This blog post is provided by Denis Meuthen and tells the #StoryBehindthePaper for the paper ‘On the use of antibiotics in plasticity research: gastropod shells unveil a tale of caution’, which was recently published in Journal of Animal Ecology. In the study, they look at how antibiotic exposure affects shell-thickness responses in the snail Physella acuta, which is known to develop thicker shells in the presence of predators.  

You might be aware of the antibiotic crisis that plagues our planet. Their prophylactic use, without consideration for aftereffects, is widespread among some animal farmers, veterinarians and even medical doctors. This practice has already led to the emergence of multidrug-resistant bacteria strains that are responsible for the death of many people. However, the consequences of prophylactic antibiotic treatment for animals are much less known.

Animals have evolved an ability that allows them to cope with sudden changes to their environment. When environmental changes happen, they can alter their hormone levels or DNA expression, and this can modify the behaviour, morphology and life-history of animals. This is known as phenotypic plasticity. This ability is crucial also in the face of predators. This is because when prey animals perceive the presence of predators, they can ensure the survival of their genes by escaping, by forming strong defences or by reproducing earlier.

A well-studied and common defence that is caused by the presence of predators are thicker shells in the snail Physella acuta (Figure 1). When these snails smell dead conspecifics, they invest energy into making their shells thicker so that predators cannot crush them anymore. However, studying these snails is not easy as many of them die in the laboratory during the course of an experiment. In a recent study, Thomas DeWitt and Heather Prestridge suggest that prophylactic antibiotic treatment may reduce snail mortality. They also singled out the antibiotic erythromycin to be the most efficient one for this purpose.

We were now interested in studying how a prophylactic treatment with erythromycin affects the ability of snails to form a thicker shell. For this purpose, we set up four treatments. Exposure of snails to either an antibiotic or a control water treatment was followed up by either exposure to the smell of dead conspecifics (high-risk) or a water control (low-risk). Antibiotic-exposed snails showed a greater difference in shell thickness between the risk treatments than controls. This was because in the absence of antibiotics, snails formed thick shells even when they experienced only low risk, which is likely the result of infection with unknown pathogens. By eliminating these pathogens, antibiotic treatment uncovered a greater amount of plasticity.

Does that mean that we should always use antibiotics as a prophylactic treatment during future studies? No – because parasites and pathogens are part of nature. If we want to understand if and how animals cope with the presence of predators or other environmental change in their natural environment, we need to consider that it may be common for them to be infected at the same time. Nevertheless, if our research questions are less concerned with the natural circumstances and are more focused on mechanics of plasticity, prophylactic antibiotic use may be quite helpful.

However, not all questions are answered with this study. For example, it remains unknown which specific pathogen or parasite in our snails could have caused them to grow thicker shells. They may do so because thicker shells do not only protect the snails but also their parasites. We know that some flatworms can cause snail shells to grow thicker, but in our study population we can largely exclude their presence. Maybe there are some microbes with similar mechanisms at work? Hopefully, future studies will help us learn more.

Read the paper

Read the full paper here: Meuthen, D. & Reinhold, K. (2023) On the use of antibiotics in plasticity research: gastropod shells unveil a tale of caution. Journal of Animal Ecology, 00: 1-10. https://doi.org/10.1111/1365-2656.13909

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