One of the most interesting aspects of the Daphnia – Chaoborus interaction is the specificity of the response. Only those Daphnia species most intimate with the predator are induced, and even then, only the instars of the appropriate size to be fed on. This speaks to the idea that such a defense has a cost, otherwise, why not have the anti-predator defense all the time? There have been a few studies that tried to determine the cost and it appears to be low (in our terms). Either the experiments have looked at the wrong units of cost (time to maturity, reduced reproductive effort?) or we underestimate the value of the currency.
To underscore this idea of specificity and cost, I begin my slow segue to adulthood (of Daphnia, not mine). In Australia we found a few Daphnia species that respond, primarily as adults, to the presence of notonectids, common aquatic insects called backswimmers. If you have ever made the error of judgment to try and handle them barehanded, you may have learned that their mouthparts are (painfully!) adapted for piercing. A notonectid captures its prey with thre forelegs and holds it until the mouthparts can be used to pierce and suck out the body fluids of the prey.
Unlike Chaoborus, notonectids prefer large prey. Still small I realize, but large among the zooplankton of vernal ponds. Consequently, adaptations that make Daphnia a little larger, like the neck teeth used to deter Chaoborus, wouldn’t do much good against a notonectid. Instead, they respond to the presence of notonectids by growing a large dorsal, cephalic crest. The function of the crest is likely the same as the neck teeth; it increases handling time or makes it impossible to grab. The crest is sometimes so large that the Daphnia looks like a swimming circle.
I tested the specificity of the response in Daphnia by subjecting Australian species and local Daphnia, both responsive to their respective predator, to the presence of the other predator. The result was that nothing happened. The local Daphnia couldn’t “taste” the presence of notonectids and the foreign Daphnia couldn’t taste the Chaoborus. Later, I came across what I identified as D. schodleri (it is almost certainly something else) with a cephalic crest living among a highly density of notonectids in vernal ponds in southeastern Texas. (Schwartz, S.S. 1991. Predator-induced alterations in Daphnia morphology. J. Plankton Res. 13:1151-1161.) With no recent genetic connection to their Australian relatives, I am convinced these populations separately evolved the same strategy with juveniles showing little or no response and adults growing large cephalic crests.
I have written a few times already but it bears repeating: the medium is the message. There are many message in the water, it just depends on who is sending the message and who is there to taste it. On ther flip side, why can’t the predators evolve a mask scent so the predators can’t detect them? Maybe this has happened countless times over the centuries like bats, their radar, and the jamming signals of the moths that would be their prey. Again, vernal ponds are nowhere near as simple as just depression filled with water and hold all sorts of lessons.
As an unsolicited testimonial, check out “An Image-Based Key To The Zooplankton Of The Northeast (USA)” as a good guide to zooplankton of that region. It contains both vernal pond and lake species. Wherever you live, the guide at the least will get you in the right direction.
