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.
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
- Picture of an adult male fairy shrimp of the species Branchipus schaefferi taken by Aline Waterkeyn
Fairy shrimp re-discovered in Belgium
Fairy shrimps (Crustacea, Anostraca) are specialized inhabitants of inland water bodies that periodically dry or freeze over. Tipped of by local conservationists we traveled to Hainaut and were able to confirm the first observation since 1997 of a member of this basal crustacean order in Belgium and the first sighting of the species Branchipus schaefferi Fischer, 1834 since 1930. Nineteen populations were found in a restricted area. The current study illustrates that populations of fairy shrimp can remain undetected, although individuals are relatively large (1 – 4 cm) and conspicuous and often characterized by bright coloration, and even in relatively well-studied and monitored regions, such as Belgium. Large branchiopods are threatened in many parts of the world and notably in Western Europe. The main reason for this is the loss of temporary aquatic habitats as a result of intensive agriculture and urbanisation, and the few remaining habitats are often degraded.
Fairy shrimp wheel track habitat
While public incentive to conserve a rare group of crustaceans may be limited, it is important to realize that temporary ponds not only house a unique crustacean fauna, but are also of vital importance for other endangered species of plants and animals (Williams, 2006). These include macrophytes, dragonflies and amphibians specifically linked with temporary waters. Substantial efforts and financial support have been directed at protecting certain endangered amphibians that use temporary ponds for breeding, such as the natterjack toad (Bufo calamita) and the fire bellied toad (Bombina bombina). Temporary pond restoration and construction projects performed for these ‘flagship’ species (e.g. EU life project Bombina) are likely to be beneficial for other typical temporary pond organisms too. For instance, different rare macrophytes were shown to re-emerge from old seed banks during pond restoration projects (Hilt et al., 2006). Due to the prolonged viability of their dormant eggs (Brendonck, 1996), it is not unlikely that large branchiopods may emerge from old egg banks present in the sediment. Consequently, a habitat oriented conservation strategy protecting the few remaining high quality temporary ponds and increasing temporary pond densities in the landscape is likely to be most beneficial as a large number of organism groups, including large branchiopods, will benefit from them.
Checking out wheel tracks and farmland ponds in the Binche area in Hainaut
Low predation pressure in combination with plenty of nutrients ensure that fairy shrimps can reach high population densities in temporary pools
Read more about the ecology of fairy shrimp and the remaining populations in Belgium in these publications:
Vanschoenwinkel et al. 2013 Natuurfocus 2013-2- Oerkreeftjes duiken opnieuw op in België
Vanschoenwinkel et al 2013 BJZ_kleur