Tolerance and hormesis--increased resistance to copper in hydroids linked to hormesis.

Cultured clones of the colonial hydroid Laomedeaflexuosa have been used over some years as an experimental model to study the dynamics of growth control [J. Mar. Biol. Ass. UK (1981a) 61, 35; Aquatic Toxicology (1981b) 1, 227; Journal of Applied Toxicology (2000a) 20, 93]. Exposure to toxic agents has been an essential element of the approach, providing the stimulus to elicit adaptive control system responses. While the work has provided interpretations of physiological interest, it has also given insights to some toxicological phenomena. It is proposed that hormesis, as a stimulation of growth due to exposure to low concentrations of copper (1-10 microg l(-1)), is due to increases in the preferred rate of the growth control mechanism. This increases the capacity to counteract inhibition and confers intolerance to the inhibitor, while overcorrections to low concentrations cause hormesis.