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.
This article introduces a whole field of behavioral ecology that is very often skipped in discussions about a species’ ecology. The idea of predator-induced defenses influencing species to be more susceptible to predation by a parasite is something that I found to be a very intriguing evolutionary adaptation. The idea that there must be a cost associated with the defended morph in the absence of predators is a concept that cannot be over shadowed when discussing the evolution of specie specific defenses. This article additionally presents the idea of predator-induced defenses as a trade-off between adaptive changes in life-history parameters and provides evidence of other organisms’ defenses. The article further presents an interesting topic of food webs, and their development by predator-prey relationships and the lack of parasitism that ties into Daphnia. This topic helps guide the investigator into the though process by which parasites are highly virulent to Daphnia hosts and that a higher susceptibility of defended Daphnia would represent a cost. The results indicated the presence of kairomones induced Daphnia to reproduce earlier and the production of offspring was larger in the first clutch. However, the maturation rate in infected Daphnia was earlier than those uninfected leading to a cost in the infection. The reporter further indicates that the mechanisms through which this inducible defense increases Daphnia’s susceptibility to parasites are unknown. The explanation of the damselfly larvae’s reduction in susceptibility by reducing grooming when predators are apparent provides and intriguing argument for an animals’ behavior influencing it’s susceptibility to predator-induced defenses. The behavior by which Daphnia finds closure in a deeper water gradient allows for the organism to be exposed to sediment that has a higher concentration of parasitic spores and may be supportive of evidence in a weaker immune system. Further research provided in the article effectively presents that their may be more than one cause for the suppression of the immune system exposing them to parasitism, including predators and food depletion. Furthermore, this study’s investigation of fish kairomones and exposure to parasite spores was developed effectively and represented a very well developed study as they indicated that further research is necessary to fully understand the process by which this defense is developed. By exploring other venues of research we may exploit other adaptations that this species or others have developed as an induced defense.
Animal behavior has always been my favorite focus of biology and one of the ideas I always found interesting was the allocation of resources for maximum fitness and that was the first thing that came to mind when I began reading this paper. Examining the costs of defense made for a very interesting study and I was actually surprised that there hadn’t been more evidence of those costs in previous literature. I didn’t know a whole lot about Daphnia prior to reading this paper but from what it stated, they clearly chose an appropriate specimen for examining inducible defensive traits.
Another part of the study I found interesting was the idea that the selective pressures of the yeast and predatory fish larvae occur at different times of the year and therefore so do the defensive traits. It just demonstrates how incredibly fine-tuned species are to their environments. The idea of an impaired immune system due to investment in defenses made sense and I would be curious to see what the results would be of the further investigation they suggested for that theory. I would also be interested to see what patterns are for inducible defenses in other species because it is such a cool topic!
I also see the logic in the paper’s explanation of how an impaired immune system might be a way in which inducible defenses increase Daphnia’s susceptibility to parasites. I think this logic appeals to me so much because I can see the parallels between how an immune system might become impaired in the Daphnia model as compared to the human model. The authors suggest that perhaps Daphnia are more prone to infection due to abandonment of ideal feeding sites and/or the energetic costs of inducible defenses. Humans also are more prone to infections when they have a poor diet and have large energy costs (though often resulting from stress, not predation).
It’s interesting that a similar study by Coors and De Meester in 2011 had entirely different results: in the presence of fish kairomones, the average amount of bacterial spores per infected host decreased. In the presented paper’s defense though, only one Daphnia clone was used in the Coors and De Meester study, and that particular clone, D. magna, has strong genetic variability. All in all, I find this paper to be extremely thought-provoking…a good read.