PARENT SESSION
Oral Session # 51: Limnology II: Plankton; Carbon.
Presiding: K Forshay
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 101.

Evolution of negative phototaxis in a Daphnia population revealed by reanimation of resting eggs.

Hembre, Leif ^{*,1, 2}, Peterson, Laura¹, Walsh, Emily¹, Forman, Rebecca², Engstrom, Dan³, ¹ Hamline University, St. Paul, MN, USA² University of Minnesota, St. Paul, MN, USA³ St. Croix Watershed Research Station, Marine on St. Croix, MN, USA

ABSTRACT- The diapausing sexual eggs (ephippia) of cladocerans (e.g., Daphnia) deposited in sediments represent biological time capsules that provide a record of the evolutionary history of populations. This study examines the egg bank of a D. pulicaria population in a lake that has been stocked annually with rainbow trout (a predator of Daphnia) for 40 years. Ephippia from sediments dating from the early 1900s to 2001 were hatched, and clonal cultures were established from the hatchlings. To evaluate whether the population had evolved over the past 40 years in response to elevated predation levels, we examined the phototactic behavior (a proxy for diel vertical migration (DVM) behavior) of clones in the presence and absence of fish kairomones. DVM is widely accepted to be a predator-avoidance behavior. Individuals migrate to depth during the daytime to avoid predation by visual predators (e.g., fish), and then typically ascend into surface waters at night when there is a lower risk of predation. Chemicals exuded from predators (kairomones) provide a proximate cue that influences the DVM behavior of individuals. Therefore, we expected that 1) the high levels of predation over the past 40 years would have selected for more negatively phototactic clones, and 2) kairomones would influence the strength of the behavioral response. Our results from assays using kairomone-free water indicate that clones hatched from ephippia produced after the trout stocking program began (1961) were indeed significantly more negatively phototactic than those produced before 1961. Interestingly, in the presence of kairomones, clones from both the pre-and post-stocking eras became more negatively phototactic, but the response was only significant for clones from the pre-stocking era. This suggests that before trout were introduced to this lake clones in the population had considerable plasticity in their DVM behavior. Their ancestors, however, appear to have evolved more conservative DVM strategies.

Key words: daphnia, phototaxis, egg bank, kairomone