title: Size-structured risk assessments govern Daphnia migration.
abstract: One of the more fascinating phenomena in nature is animal mass migrations and in oceans and freshwaters, diel variations in depth distribution of zooplankton are a phenomenon that has intrigued scientists for more than a century. In our study, we show that zooplankton are able to assess the threat level of ultraviolet radiation and adjust their depth distribution to this level at a very fine tuned scale. Moreover, predation risk induces a size-structured depth separation, such that small individuals, which we show are less vulnerable to predation than larger, make a risk assessment and continue feeding in surface waters during day, offering a competitive release from down-migrating larger animals. Hence, we mechanistically show that such simple organisms as invertebrate zooplankton are able to make individual, size-specific decisions regarding how to compromise between threats from both predators and UV radiation, and adjust their diel migratory patterns accordingly.
authors.affiliation: Institute of Ecology/Limnology, Ecology Building, Lund University, 223 62 Lund, Sweden. lars-anders.hansson@limnol.lu.se
authors.forename: Lars-Anders
authors.initials: LA
authors.lastname: Hansson
authors.forename: Samuel
authors.initials: S
authors.lastname: Hylander
mesh_terms: D025041:Animal Migration / Q000502:physiology*; D000818:Animals; D049628:Body Size; D003621:Daphnia / Q000033:anatomy & histology / Q000502:physiology* / Q000528:radiation effects; D003657:Decision Making; D012309:Risk-Taking; D014466:Ultraviolet Rays