The effects of variations in temperature, salinity and water movement on the laboratory growth and reproduction of a single clone of the bougainvilliid hydroid Clavopsella michaeli were examined. Unfavourable conditions resulted in reproducible oscillations of growth rate. They ranged from overall stimulatory effects (hormesis) to irreversible inhibition and shrinkage of colonies. Exposure to low concentrations (0.1-5 μg l-1) of copper and mercury ions produced similar responses, which were therefore regarded as non-specific. A control mechanism, dampening these growth rate oscillations, supported higher colonial tolerance, that is adaptation, to environmental stresses of low intensity. When the counteractive capacity of the control mechanism was exceeded, inhibition occurred. Higher frequencies of gonozooids were observed in stressed colonies and were interpreted as an adaptive response. © 1991 Kluwer Academic Publishers.
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