Competitive trade-off strategies in ArcticDaphnia linked to melanism and UV-B stress

Abstract

Different high-Arctic, freshwater localities at Ny-Ålesund, Svalbard (79dgN) were examined for their UV-absorbing properties, depth and presence of melanic Or non-melanic morphs of the planktonic crustaceanDaphnia pulex. Light regimes in two localities with each of these morphs were measured by using underwater spectroradiometer. Most localities have low absorbance of short-waved light, but no clearcut relationship between UV transparency and occurrence of melanic morphs was detected. Yet, in the laboratory, the melanic morph showed far lower growth rates, thus being competitively inferior to the non-melanic. Conversely, the melanic morph was more resistant to UV light, suggesting a trade-off between the metabolic tax paid for the melanin synthesis and its UV-protecting abilities, or a staggered growth capacity possibly owing to polyploidy. Frequency of melanic or non-melanic clones could thus be directly linked to ambient UV-B stress and serve as an indicator thereof, but the apparently extensive need for UV protection under the Arctic lighl regimes is still puzzling, and the role of melanism and polyploidy in these organisms cannot be considered finally settled.