I left off with the idea that growing up in a vernal pond is not necessarily straightforward. Depending on the presence of particular predators, juvenile Daphnia can alter their shape, a phenomenon termed predator induced defense. In Daphnia pulex it takes the form of neck teeth or nachenzahne (I’d include a picture but I couldn’t find any on the web and haven’t digitized my slides). These are small, tooth-like protrusions on the dorsal side of the animal at about the level of the heart. The effect is to make the juveniles slightly wider and more difficult for Chaoborus to handle. As handling time increases so does the chance for escape.
There are a number of interesting facets to this induction. Chaoborus is not the only predator of Daphnia. I have previously discussed Hydra as an example and there are others. In the northeastern US only Chaoborus seems capable of causing this particular effect. In other parts of the country, and the world, there are other predators and different forms of induction. I’ll discuss those later.
Those Daphnia species, or even D.pulex populations, that don’t live with Chaoborus do not have the ability to grow these teeth. For example, Daphnia obtusa, easily confused with pulex, cannot be induced to grow the teeth. I’m not sure why this should be the case, but it may have to do with the types of ponds the species inhabit or the timing of their occurrence compared with Chaoborus larvae. The honest answer is I don’t know.
In the absence
of Chaoborus, juvenile D. pulex look like miniature adults, the normal state of development. The neat part is that if predators are present the only instars that are induced to the altered shape are those small enough to be edible. Once the juveniles outgrow the threat of the predators, that is they are too large to be forced into the maw of Chaoborus, they lose the neck teeth. An exception I remember were Daphnia we collected in northern Michigan (D. minnehaha, picture at left) that lived with extremely large Chaoborus larvae (I understand that “extremely large” in vernal pool terms means they are an inch long). In that case the adult Daphnia were also induced.
I have written previously about the medium being the message in vernal pools. Predator induced defenses are the ultimate example. The message is a chemical produced by the predator. Every animal that lives in the water is leaky (so are we for that matter) and whatever they leak in the water announces their presence if a species has receptors for the signal. The hard part is evolving the appropriate receptors and so many pond dwelling species lack the ability to sense the message in the water.
I spent way too much time trying, in vain, to isolate the message by extracting liquids from large quantities of Chaoborus. I learned it has a small molecular weight and could take a fair amount of abuse including drying, freezing and heating. That isn’t much to go on. After my stumbling attempts failed, the project was handed off to a natural products chemist. Pounds and pounds of Chaoborus were shipped to this other lab but even with all their equipment, they couldn’t isolate it either. Almost 20 years later I don’t think we know any more than we did about the inducing chemical. We do know vernal pool water is much more complicated than a casual observer might imagine.
More on induced defenses in my next post.
