Experiments indicate 30–31°C as the upper thermal limit for perch since, irrespective of the level of their temperature acclimatization, they cannot survive hotter conditions for more than a few hours. Perch that were gradually heated in lethal temperature tests indulged in agitated exploratory behaviour as they neared their lethal levels of temperature. Just below the lethal, balance and coordination became seriously impaired, and the final phases of activity were marked by violent and disoriented swimming.
Among other changes, atrophy of the interrenal tissue accompanied exposure to various levels of temperature. These changes were related to severity of thermal stress, being most marked when high temperature was prolonged during acclimatization at 28–30°C, when hyperplasias could also occur, or when temperature was raised rapidly to exceed the animal's thermal tolerance in lethal tests. At moderately high temperatures it sometimes appeared that interrenal cell hypertrophy could take place. Changes in other tissues—viz. renal tubules, head kidney, liver, muscle—generally only occurred at the higher levels of prolonged exposure. While the changes varied in character between tissues all are considered essentially degenerative, and due to some kind of breakdown of the body's normal defences against stress.
To test the postulated function of interrenal tissue in combating various forms of stress, perch were chilled, made anoxic, and placed in diluted sea water, as well as heated. These experiments, with others in which perch received mammalian ACTH (both to test its general histological effects and in an attempt to influence lethal temperature) indicate an important function for interrenal tissue in offsetting stress.
It is tentatively suggested that the complex of histological changes observed considerably resembles that of the “General-Adaptation-Syndrome” of mammals, that most of the histological changes are due to either adaptive or maladaptive adjustments of the perch's organ systems or tissues, and that interrenal tissue occupies a similar functional position in the perch to that of adrenocortical tissue in mammals in opposing stress, including the stress of heat.