An investigation of potential immunological and metabolic indices of environmental water quality in the shore crab, Carcinus maenas

Abstract

The aim of this study was to investigate the potential of the shore crab, Carcinus maenas. to act as a biomarker of water quality using a range of metabolic and immunological indices of organism condition. Haemolymph total sugar, glucose, trehalose and hydrogen peroxide concentrations were measured to indicate the metabolic status of C. maenas whilst immunocompetence was inferred from haemocyte counts and phenoloxidase enzyme activities. Before any of these indices could be used to monitor changes in organism condition following exposure to artificial stressors the responses of these indices to natural environmental perturbation were recorded. Baseline variability associated with tidal cycles was assessed in a computer controlled tidal tank with an effective excursion of 0.9m. Entrainable circatidal rhythmicity was demonstrated in the indices of total haemolymph sugar, hydrogen peroxide, haemocyte population and for the first time rhythms in the activity of the phenoloxidase enzyme were recorded. Furthermore the trends associated with tidal height were completely reversed when crabs were subjected to aerial exposure during low water. No predictable cycles were recorded for any of the indices associated with the seasonal cycle, probably because of the number of environmental variables operating simultaneously. However, a significant positive correlation was seen between salinity and total haemocyte count possibly as a function of osmotic requirements and a positive correlation between bacterial load and phenoloxidase enzyme activity was recorded for the first time. Having defined baseline variability, the responses of C. maenas to a pathogenic infection with Listonella anguillarum and a 50% dilution of the water soluble fraction of Wyche Farm crude oil were investigated. Significant and predictable decreases in the circulating haemocyte population were recorded following exposure to both stresses. Very little change in the metabolic status was seen following exposure to bacterial infection or organic pollution. Predictable decreases in the activity of the phenoloxidase enzyme were seen following bacterial infection which were followed by a recovery suggesting the possibility of immunostimulation. Hydrocarbon exposure appeared to have no affect on the phenoloxidase enzyme system in isolation. Synergistic exposure to hydrocarbon pollution and bacterial infection also produced very little change in the metabolic status of C. maenas but again produced predictable decreases in the circulating haemocyte population. Combined exposure to the two stressors seemed to prevent the phenoloxidase enzyme system responding to the presence of a bacterial infection and the idea of immunosuppression of the phenoloxidase system was suggested for the first time in marine crustaceans. The potential use of C. maenas as a biomarker of water quality and the value of the majority of the indices tested was questioned because of their inherent resilience to the stresses imposed.

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