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House of Plastic: How some of the earth’s smallest inhabitants are using microplastics as building materials

By September 24, 2020No Comments
© Courtesy of Artur Lik Photography

Last week we caught up with researcher Sonja Ehlers, PhD student at the University of Koblenz-Landau and the German Federal Institute of Hydrology, to talk about how plastic is affecting some of the earth’s tiniest aquatic creatures.

Chances are you may have never heard or much less seen them, but caddisfly (Lepidostoma basale) have just become the latest example of how our mismanaged plastic waste is affecting aquatic life. Observed in their natural environment and laboratory experiments, the larvae of these tiny insects use plastic fragments to build tubular shelters to protect them from predators and the elements – but they are not ideal.

Research beginnings

Originally from Germany, Ehlers was first confronted with the problem of plastic pollution while she was carrying out research for her master’s degree in marine ecology in Nova Scotia, Canada. She recalls, “whenever I went to the field to carry out the experiments, I always saw plastic bottles, containers, especially in the intertidal areas and thought this really needs to be investigated further.” Noticing the abundance of plastic, she decided to pursue a PhD on the impact of plastic pollution on aquatic lifeforms.

Returning to Germany she landed a placement with the German Federal Institute of Hydrology and the University of Koblenz-Landau. Tucked away between two rivers, the Rhine and Moselle, Ehlers found the perfect location to carry out her experiments. Her idea for the research started with a single observation: “We know that many species of the caddisfly use materials from their immediate surroundings such as sand and plant materials to build their cases to protect their bodies, so why should they not be using plastics which sink down to the bottom of aquatic habitats?” Her study is unique in the sense that, “most of the research around microplastics and animals focuses on consumption, but rarely do we know how they interact with them.”

The caddisfly and their shelters

There are roughly 7000 species of caddisfly, the larvae of which develop in aquatic environments, such as rivers and streams. In her description, Ehlers likens the caddisfly to butterflies who undergo a series of stages of development, turn into a pupa and then emerge as a flying terrestrial adult. Once in the water, she continues, “the larvae cover their bodies using plant material, wood sticks, leaves, snail shells, pebbles or other materials.” The material chosen depends on the species but also their surroundings. The quality of the cases, which are built as emergency or portable homes are critical to their survival as they protect them from predators and environmental influences. She explains, “the cases or shelters are used to protect their soft bodies from predators such as a dragonfly larvae and fish, and some of the species use the cases to channel water along their gills to filter the oxygen out of the water.”

However, as the larvae build their shelters based on what is available to them, the abundance of microplastics in aquatic ecosystems means that plastic has become an option – not necessarily to their benefit.

© Courtesy of Sonja Ehlers. The river where the caddisfly larvae were collected for the experiment.

The impact of plastic

Ehlers’ experiment which involved putting PVC and PET plastic fragments, together with sand, into the water tanks, showed that most of the larvae began making their shelters using the plastic fragments first. She suspects, “the plastic fragments are much lighter than sand grains, which may have been why these materials were initially chosen, as they were quick and easy to build with.” However, the larvae began using the sand grains once they discovered that their shelters became positively buoyant, lifting at an upwards slant towards the water surface. The heavier sand grains anchored the structure down, making them more stable.

© Courtesy of Ehlers et al. (2020). A. Typical field-collected original case. B. Portable case built from sand under laboratory conditions (treatment: ‘only sand’). C Portable case built from sand and black PVC particles under laboratory conditions (treatment: ‘PVC hc’). D. Portable case built from sand and blue PET particles under laboratory conditions (treatment: ‘PET lc’).

Although caddisfly larvae incorporate a wide variety of materials in their shelters, the use of plastic proved to be problematic. Ehlers’ experiment demonstrated that with an increasing percentage of plastic fragments in the shelters, case stability decreased – in other words – they were more likely to break under pressure. Yet it is not just the fragility of the plastic which make them more prone to predators, but also their brighter colours. She surmises that their lightweight could also make them more vulnerable to river currents. As the larvae undergo crucial stages of their development within these cases, plastic leaching may also prove to be problematic for them and other animals up the food chain.

Looking ahead

Like most researchers passionate about their field of study, Ehlers’ ideas for a follow up project are already brewing. How larvae interact with different plastic types and the effect of toxins have sparked her interest, but her short-term focus will be on submitting her PhD thesis in just a few months’ time. An image of the larvae wrapped in plastic is set to be published by Frontiers in Ecology and the Environment this year. A previous study by Ehlers, published in Aquatic Biology, documented the presence of microplastics in caddisfly cases in their natural environment.

Ehlers’ research illustrates that the caddisfly larvae are not unique in that they simply make do with their changing environment, but they are a reminder of how our mismanaged plastic waste is affecting some of the tiniest creatures on earth. The images of the delicately designed plastic shelters feel almost bittersweet. They mirror our own need for a safe and secure place to live, but at the same time represent the more distasteful side of human behaviour.

For more about Sonja Ehlers and her research, find her website here.