Greater than 99% of Vibrio alginolyticus were cleared from the haemolymph of Penaeus monodon within 4 h of exposure to the heat-killed bacteria. Bactericidins and other humoral factors (possibly lectins) appear to be induced within 1 day post-treatment. The appearance of “lectins” was short-lived and only detected in bacteria-treated shrimps. Bactericidins, however, peaked at day 2 and persisted in the haemolymph until day 5. They showed partial specificity. In addition, bactericidins were briefly induced at day 2 in control shrimps which had been injected with saline. Haemagglutinins active against sheep red blood cells were detected at a variety of titres in all groups of shrimps.
These results are discussed with reference to the responses of other crustacea and of insects to exposure to bacteria.