ASSIMILATION OF SELENIUM IN THE MARINE COPEPOD ACARTIA-TONSA STUDIED WITH A RADIOTRACER RATIO METHOD

Assimilation of selenium in the marine copepod Acartia tonsa, feeding on the diatom Thalassiosira pseudonana, was studied using a new dual-label radiotracer ratio method. Copepods were fed diatoms labeled with the gamma-emitting isotopes Se-75 and Am-241. The diatoms had concentrated selenite Se-75 1.1 x 10(5) times and Am-241 2.9 x 10(5) times out of seawater. Mass balance calculations indicated that only about 1% of the ingested Am-241 was retained in the copepods after gut evacuation, suggesting this isotope as an excellent tracer of bulk ingested material. The assimilation efficiency of the ingested Se-75 was determined by relating its activity to that of Am-241 in the food and fecal pellets of the copepod. The Se-75 concentration in the feces was consistently lower than that in the food by over an order of magnitude. The assimilation efficiency of selenium in A. tonsa was found to be 97.1 +/- 1.5% over a series of experiments, and remained constant over feeding periods ranging from 6 to 49.5 h. Mass balance calculations of selenium assimilation efficiency gave comparable values to those generated with the ratio method. The high assimilation efficiency of selenium in planktonic animals may account for its relatively long residence time in surface waters. The application of a dual gamma-emitting radiotracer ratio method for determining assimilation efficiencies of ingested elements could be readily extended to many other elements and animals.