Natural Food Availability and Assimilation in the Bivalve Choromytilus meridionalis

Samples of seawater overlying a bed of the mussel Chorornytilus meridionalis (Kr.) at Bailey's Cottage, False Bay, South Africa, were analysed for organic and inorganic content in the particle-size ranges 2-100 pm and 100-200 pm diameter.Organic matter comprised 10-30 % of total sample weight, the remainder being predominantly sand.Changes in weight of particulate matter in samples taken throughout the year showed no clear seasonal pattern.Particulate organic material considered available as food to the nlussels averaged 2.65 mg I-'.Particulate inorganic matter averaged twice this amount.The energy value of the organic material averaged 6.1 kJ g-l.Phytoplankton was present in 34 % of the samples and is not cons~dered an important constituent in the mussels' diet.The assimilation efficiency of mussels feeding on natural detritus averaged 40 % over the ration levels 3-18 mg I-' dry weight of particulate matter.However, assimilation efficiency on pure Dunaliella primolecta culture declined to zero at a ration of 3 mg 1-' while pseudofaeces production was initiated at 6 mg I-'.It was concluded that the presence of particulate inorganic matter may be an important dietary requirement, 'diluting' the food and enabling assimilation to continue at the high ration levels experienced in the field.