Digestive Associations Between Marine Detritivores and Bacteria

Although bacteria can be associated with animals in a wide variety of ways, nutritional interactions are by far the most ubiquitous. We define them simply and inclusively as interactions in which proximity allows nutrient transfer in one or both directions, with the most obvious associations involving the alimentary tract and feces, and food just prior to ingestion. Transient or indigenous, attached or free-living bacteria may be obligately or facultatively associated with their animal counterparts. Obligate interactions of economic importance, e.g. in ruminant and termite guts, are clearly the best studied. A few other strong interactions, such as competition for food between bacteria and carnivores, are reasonably well understood intuitively. We purposely choose here a system marine detritivory in which intuition is a less adequate guide, and mutualism, competition, and predation all are suggested interactions. Because marine detrivores ingest bacteria-covered particles, interactions are virtually assured; we look for some methodical approach by which their existence and importance can be predicted and analyzed. The one we adopt for this review is a cost-benefit analysis that adapts and integrates disparate approaches developed for other applications. We evaluate its predictions through literature review. Then we evaluate the ecosystem consequences of our tentatively concluded interactions