To summarize, I presented this paper on an experiment conducted using two species of Daphnia to look at trade-offs between predatory induced defenses. Specifically, the paper aimed to show that organisms exposed to fish kairomones (any chemical signal directly or indirectly associated with the presence of fish) were at a higher risk of infestation by a parasitic yeast species.
The experiment was a very simple set up, in that individual Daphnia were placed into either a control media or kairomone media and then exposed to the yeast parasite. The yeast was allowed to colonize for a period of about three weeks and Daphnia were then examined to see how the fish kairomone affected the prevelance of infection and the parasite spore load. The direct effect of fish kairomone on Daphnia life-history strategy was also examined in the experiment.
The results showed that Daphnia exposed to the fish kairomone reproduced at an earlier age (therefore smaller body size) and produced more offspring in their first clutch than the individuals in the control media. Also, Daphnia exposed to kairomones had a much great prevalence of infection and parasite spore load than the individuals reared in the control media.
The study showed that Daphnia exposed to fish kairomones did indeed suffer a cost of defense by becoming more susceptible to the yeast parasite. The paper goes on to discuss that ability for Daphnia to have evolved these off-setting traits in their natural habitat because these two selective pressures (the fish and the yeast parasite) peak at different times of the year, the yeast in autumn and the predatory larval fish in late spring/early summer. Therefore the Daphnia have the ability to defend themselves against both predators in time (Daphnia only live a few months, at most). So, each generation of Daphnia have the ability to either invest in morphological and life history defenses in the presence of the fish predator, or to shut off the inducible defense and invest more into somatic maintenance and the immune system in the presence of the yeast parasite.
In class, we discussed the underlying evolutionary importance in this experiment, mainly that the Daphnia have to have the ability to express these defenses within their genome, however because of the cost of expression the traits lie dormant unless they become necessary. We discussed how inducible defenses, in general, must have a cost associated with them, otherwise they would be continually expressed. We also discussed how alien Daphnia are, and how they swim.