A short, layman’s introduction to the wonderful world of waterfleas

Small crustaceans called waterfleas form the basis of much of the research in the research group of Prof. Dr. Luc De Meester. Much of the experimental research on evolution and on how evolution impacts ecology uses the water flea Daphnia as a model system. Below, we outline through cartoons why Daphnia is such an excellent model system and we provide insights in some of our research topics and results. This website was developed by and is in support of the research done by KU Leuven Research Fund project C16/2017/002 and is in continuous progress.

Here's a start: this is what waterfleas look like in a scientific drawing:

From "On the freshwater crustaceans occurring in the vicinity of Christiania" by George Ossian Sars.

and here is a picture of what they look like in real life:

Daphnia magna, picture by Joachim Mergeay.

Waterfleas are small aquatic crustaceans with a key role in the aquatic food web. They feed on algae and are an important prey for larger invertebrates and fish. Their global occurrence and their importance in pond and lake systems make them a relevant model system in the research fields of ecology and ecotoxicology and their funky life cycle makes them highly suitable for studies in evolutionary biology and genomics.

What exactly makes them special?

They have a special kind of life cycle (called parthenogenesis): they clone themselves when things are going well. In this asexual phase of reproduction, waterflea populations are all-female. When conditions turn unfavorable (for example when winter sets in or food becomes scarce) they induce the production of males and sexual females. In this sexual phase of the life cycle, males and females produce resting eggs that can survive the harsh conditions by "hibernating" in the sediments. Once these (sexually recombined) resting eggs get cues that everything outside is favorable again, they hatch and everything starts all over.

Their small body size and short life cycle make them easy to culture in a lab. Moreover, the fact that they clone themselves means you can culture an individual under a certain condition (for example in the presence of a pesticide) and culture her genetically identical sister in another condition (the absence of a pesticide), to study the exact effect of the condition, without interference of the genome of the individual. Also, as long as you keep culture conditions favorable, you can keep clonal lineages in the lab indefinitely, so you can do many experiments on the same set of clones!

The asexual phase of its life cycle is not the only thing that makes this organism such an ideal study system. When Daphnia start to reproduce sexually, they produce resting eggs that are encased in a protective envelope. The intriguing thing about the resting eggs is: they are deposited in the layered sediment, and not all eggs hatch when conditions turn favorable again. Some stay dormant. These are slowly buried in the layered sediments, and when resurrection ecologists come by, they take sediment cores, digging up eggs from the 50's and 60's, as shown in the picture below. You can still hatch some of these eggs, and do experiments on a clone whose parents lived long before the French revolution!

So ... cloning, resting eggs, that's all very nice and a lot of fun. But is all of this relevant? Ah! Daphnia isn't "just" an ideal model system in science (which needs model systems of every kind, such as the bacterium E. coli, or the fruit fly, mice and rats, several species of plants, etc…), Daphnia also happens to be a "keystone species" in freshwater ponds and lakes. They are grazers, so they eat algae and keep the ponds and lakes from turning green. In turn, they are an important prey item for higher trophic levels (fish and invertebrates), which gives them a central position in the aquatic food web. So, given their importance in worldwide aquatic systems, it's not just cool but also very useful to study them.

A huge amount of knowledge is already available on Daphnia, since it has been a model system in (general and evolutionary) ecology for a very long time. We know, for example, Daphnia are strongly affected by various kinds of pollution.

Plastic and chemical pollution affecting Daphnia.

Similar to so many other species, Daphnia are also under a lot of other anthropogenic selection pressures. Rising temperatures due to climate change have strong effects on Daphnia. More subtly, recent research shows that urbanization also affects Daphnia, by (amongst many other effects) creating heat islands in cities, and by decreasing dispersal possibilities for the Daphnia.

Resurrection ecology gives us a very strong research tool to study the effects of these (fairly) recent anthropogenic pressures on natural populations, because we can compare populations from today and populations from a century ago. This allows us to, for example, compare evolutionary potential of past and present populations to respond to urbanization and climate change.

Recent studies have shown that the fitness of Daphnia is strongly affected by their gut microbiome. The gut microbiome is also very important for human health, and studying microbiomes is thus very useful.

If you want to know more about currently running projects on waterfleas, check out the rest of this website! In particular, in the KU Leuven C-project, the waterflea, its microbiome and one of its main predators, the damselfly, are jointly studied in experiments to gain a more thorough insight into their exact responses to climate change, pollution and toxic cyanobacterial blooms.

In one of these studies, for instance, the interaction between predation by a damselfly on the waterflea and urbanisation is studied: