My last topic regarding resting eggs is what triggers their hatch. This esoteric topic has important consequences for pond conservation. The triggers remain far from clear but involve some combination of dark, light, temperature, drying, and oxygen as well as the carbohydrate trehalose. If this sounds unclear, it is.
I studied Daphnia hatching cues across a geographic range in a lab that kept clones collected from around the world. Because Daphnia reproduce asexually, whatever genes and alleles they carry for hatching stay intact in the lab. I compared 36 clones from 7 species in a series of experiments where I varied the temperature at which the eggs were stored, the temperature at which I attempted to hatch the eggs, and the duration of light the eggs received. I won’t go into the details of the experiment but if you are really interested you can either contact me for a pdf copy of the paper or find it yourself (Schwartz, S.S. and P.D.N. Hebert. 1987. Methods for the activation of the resting eggs of Daphnia. Freshwater Biology 17: 373-379).
What I found to be interesting is that cues for hatching varied at every scale. At the largest scale, among geographical regions, this variation makes sense as different areas have different conditions at the start of the pond season. One cool result was clones from the arctic require much cooler temperatures to hatch than clones from southern climes. Arctic clones hatched poorly at temperatures over 7 C whereas clones from temperate regions required much warmer temperatures. Unfortunately, I didn’t have access to clones from the southern US to make a continental comparison.
At the smallest scale, within a pond, we found that clones from the same pond had different optimum hatching cues. This result is important in the context of seasonal succession of clones, a result of seemingly limited importance to those ponds where there are truly asexual Daphnia, those never having sex. My explanation for these details will have to wait for another blog.
It is the intermediate geographic level that I think is so important to pond lovers. Hatching optima varied between locally collected populations, those from different ponds that were in the same neighborhood. The significance is each pond acts as an evolutionary center where populations become finely adapted to conditions in that one, particular pond. The pond a few meters away has subtly different physical conditions and may have been colonized by different clones. I describe this for one species but it is likely true for all the others. I want to shout the meaning: “the biota of each pond is unique and needs to be protected.”
Sure, a bunch of ponds may have Daphnia and other species in common, but even with similarity at the species level, the biota of each pond is genetically unique. Given the extremely poor dispersal capabilities of so many pond dwelling invertebrate species, the destruction of any one pond means the loss of a distinct genetic lineage. I am mostly ignorant of regulatory matters, but amassing proof of just how unique each pond is seems important to me. I’ll return to the theme of dispersal again, but for now, salamander lovers, I supply you with crucial secrets locked in the sediments of vernal pools that may help save the entire lot.
So I started with a resting egg and end up hoping to save ponds. I continue to be amazed at the extent to which everything is connected.
Next I’ll discuss how Daphnia grow up.
