Elevation of plasma cortisol and hypertrophic response of inter-renal and chromaffin tissues of rainbow trout, Oncorhynchus mykiss (Walbaum), to the harmful diatom Chaetoceros concavicornis

Abstract. Changes in plasma cortisol concentrations and inter-renal and chromaffin tissues of rainbow trout, Oncorhynchus mykiss (Walbaum), exposed to harmful concentrations of the diatom Chaetoceros concavicornis were investigated. The plasma cortisol level of trout exposed to C. concavicornis was over 10-fold greater than that of untreated trout. Computer-assisted feulgen-microspectrophotometry was employed in order to assess the activity of inter-renal and chromaffin cells. The total areas of inter-renal and chromaffin nuclei in trout treated with C. concavicornis significantly increased compared to those of untreated fish. In contrast, the chromatin density of inter-renal and chromaffin nuclei in treated trout decreased significantly. This decrease of chromatin density in treated fish was caused by an increase in the low-density components, and a decrease in the middle-density and high-density components. Although the hypertrophic changes to the inter-renal and chromaffin nuclei of treated fish exhibited the same general trend, the extents to which the changes took place were different, occurring more rapidly within inter-renal tissues. The rainbow trout exposed to harmful concentrations of C. concavicornis were experiencing a severe physiological disturbance to which they tried to adapt.