Annual Viral Expression in a Sea Slug Population: Life Cycle Control and Symbiotic Chloroplast Maintenance.

In a few well-known cases, animal population dynamics are regulated by cyclical infections of protists, bacteria, or viruses. In most of these cases, the pathogen persists in the environment, where it continues to infect some percentage of successive generations of the host organism. This persistent re-infection causes a long-lived decline, in either population size or cycle, to a level that depends upon pathogen density and infection level (1-4). We have discovered, on the basis of 9 years of observation, an annual viral expression in Elysia chlorotica, an ascoglossan sea slug, that coincides with the yearly, synchronized death of all the adults in the population. This coincidence of viral expression and mass death is ubiquitous, and it occurs in the laboratory as well as in the field. Our evidence also suggests that the viruses do not re-infect subsequent generations from an external pathogen pool, but are endogenous to the slug. We are led, finally, to the hypothesis that the viruses may be involved in the maintenance of symbiotic chloroplasts within the molluscan cells.