods and adult harpacticoid copepods in the stomach contents of oysters. Nelson (1933) reported that one oyster Ostrea edulis stomach contained 160 live nematodes, 471 dead but intact specimens, and 5842 partially or wholly digested worms, all Chromadora spp. He postulated that the acquisition of protein from the nematodes was important to the oysters after they had spawned.Cowdenetal.(1984)foundthat Mytilusedulis L. inlaboratorysettingsingestedlarvaeofpolychaetes,aster- oids, gastropods, and echinoids. Kimmerer et al. (1994) stated that, within one year of the introduction of the clam Potamocorbula amurensis into the San Francisco Bay estuary, the abundances of adults of three common estuarine copepod species had declined ¢ve to ten fold. Krsinic (1980) foundthat tintinnines were an important factor in the nutrition of oysters in the Adriatic Sea. Similarly, while working in the same area as Krsinic, Jasprica(1997)foundthatmussels( Mytilusgalloprovincialis ) alsoingestedtintinnines. However,withtheexceptionofKimmereretal.(1994), allofthesereportsofbivalvesingestingzooplanktonhave beenconcernedwithmicrozooplankton.Inamorerecent study,Davenportetal.(2000)foundthat M.edulis, when hunginmeshbasketsfromapieringestedmesozooplankton includingnematodes,polychaetes,amphipodsupto6mm and Carcinusmaenas zoeaeof2mmlength. This study compares the species and sizes of zoo- plankton ingested by mussels suspended in the water column with those species present in the water column at time of sampling. In addition, benthic cages were used to determine whether mussels ingested benthic animals.Two other species of bivalves, Cerastoderma edule (L.) and Aequipecten opercularis (L.) were investigated for zooplankton ingested, both in the ¢eld and under manipulatedconditions. MATERIALSANDMETHODS Bivalvecollection Three bivalve species were considered for this study because of their relatively common occurrences around Great Cumbrae Island, Scotland (55.468N 4.558W) in May 2001. Mytilus edulis were collected fromWhite Bay. Collectionsweremadeatlowtide.Byssusthreadswerecut with scissorsto avoid damage to the mussels which were transported to the University Marine Biological Station Millport, (UMBSM) and maintained in running un- ¢ltered seawater overnight. Aequipecten opercularis were collected by dredging o¡ White Bay at a depth of 30m usinga1-mdredgetowedbyRV‘Aplysia’.Eighteenscallops were each injected with 10ml 70% alcohol as soon as collected to preserve stomach contents, while a further18 were maintained in seawater and returned to UMBSM. Cerastodermaedule were gathered at low tide from Balloch- martin Bay at low tide. Cockles and queen scallops were kept in un¢ltered seawater until use later on the date of collection. Experimentalarrangementsandprotocol Bivalves were removed fromthe running seawater and suspended in plastic coated wire mesh cages from the UMBSM pier for a period of two to four hours for mussels andcockles and overnight for the queen scallops beforesamplingforstomachcontents(seeTable1forcage dimensions). Davenport et al. (2000) found that gastric J.Mar.Biol.Ass.U.K. (2002), 82,615^619 PrintedintheUnitedKingdom JournaloftheMarineBiologicalAssociationoftheUnitedKingdom(2002)