Evaluation of the neutral red retention assay as a stress response indicator in cultivated mussels (mytilus spp.) in relation to seasonal and environmental conditions

The neutral red assay (NRA) was evaluated as an indicator of stress response in mussels that were held under various culture situations. The NRA measures retention time of neutral red dye in the hemocyte organelle, the lysosome, which can be correlated to the condition of a mussel under stressful circumstances. Shelf life and standard meat yield also provide an indication of mussel condition. The objectives of this study are to compare and evaluate mussel stress response in relation to: (1) seasonal and environmental changes and (2) postharvest handling. Neutral red retention (NRR) levels and shelf life were reduced in late summer mussels (postspawning) compared with early summer mussels (prespawning), and increased in autumn/early winter mussels, indicating a seasonal pattern of stress response associated with reproduction. Harvested mussels exhibited a decrease in NRR during extended air exposure, especially when held at air temperatures above and below air temperatures comparable to ambient water temperatures. The results demonstrated that NRA was a useful, sensitive index of physiological stress response in mussels subjected to various culture practices and conditions. The implication of this work for mussels growers is that reduced air exposure following harvest and reduced handling during certain seasons will result in less stressed mussels and hence a better quality product.