@ 1978, by the American Society of Limnology and Oceanography, Inc. The herbivorous habits of Mysis dicta in

IIerbivorous feeding by Mysis relicta on three size fractions of natural phytoplankton from Lake Michigan was measured from the change in concentration of chlorophyll a. Mysis fed primarily on the largest size fraction. When offered only the largest size fraction, Mysis frag-mented the material and the fragments were observed as increases in the smallest size frac-tion Diuptomus ashlandi, which could feed only on the smallest size fraction, is a possible recipient of such fragmented cells or chains. A simple compartmental model indicated that filtering rates of Mysis were significantly greater on the largest than on the smallest fraction, and that during feeding 56 % of the largest size fraction was broken up into the smallest size fraction. Mysis may feed on large phytoplankton by migrating at night into a chlorophyll a maximum in the thermocline containing mostly the largest size fraction. Moonlight inhibited the ascent of Mysis at night and may indirectly affect phytoplankton mortality due to mysid grazing. The opossum shrimp Mysis relicta is found in all of the Laurentian Grea