The Adaptive Significance of Calanoid Copepod Pigmentation: A Comparative and Experimental Analysis

A two—part study was undertaken to investigate the adaptive value of carotenoid pigmentation to calanoid copepods and the costs in terms of visually selective predation. A comparative survey of copepod populations documented the distribution of pigmented forms and suggested the importance of water temperature, lake depth, and elevation to copepod pigmentation. Estimates of photodamaging irradiance and yearly temperature regime were constructed from the primary survey variables and compared for their correlation with copepod pigmentation. Yearly temperature regime is directly related to elevation and is most important in explaining variation in copepod pigmentation. Copepods are most darkly pigmented in the coldest lakes. Phytoplankton pigment concentration is not limiting to the pigmentation of calanoid copepods. Laboratory experiments demonstrated that carotenoid pigments can be photoprotective to calanoid copepods exposed to strong sunlight, yet field experiments failed to demonstrate photodamage when copepods were allowed to migrate vertically in enclosures. Respirometry experiments have shown that pigmented copepods respire faster in the light than in the dark, while unpigmented copepods do not. This metabolic facilitation by light helps to explain literature accounts of the sunlight—related behavior of pigmented zooplankton. Laboratory and field experiments with predators confirmed that vertebrate predators select pigmented calanoid copepods, but that common invertebrate predators are not visually selective. Trout are highly visually selective predators. Although salamanders are visually selective in laboratory enclosures, they are inefficient at capturing calanoid copepods in nature. Pigments are adaptive to calanoid copepods because they facilitate a metabolic increase and provide photoprotection when the animals are illuminated. Pigments are rarely found in the zooplankton, however, because zooplankton are continuously exposed to predation by visually selective predators. In cold environments with short growing seasons, as at high elevations, the advantages of pigmentation at least partially outweigh the costs in terms of selective predation, thus shifting the balance in favor of pigmentation.