POTENTIAL DIGESTIVE FUNCTION OF BACTERIA IN KRILL EUPHAUSIA-SUPERBA STOMACH

Evidence is presented that bacteria in the stomach of live Antarctic k rill Euphausia superba participate in the digestion of host dietary co mponents. Total (AODC: Acridine Orange Direct Count) and culturable (C FU: Colony Forming Units) bacteria in this organ in fresh adult krill were compared in numerical and physiological terms with those in conte mporaneously collected seawater samples in Admiralty Bay, King George Island, South Shetland Islands. On a per unit volume basis, AODC and C FU numbers in the stomach were greater than in the seawater, and highe r than could be accounted for through selective prey filtration alone. CFU in the stomach and seawater averaged similar to 4% and <0.01% of the respective AODC, and in the stomach similar to 6% of AODC were div iding, compared to less than 1% in seawater, observations suggesting t hat bacterial growth occurs in the stomach. Biovolume of cells in the stomach exceeded that of cells in seawater (t(2854) = -6.262, P < 0.00 1), but the former were too small to have been selectively filtered. M altose and gluconate assimilation, and lipase (C-14), 'trypsin'-like, and acid phosphatase activities were significantly more prominent in c ulturable bacteria from the stomach than seawater, further indicating selection. Conversely, a-fucosidase production appears host derived. Y easts isolated from the stomach were identified as Leucosporidium anta rcticum and Metschnikowia australis.