Suspension-feeding bivalves, such as mussels and oysters, are abundant species in many coastal and estuarine waters and are also cultured and harvested for human consumption. The bivalves have a large filtration capacity and are able to accumulate high numbers of microorganisms and are therefore frequently involved in food-borne diseases. To develop monitoring programmes, a lot of information is needed to understand the uptake, retention time and depuration routes for different compounds. The individual variances of the contents of both alga-toxins and pathogenic microbes in bivalves are well known but good explanations for these observations remain. The aim of this study was to develop a laboratory model that can be used to quantify the individual variation in uptake and transit time for particles in the bivalves. The principle of the method is to use particles labelled with a gamma-radionuclide which is taken up by the organism through its normal feeding route and during the experiment visualise the amount of particles and their distribution within the mussel. By outlining the region of interest of the image displayed, the values of radio-activity from the certain area will be measured. In this study, the mussels were given so called microspheres, (non-degradable latex particles, 15 µm in size, labelled with 57Co). The region outlined to be measured was the stomach-area. The accumulation of radioactivity in this region was calculated as uptake and the reduction was calculated as the elimination. The uptake of the microspheres was shown to be comparable among the 21 mussels included in the study, while the variance of the elimination between individuals was high. In order to investigate the variances within individuals, a number of four mussels were randomly chosen for repetitive measurements of uptake and elimination. The temporary variance of the transit-time within a mussel was shown to be as high as the variance between individuals. Consequently, the individual variances, in terms of alga-toxins or harmful microbes in mussels, could be explained by the arrhythmic activity in the digestive glands. The experiment was carried out independent of diurnal and environmental conditions like temperature, salinity and food availability, indicating that neither of these factors was predominating in regulating the activity.

The study showed that non-degradable labelled particles could be used as markers for uptake and transit times as stated above. The method is non-destructive and thus allows you to do repeated measurements on the same individual. The gamma camera technique could also be used to study biodistribution of radio-labelled organisms such as phytoplankton and bacteria. Particles of various sizes and shapes could be used and two different particles labelled with non-interactive radio-nuclides could be measured in parallel.