Abstract
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 super(75)Se and super(241)Am. The diatoms had concentrated selenite super(75)Se 1.1 x 10 super(5) times and super(241)Am 2.9 x 10 super(5) times out of seawater. Mass balance calculations indicated that only about 1% of the ingested super(241)Am 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 super(75)Se was determined by relating its activity to that of super(241)Am in the food and fecal pellets of the copepod. The super(75)Se 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 plus or minus 1.5% over a series of experiments, and remained constant over feeding periods ranging from 6 to 49.5 h.