Contribution of food was examined in a laboratory tracer experiment as to the accumulation of ¹³⁷Cs, ⁵⁴Mn, ⁶⁵Zn and ⁶⁰Co by sea urchin Strongylocentrotus nudus. Concentration factors of sea urchin from sea water were estimated to attain 320, 66, 11 and 4 for ⁶⁵Zn, ⁶⁰Co, ⁵⁴Mn and ¹³⁷Cs respectively. While, concentration factors through food were calculated to be 1200, 1100, 270 and 4 for ⁶⁵Zn, ⁵⁴Mn, ⁶⁰Co and ¹³⁷Cs respectively, when Ulva pertusa was used as food. And other food such as Eisenia bicyclis, Chondrus ocellatus and Tapes japonica were also recognized to contribute much to the accumulation of the radionuclides by the organism. Furthermore, the concentration factors through food for sea urchin proved to be changed widely by kinds of food.
Distribution of each radionuclide to shell and spine of sea urchin was different between the uptake pathways, but those to other tissues and organs were considerably similar to each other regardless of the uptake gathways. Distribution to digestive tract and gonad shows a temporal decreasing and increasing change common to both pathways respectively. Therefore, each radionuclide is estimated to be metabolized similarly by sea urchin regardless of the uptake pathways.