Always nice to deal with unfinished business. After more than 15 years, the data of my only unpublished PhD chapter are now finally published. In the end it took us a physical model, a demographic model, two field experiments and three lab experiments to show that fairy shrimp can avoid extinctions by ensuring that their dormant eggs can hatch at various moments in the future and that this is an evolutionary risk spreading strategy.
Pinceel, T., Buschke, F., Geerts, A., Vanoverbeke, J., Brendonck, L., & Vanschoenwinkel, B. (2021). An empirical confirmation of diversified bet hedging as a survival strategy in unpredictably varying environments. Ecology. (SCI: 5.499), Q1
Annual fishes of the genus Nothobranchius are endemic to temporary ponds. When exposed nonlethally to a predation risk, their reproductive efforts are increased, likely to reproduce as much as possible before being predated upon.
This experiment was done by Arnout Grégoir, who recently defended his PhD. We are learning more about the interesting life histories of these remarkable vertebrates, that, in terms of certain life history aspects, have more in common with aquatic invertebrates than fish.
In a new paper Tom Pinceel shows that crustaceans from ephemeral water bodies have different egg hatching frequencies depending on local climatic conditions. If the climate is harsher and less predictable, a lower percentage of eggs hatches after rains. This ensures that more long lived eggs are left that may grow during future conditions!
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.
After four years of frog hunting and intense experimenting, Jane Reniers defended her doctorate on amphibian life history strategies. The title of her doctorate was Managing Reproductive Challenges in Time Constrained Environments. Amphibian life history variation from clutch to landscape. I believe PhDs are all about managing challenges in a time constrained environment. And just like the amphibians she studied, Jane managed to overcome a lot of challenges and bad luck but still defended succesfully after just more than four years. A great job with a nice booklet to show for it… and a lot of great manuscripts still waiting to be published!
While she is moving on to new challenges, we will miss Jane’s spirit and laughter in the lab. But no doubt we will continue to collaborate to publish the remaining chapters of her PhD. We wish you all the very best Jane!
After just three years, Falko Buschke‘s Erasmus Mundus PhD fellowship came to an end. Just months later he succesfully defended his PhD thesis. Initially drawn to Belgium with the prospect of doing a thesis on community dynamics with a lot of empirical work, Falko soon settled into a different niche. Making use of the IUCN database he set out to explain the distribution patterns of terrestrial vertebrates in Africa. For this he used a very diverse set of statistical tools. He reconstructed biogeographical patterns in Africa based on how species present in different locations respond to spatial and environmental gradients. He experimented with novel ways to define regional species pools and investigated the drivers of patterns of alpha and beta diversity. Finally, he also experimented with spreading dye models and built a neutral metacommunity model to explain different biogeographical patterns in this realm. Overall, it was an exciting journey exploring the interface between community ecology and macro ecology. Falko, it was great having you here. We will miss your wit and humour now you have returned to South Africa… and will continue to follow your adventures on http://solitaryecology.com/
Just two weeks later, Tom Pinceel joined Falko in the league of doctors. After doing a MSc working on genetic patterns in rock pool fairy shrimp, Tom continued along this path and started to explore the hatching strategies of these enigmatic inhabitants of temporary pools worldwide. Tom showed adaptive variation in hatching strategies of pool invertebrates along a gradient of habitat stability. He also revealed that the ancient diversification of fairy shrimp on the Australian continent coincided with a period of intense aridification. When Australia lost most of its rainforests, desert adapted fauna like fairy shrimps seem to have benefited and responded with a spectacular adaptive radiation. This resulted in a nice little booklet with most of his chapters already published. Tom is now continuing his research into delayed hatching as a survival strategy in extreme environments as a prospective post doc. We can only hope he will be able to continue his work in the near future.
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.