1. Experiments have been done to determine the oxygen utilisation by male, female and Stage V Calanus under different environmental conditions.
2. An initial fall in the respiration of adult Calanus was observed during the first few hours after capture. Stage V do not show this clearly. It is found more often in winter than in summer.
3. The lethal temperature varies from 24° C. in winter to 26° C. in summer. Stage V Calanus are more resistant to high temperatures than adults.
4. Respiration rises with increase of temperature from 0° C. to 20° C. The increase does not follow van't Hoff's law. The oxygen consumption of males and females is about the same, while that of Stage V is lower. Above 20° C. there is a harmful effect.
5. Within the limits studied (pH 7.4–pH 8.5) change in hydrogen-ion concentration has no effect on respiration.
6. Calanus are unaffected by an increase in the oxygen content of the water, but are sensitive to low oxygen tensions. Below a concentration of about 3 ml. per litre the respiration decreases. At concentrations between 1 and 2 ml. per litre they are killed. They are more resistant at 5° C. than at 15° C. and Stage V are more resistant than adults at both these temperatures.
7. Calanus can become acclimatised to salinities as low as 35–40% seawater (S=12‰–13.6‰), but their respiration is lowered at a salinity of 50%.
8. Light has a striking effect on Calanus. It may increase the respiration by 100% or more. This effect can be detected also in the sea, but not below 5 metres. Continuous exposure to light is harmful.
9. The bearing of these results on the distribution of Calanus is discussed.
10. From the amount of oxygen used in respiration, calculations of the food required are made and these are compared with the results given by Putter for Calanus and other copepods.