Metabolic stimulation by light in a pigmented freshwater invertebrate.

The pigmentation of terrestrial invertebrates has been shown to function in the optimization of solar warming, yet pigmented aquatic invertebrates have never been examined for the ability to utilize solar illumination in a similar manner. The degree of carotenoid pigmentation in calanoid copepods is easily quantified, and comparisons between populations suggest that variation in copepod pigmentation is related to variation in water temperature, with the most darkly pigmented copepods occurring in the coldest lakes. Darkly pigmented copepods show significantly higher metabolic rates in the light than in the dark, whereas lightly pigmented copepods do not. The metabolic experiments provide evidence that dark coloration is adaptive to cold-water copepods because it facilitates significant internal warming of tissues by sunlight. This hypothesis may be used to explain the pigmentation of many other cold-water invertebrates.