UTILIZATION OF FRESH DETRITUS DERIVED FROM CYSTOSEIRA-MEDITERRANEA AND POSIDONIA-OCEANICA BY THE DEPOSIT-FEEDING BIVALVE ABRA-OVATA

Ingestion rates and absorption efficiencies of the deposit-feeding bivalve, Abra ovata, fed on C-14-labelled detritus were calculated. The detritus were freshly derived from two macrophytes: Cystoseira mediterranea and Posidonia oceanica and radiolabelled with C-14-formaldehyde. Compartmental analysis was coupled with analog modelling of the exchanges of radioactivity within the studied systems to compute ingestion rates and absorption efficiencies. Detritus derived from C. mediterranea was ingested at a rate less than that derived from P. oceanica (8 x 10(-4) mg Organic matter (OM).mg-1.h-1 vs 1.6 x 10(-2) mg OM.mg-1.h-1, respectively, for the 50 mg experiments). Ingestion rates of detritus derived from C. mediterranea increased linearly (IR = 1.62(-5) x [OM quantity]) with food concentration whereas ingestion rates of detritus derived from P. oceanica seemed to reach an upper limit within the range of tested concentrations (10-50 mg OM.300 ml-1). Absorption efficiency of the detritus derived from P. oceanica was rather low (about 2.5%). Absorption efficiency of the detritus derived from C. mediterranea was much more difficult to assess due to low ingestion rate. These results are discussed in the context of comparing utilization rates (i.e. ingestion rates and absorption efficiencies) of different food sources by benthic deposit-feeders. Because of differences among forms of detritus in the relationship between food concentration and ingestion, it is concluded that such comparisons should be conducted on a wide range of food concentrations.