Zooplankton dormant eggs are time capsules that can transport offspring to distant futures. However, after decades of study we still don’t know very well how this mechanism has evolved and how it works from a mechanistic point of view.
In new paper, Tom and I decided to use the VUB’s micro CT scanner to have a look at the internal structures of zooplankton resting eggs. Why would we want to? Well, in the past, the only way to look inside them was to freeze dry them, cut them and look at them with a scanning electrone microscope. This means that you’d have to kill the embryo and that the procedure might result in artefacts. You might see structures that don’t look that way in real life. Given that we are doing a lot of experiments on the evolutionary importance of differential hatching from resting eggs we were really keen to have a look at exactly what’s going on inside these eggs before they decide to hatch.
This pilot experiment showed that the method can yield useful images although the resolution is less than SEM. In addition it turns out that the embryos in the eggs also don’t seem to suffer too much from the X rays and most of them still hatch afterwards. More information, is likely to follow as soon as we can start to link embryonic and egg traits to the hatching behavior of eggs.
3D reconstructions of resting eggs obtained via X ray scanning. Top left: a cyst of the fairy shrimp Branschipodopsis wolfi, Top right: a cyst of the tadpole shrimp Triops. Bottom: an ephippium with two resting eggs of the water flea Daphnia magna.
In a new paper out in Scientific Reports, we use a matrix population model to test how sensitive populations of fairy shrimps are to changes in climate. The stepwise modeling procedure allows to calculate the long term population growth as a measure of fitness. If it is positive, the population will survive, if it is negative it will not. It does this by calculating, for each generation, how many eggs would be produced based on known life history traits of the species and a measure of environmental quality of the inundation (in this case represented by inundation length).
For most species it is very difficult to know how they would respond to changes in climate. However, for our fairy shrimp we have a lot of background information that allows us to make educated guesses about which life history traits could be important. We know for this species that it requires a specific amount of time to reproduce which is related to how long a pool can hold water and on the conditions they need to hatch. We also know how much eggs they can produce per day, how many eggs hatch during each inundation etc…
Population of the fairy shrimp Branchipodopsis wolfi in a temporary rock pool on a mountaintop in South Africa
The length of these inundations is one environmental parameter (of many) that will change under changing climates. But it is an important one that is directly linked to fitness. Shorter inundations means less inundations that are long enough for reproduction.
We were – and are – still ignorant about how these species will respond to these changes. However, the model does allow us to test which life history traits could be important to maintain long term survival of the populations. As such it shows which traits could help populations to survive.
One of the conclusions of the study is that, when inundations are short, it would be beneficial to make sure that a lower fraction of eggs would hatch during a given inundation. Such a mechanism could be an example of a risk spreading theory that is consistent with predictions of evolutionary bet hedging theory.
It is still a simplistic model, so it does not tell us how things will go in the future. It does not capture tradeoffs among life history traits nor the evolutionary potential of the populations. Yet, it still narrows down the range of possible future scenarios of these populations by showing what the consequences for population survival would be if populations could respond adaptively or plastically and change there life history traits.
It has been a hectic year for most of us, not just for me. Roughly two years ago we started from scratch. No money, no projects, no equipment. Now a lab has emerged. Last year field work was performed and animals were studied on and from five continents (Europe, Central America, Africa, Australia and SE Asia). I have seen more invertebrate orders and families last year than in any of the previous years. Elaborate field experiments were set up (Celina, Beth, Hendrik) sometimes with so many treatments that it was difficult not to get lost. We abandoned plankton as a core group and embraced more invertebrate and vertebrate groups than ever before. Our taxonomical expertise has increased tremendously and so has the literature we have on groups we never tackled before. Yannick and Hendrik made their own field guide for rock pool invertebrates from Western Australia, Mario personally made a key for invertebrates from moss islands in Belgium and nobody is more skilled in finding cryptic species than Gisela. I cannot tell you how much I appreciate this because this quality control and extra taxonomical resolution makes all the difference and allowed us to detect a lot of patterns that would have remained obscured otherwise.
Many of you also did exceptional things to gather data. Some people crawled through dark holes full of feces (Barbara) to get data, others will face or have faced the treacherous mud of wetlands (Evelien, Lise). Some of you have used slave labor to collect samples and aid with lab work (Celina) or seduced Greek fishermen to get free transport (Sofie). Several of you have struggled with terrible bureaucracy, permits, tropical parasites or a combination of all four. Some people said I was foolish to take on so many MSc students by the start of the year and they were right. But I was convinced by all of your plans and have not regretted it.
I’m also happy that overall we are doing well. Despite the fact that I never had less time to write papers than last year, we scored important papers in Global Ecology and Biogeography and Scientific Reports… and strangely enough in Alzheimer’s & Dementia (don’t ask me how, I forgot). Our website got more than 10 000 unique visitors.
Valerio discovered something amazing in reptiles (I cannot write what, not published yet). Mathil got a PhD fellowship and lead a successful expedition into unknown territories. Evelien’s connectivity analyses are being explored in other systems and datasets from moss mites and coral reefs to pelicans. Karen found that predator avoidance strategies in the African savannah affect the shape of drinking holes and the vegetation around it… because antelope tend to approach water upwind to avoid being detected. With Melissa, we used a supercomputer to reconstruct interaction strengths in food webs. We build a matrix population model that showed that evolutionary bet hedging could help populations to cope with climate change. We joined the Bromeliad Working Group and are planning more exchange with Canada and Brazil. We used X rays to peer into the darkness inside the time capsules of dormant plankton and are only beginning to understand how they manage to use time travel to cope with environmental stochasticity. We are collaborating with Bio Engineers (the ecology of intracellular interactions), Physicists (optics), Archeologists (distribution models of ancient settlements) and Geographers (dispersal, urban ecology) on interdisciplinary research themes. These are just few of many highlights of my year.
Thanks to all my students and collaborators for helping us with starting up this lab!
In freshwater zooplankton, that survive unfavorable periods of winter cold or drought as dormant eggs in the sediment, light is an important cue that may activate the embryo to hatch. If no light is detectable then the egg is probably buried and it would be a bad idea to hatch. We investigated the light-activation process of zooplankton resting eggs using a rock-pool fairy shrimp as a model. We showed that light activation entails a relatively simple mechanism involving a light-energy threshold. These results illustrate the potential adaptive value of light activation but also highlighted the possible role of variation in eggshell pigmentation as a risk-spreading strategy. How does this work?
Much like a pair of sunglasses, the egg shell modulates how much light is absorbed. Consequently embryos in eggs with a darker egg shell should be less responsive to light. This is exactly what we found. In darker eggs, the embryo responds later, presumably because the light energy threshold is reached later. Given that there is often strong variation in the color of eggs in populations and in clutches of eggs, this simple ‘sunglasses effect’ can ensure that not all eggs will hatch at the same time. As a result the emerging larvae that use different food sources when they get older are less likely to compete with one another. As such, it could represent a simple, yet potentially effective risk spreading strategy.
While the effectiveness of this strategy within inundations was demonstrated, its potential role in spreading hatching over different inundations remains unknown. Tests are needed to assess whether degradation of pigments over time may be an adaptive mechanism that prevents resting eggs from becoming locked in diapause. Additionally, given the similarities in observed responses to light activation in both crustacean resting eggs and plant seeds, parallel patterns in these taxonomically distant groups might possibly reflect an old evolutionary mechanism tapping the same biochemical pathways, but this hypothesis also remains to be confirmed.
The paper is accessible via this link:
Pinceel et al 2013_light induced dormancy termination