Fish-mediated trait compensation in zooplankton

1. Environmental factors fluctuate spatially and temporally, and organisms that can alter phenotype in response to these changes may increase their fitness. Zooplankton are known to be able to induce body pigmentation in response to ultraviolet radiation (UVR) and to reduce the pigmentation when exposed to fish predators. Hence, reduced pigmentation because of the presence of fish could potentially lead to UVR damage, which calls for alternative protective echanisms.
2. We exposed zooplankton to fish cues and UVR stress to assess whether body pigmentation and cellular antioxidants are flexible predation and UVR defences.
3. Zooplankton exposed to fish predator cues (no direct predation) reduced their pigmentation by c. 30% in 20 days. However, they were able to rapidly counteract negative UVR effects by increasing the activity of antioxidant defences such as glutathione S-transferase (GST). When exposed to UVR, the GST activity increased by c. 100% in zooplankton that had previously reduced their pigmentation because of fish cues. Transparency in the zooplankton did not lead
to considerably higher UVR damage, here measured as inhibition of olinesterase (ChE).
4. We conclude that zooplankton pigmentation and antioxidant enzymes are flexible UVR defence systems, which can be induced when needed. Zooplankton may employ antioxidant defences when pigmentation is reduced to counteract predation risk and thereby rapidly respond to detrimental effects of UVR exposure, that is, they can compensate one trait with another