The asset protection principle predicts that the more energy or resources an individual has, the less gain they will receive in contrast to the risk of fitness loss through further foraging. In other words, the more starved an individual is the more it will risk by continuing to forage. Zooplankton perform one of the largest foraging movements in terms of biomass on the planet, known as diel vertical migration. The size of most zooplankton, however, has hindered the study of foraging decisions at the individual level. Therefore, the trade-off between starvation and extrinsic risk has remained implicit. Here, we tested whether the asset protection principle applies to mm-sized crustaceans. Water fleas, of the species Daphnia magna, were subjected to either 0, 24 or 72 hours of food restriction before being behaviourally assayed. Utilising a proven tracking platform for mm-sized aquatic organisms, swimming behaviours were recorded in an experimental arena when exposed to an ultraviolet (UV) threat from above, either in the presence or absence of a spatially explicit foraging patch at the surface.
Results/Conclusions
The presence of food drastically reduced the average swimming depth (p < 0.001). Individuals in the behavioural assays with a food patch tended to stay relatively near the surface waters with an average swimming depth of 83 mm whereas individuals in control experimental conditions had a swimming depth of approximately 6.5 times deeper during UV exposure. Despite the food patch altering the average vertical position, when exposed to UV, individuals with varying energetic states did not differ in their depth (p = 0.23), therefore we found no support for the asset protection principle. We also tested speed as a proxy for energetic availability to ensure that the treatment effects were sufficient and found that the food restriction period clearly reduced the the speed of individuals (p < 0.05). This indicates irrespective of energetic state or the condition of the individual, food availability is more important for D. magna than UVR in determining their distribution.