Immune state correlates with histopathological level and reveals molluscan health in populations of Modiolus kurilensis by integral health index (IHI).

Quantitative analysis of the histopathological and immune parameters of bivalve Modiolus kurilensis collected from water areas with different level of ecotoxicological stress was performed. Significant differences between samples from polluted and non-polluted sites were revealed for total haemocyte count; percentage of agranulocytes; size and internal complexity of agranulocytes and granulocytes; phagocytic activity; percentage of NBT-positive cells; hemolytic activity and plasma protein concentration; percentage of the optical density of haemolymph major polypeptide bands at 55 kDa, 78 kDa, and 124 kDa; concretion coverage area in the kidney tubules; thickness of the tubular basement membrane; nephrocyte shape; and karyopyknosis of the kidneys; and hypervacuolisation; necrosis; karyopyknosis; haemocyte infiltration; fibrosis; and invasion of the digestive gland. Analysis of the global histopathological condition index based on the weighted indices also revealed that both the digestive gland and kidneys showed significantly greater histopathological changes in the bivalves collected from polluted water. Bivalve histopathology is an established tool in aquatic toxicology. However, it reflects a morphological picture of change, which, as a rule, can be clearly recorded only at the later stages of pathology, and in some cases, indicates an adaptation to stressors within the physiological norm. In this respect, a promising and highly sensitive biomarker of the functional state of bivalves, in terms of norm and pathology as well as their habitat, is the evaluation of immune status in combination with morphological changes. However, the use of different methods and scales of assessment and the diagnosis of biomarkers, characterised by different profiles of the stress response, makes it difficult to compare the results of different studies. We propose a reliable and powerful system for assessing the physiological state of bivalve molluscs, expressed in the integral health index (IHI) and based on the standardisation of the numerical values for all parameters that have significant differences between animals collected from impacted and non-impacted water areas. In our study, IHI calculated in three variants (for histopathological parameters, for immunological parameters, and in combination) showed the most significant differences in each of the cases, but the strongest difference (-4.07) was in calculating the total IHI, which included both the immune and histopathological parameters (p = 0.00005).