Ingestion of bacteria by cod (Gadus morhua L.) larvae. In: The propagation of cod Gadus morhua L.: an international symposium, Arendal, 14 - 17 June 1983

The uptake of Rhodamine-labelled bacteria by cod larvae was demonstrated using a fluorescent microscopy technique, which allowed a quick estimate of bacterial uptake and gut clearance. Live or heat-killed bacteria, isolated from cod-eggs, were rapidly ingested by larvae at yolk-sac and older stages. A substantial uptake could be seen after 15 min exposure, and eventually the bacteria became densely packed in the digestive tract. Some morphological details in the developing gut of the cod larvae could be observed after ingestion of the fluorescent-labelled bacteria. In yolk-sac larvae, bacteria were bound in discrete areas in the midgut, indicating clustering of receptors or internal folding or segmentation of the gut. In older larvae, which had ingested bacteria or other cells, the hindgut was never emptied, not even after starvation. The results indicate that particles of undigested material are retarded in the hindgut, and that probably absorption by endocytosis takes place in this area. The results reported in this paper point to a possible role for bacteria in the early life of cod larvae. Aspects discussed include the mechanism of uptake of bacteria, entrapment in the gut, digestion or elimination of bacteria by extracellular or intracellular processes, and the role of an indigenous microflora with respect to feeding strategies and health hazards of cod larvae