MARTIN, G.G.*; CHIU, A.: Barriers to pathogens penetrating the intestine of the penaeid shrimp, Sicyonia ingentis.

The intestines are the longest section of the digestive tract in shrimp and their function is not clear. They are not involved with digestion. Shrimp ingest detritus with a high concentration of micro-organisms. What mechanisms prevent microbes from penetrating the intestinal wall? Extending from the lumen to the outer wall of the intestine are the following layers: peritrophic membrane, simple columnar epithelium, basal lamina, zone of large granule hemocytes, muscle layer, and a loose connective tissue layer containing blood sinuses. Microbes reaching the latter layer could be dispersed throughout the body. Defensive responses within these layers include the following. The peritrophic membrane contains chitinous fibrils and may resist pathogen penetration. The epithelial cells are elongate and unusual clusters of cylinders plug their nuclear pores. The particles may be an organelle or a virus. Preliminary studies suggest that these particles are present in shrimp collected off Palos Verdes (near DDT contaminated sediment and a sewage outfall), but not in shrimp collected from cleaner water off Santa Barbara. When bacteria in the genus Vibrio are injected into shrimp intestines, the epithelial cells detach from the basal lamina and we are determining if non-pathogenic bacteria have a similar effect. Beneath the basal lamina is a layer of regularly spaced, fixed, large-granule hemocytes. Because these cells make up only 10% of the circulating hemocytes, their abundance along the basal lamina may indicate a specific defensive role and suggests that they represent the end stage of maturation of granulated hemocytes. The granules in these cells contain chitin and the defensive role of this molecule should be studied. Further studies on the intestine may help our understanding of health and disease in crustaceans.