Phenotypic plasticity in pigmentation in Daphnia induced by UV radiation and fish kairomones

Summary

• 1
  Planktonic organisms are exposed to harmful ultraviolet (UV) radiation. Pigmentation offers protection but at the same time increases visibility, and therefore vulnerability, to visually orienting predators such as fish. As an adaptation against fish predation, zooplankton should be transparent, though this would leave them less protected against UV radiation. Thus both adaptations would appear to be mutually exclusive. However, phenotypic plasticity in pigmentation could allow flexible adaptation to both environmental situations.
• 2
  We tested the hypothesis that Daphnia should be able to change their level of pigmentation in response to fish kairomone and/or UV radiation using four species of Daphnia.
• 3
  Daphnia hyalina Leydig increased pigmentation under UV radiation and D. pulex Leydig reduced pigmentation in the fish kairomone treatment. Both species live in habitats with variable UV and fish impact.
• 4
  Daphnia cucullata Sars and D. middendorffiana Fischer showed no reaction, probably because of their extreme adaptations: D. middendorffiana is strongly pigmented and seems to be adapted to high UV-B impact and an absence of fish in its arctic habitat. In contrast, D. cucullata has evolved in coexistence with fish. It can afford being nearly transparent because it lives in eutrophic lakes where UV-B is not relevant.
• 5
  Our data on four species suggest that plasticity in pigmentation might be common in Daphnia adapted to environments with contrasting or variable selection pressures.