Tiny freshwater crustaceans called Daphnia, sometimes called water fleas, are common in North American lakes. These delicate, transparent creatures are indispensable to the health of our freshwater ecosystems. They graze on algae and serve as food for fish, creating an important link in the food web. New research finds that Daphnia young subjected to a fungal-like parasite known as microsporidia can experience a sudden increase in mortality. This is counterintuitive since their mothers show only slight signs of infection.
Daphnia are frequently seen swarming en masse, with hundreds showing up in a single scoop of lake water. Their internal organs usually have a noticeable greenish tint. This tube—as the gut is often called—typically travels in a simple line just a single cell deep along the middle of their bodies. After being infected by microsporidia, Daphnia’s gut would go through macro visible metamorphosis, acquiring opaqueness and crystallized crystalline structure. This transformation is a good indication that the parasite is there. For adult Daphnia, the reproduction and health effects observed are usually small.
The Hidden Costs of Infection
Meghan Duffy, a professor at the University of Michigan, studies symbiotic relationships among freshwater plankton. Her surprising discoveries have deep implications for future generations of Daphnia. Adult Daphnia might appear healthy despite being infected. The toll on their children can be catastrophic.
Duffy reminds scientists that they’ve spent a lot of time studying virulence. This idea is often operationalized as the impact of a parasite on the fitness of its host. All of that often only takes into account the effect on the specific host organism that’s infected. This oversight can result in a limited understanding of the role parasites play in regulating populations over longer time scales.
The significance of this research goes beyond biology. This is an environmental health and ecological stability issue. Scientists are looking to understand the multigenerational impacts microsporidia. One key area they’re focusing on is the impact maternal health has on offspring survival rates. Duffy explains the ways a mother’s hunger can deeply affect her progeny. Most obviously, the absence of resources has a huge impact on their evolution. So if she’s exposed to a predator, that is going to have consequences for her offspring. We know that these effects can have intergenerational effects.
Ecological Importance of Daphnia
These microscopic crustaceans are crucial filters, maintaining the balance of our lake ecosystems. By filtering huge quantities of algae out of the water, they serve to keep lakes clear and not a soupy “pea soup.” In turn, these macroinvertebrates support the dominant food web which includes juvenile bluegill among other species of fish.
They’re a very crucial connector in the lake food web. Since they yum up the algae, these critters prevent our lakes and ponds from devolving into green pea soup. Then, fish eat them,” Duffy explains. A billion Daphnia swarm each lake. They do eat, and eat a lot – actively grazing on tons of algae every day.
Their ecological contributions as indicators, commanders, and cultural connections emphasize the necessity of understanding their health and survival from parasitic infections. Duffy notes, “There are billions of them in a lake, so they can eat a lot of algae in a day. Then, juvenile bluegill think that they’re just scrumptious.” Due to the interconnectedness of all these species, any disruption in the Daphnia population would cause a cascading effect throughout the ecosystem.
A Call for Broader Research
Duffy’s work not only calls into question how scientists think about and measure virulence, but has important implications for understanding generational effects of parasitic infections. Current models often fail to consider intergenerational impacts. This oversight fails to capture the full effects of parasites, like microsporidia, on their hosts’ populations.
She implores, “We want other scientists to start looking into this space. If we only measure virulence in the standard way over just one generation, we might miss the parasite’s greatest effects. Our calls to action here highlight the compelling need for rigorous research. We need to conceptually broaden our thinking about how infections impact not just single organisms but across entire populations through time.