When The Wait Is Over

Before I get back to the life history of Daphnia, I want to finish two more lines of thought on resting eggs (who knew there was so much to say about resting eggs?).  I mentioned in my previous post an experiment where I monitored the hatching of five types of pond zooplankton from sediment.  I concluded there are hundreds eggs in the sediment samples and all species exhibit bet-hedging, with a portion hatching at each wetting rather than all at once.

I made another observation during these experiments.  The eggs of the five species hatched in a specific order over a period of about 5 days.  Within a day, the eggs of the clam shrimp hatched followed by Daphnia, calanoid copepods and finally ostracods about 4 days later.  This pattern repeated itself in every replicate during each sequence of wetting and drying.  Understanding the sequence requires knowledge of the life cycles of each species and since that is where this story began a few days ago, I’ll have to return to it later.  I have little doubt there is a similar sequence, depending on what species are present,  in vernal ponds everywhere.

A last point has to do with my earlier story about eggs hatching from sediments collected 10 years previously.  I mentioned that while interesting, it is not exceptional.  What is much more interesting along these lines is the work of Charles Kerfoot at Michigan Tech and Nelson Hairston Jr at Cornell.   They both study the longevity of resting eggs, primarily in lakes, and have hatched resting eggs are over 200 years old.  One of the neat implications is that it is possible to bring back, or resurrect, entire populations and their genes to allow a comparison of the genetics of populations from the past and compare it those of the present.  If there are changes, are they directional or random?  Has some sort of change in the lake’s chemistry or biota driven the change?  This field is now called resurrection ecology, a term coined by Dr. Kerfoot and there have been attempts made in lakes worldwide to resurrect lake populations of zooplankton.

I am not aware of successful attempts in ponds.  In preparation for a grant proposal a few years ago, a group of colleagues and I tried to hatch eggs collected from sediment cores of Sand Hills’ lakes and ponds in western Nebraska.  We had no success and found that most of the ephippia, of which there were many, were empty.  The discrepancy in success between the ponds of Australia and Nebraska remains a mystery.  Perhaps the explanation has to do with the arid conditions of Australian ponds when we collected versus the moist conditions of the Nebraska ponds.  What might be happening in the ponds of New England, or elsewhere, is unresolved.

One last tantalizing point that needs study is the age of vernal pools.  Some may be thousands of years old.  Are there mysteries and stories locked in their sediments?