Report No FR/D0024

Oct 1995

Organisms occupying littoral and estuarine habitats are subjected to a variety of contaminants including elevated levels of metals and lipophilic xenobiotics including hydrocarbons. Chronic exposure to sublethal concentrations of environmental toxicants is suspected of increasing susceptibility to diseases in shellfish and fish by suppression of the immune system. Filter feeding bivalve molluscs and bottom-living fish are particularly vulnerable to the risk of contaminant effects due to their close proximity to the sediments where the majority of toxic compounds accumulate. This project has focused on the development of in vitro tests, which can be applied under field conditions, to assess the competence of the immune system and health of populations of bivalve molluscs and fish from clean and contaminated environments. Laboratory studies, using specific compounds under controlled conditions, are a necessary prerequisite in order to interpret the findings in the field situation.

All organisms including molluscs and fish have developed an internal defence (immune) system which is capable of eliminating potential pathogens from tissues. A non-specific defence mechanism common to both mussels and fish for the removal of foreign particles involves phagocytosis which comprises recognition, chemotaxis, attachment, incorporation and destruction of the invading pathogen. The suite of assays investigated for quantifying immune function in the fish and molluscs have included characterisation of the population of immune cells, phagocytosis and ability to destroy potential pathogens.

A comprehensive suite of assays has been developed to measure immune capability and used in laboratory exposure experiments and field sites with known contaminant levels. The laboratory experimental results show that exposure to hydrocarbons and metals, particularly copper and cadmium, may significantly influence various aspects of immune function in both mussels and fish. The use of a suite of assays has produced a broad assessment of immunocompetence. The introduction of a pathogen following contaminant exposure resulted in a significant increase in mortality compared with non-contaminant exposed mussels, giving direct evidence of immune suppression caused by contaminant exposure. The variability in the degree of suppression or enhancement of different aspects of the immune response, and the difference in the responses observed as compared to those in different species, emphasise the need for a multiassay and multi-species approach to analysis of contaminant effects.

The principal studies carried out to apply the assays of immune response in field situations were carried out, together with a UNESCO project measuring contaminant levels, within the Venice Lagoon. Application of the assays on board ship, sampling fish around the British coast, was also investigated. The Venice Lagoon supports fishery and aquaculture industries. It also serves as a drainage basin for surrounding farmland and industrial sites and, due to the shallow water and inadequate tidal flushing rates, many effluents and contaminants accumulate within the water and sediments. The results from the present study indicated differences in immune response in mussels and fish from the different sites. These differences fluctuated according to the time of year; however, clear correlations between contaminant levels and immune response could be detected.

The assays developed in this project, when combined, may therefore represent a sensitive means of testing for environmental stressors and their possible effects on disease susceptibility in aquatic organisms. Further research is needed to test for the potentially more harmful effects on disease susceptibility of long-term exposure to contaminants and in particular the effects on early stages in the life history.

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