The acorn barnacle Elminius modestus (Darwin) was shown to be an excellent organism in which to study the effects of pollution throughout the life cycle. Effects of copper were demonstrated on both larval and adult forms at acute, chronic and physiological levels. Acute effects of copper on E. modestus nauplii were shown through acute toxicity tests (100 ppb-500 ppb). The test procedure was standardised and subsequently used to identify the effects of various factors on copper toxicity. Rises in temperature from 12.5*C to 22.5*C increased copper toxicity to the nauplii. Parental maintenance could affect naupliar tolerance to copper. The organic content of seawater reduced copper toxicity to the nauplii. The tolerance of nauplii from the copper-contaminated Dulas Bay had a genetic origin. The complex interactions between food, feeding behaviour and copper toxicity were investigated throughout the development of E. modestus larvae. The major findings pinpointed 'nutritional stress' as the main factor that influenced the action of dissolved copper. In the laboratory, chronic effects on adult E. modestus barnacles were evidenced through reduced survivorship, growth and reproductive effort in a population exposed to 50 ppb copper for more than four months. Exposure of E. modestus adults to 20 ppb copper reduced feeding rates and altered feeding behaviour. Further physiological effects of copper were demonstrated on E. modestus nauplii through variations in oxygen consumption. At 16*C, nauplii exposed to 50 ppb had higher respiration rates than 'control' nauplii. The oxygen consumption of nauplii at higher concentrations was equivalent to that of uncontaminated nauplii. The result is interpreted as evidence that regulation (copper sequestration) incurs an energetic cost.