To boldly go – microplastics make hermit crabs more risky

Exposure to microplastic pollution may affect hermit crabs’ sensitivity to predators as they behave bolder and in more predictable ways, according to a new University of Essex study.

The contamination of natural systems with plastic debris has become one of the most pressing global environmental issues. Microplastics are of particular concern as their abundance and small size means they can be ingested by a wide range of marine life.

Whilst many studies have investigated the toxicity issues around microplastics, this Essex-led research is the first to look at the potential impact of microplastic exposure on different levels of animal behaviour.

Published in the Journal of Hazardous Materials Letters, the research looked at the potential effects of microplastic pollution on the behaviour of European hermit crabs, Pagurus bernhardus. In particular, the scientists were interested in the startle response – the main risk-avoidance mechanism of hermit crabs, where the animal retracts into its shell when exposed to a potential threat. The length of time it takes a crab to re-emerge is often used as a measure of individual boldness, and predictability of this behaviour is believed to affect predator-prey dynamics, as more predictable individuals are easier to catch.

Dr Gerrit Nanninga, who led the study, said: “We found that increasing microplastic exposure led to a significant decrease of startle duration in general and a reduction in the variability in this behaviour. In other words, crabs became less risk averse on average by re-emerging faster from shelter and their behaviour became more predictable with increasing microplastic concentrations. These findings indicate that microplastic pollution could affect these crabs’ sensitivity to predators.”

Microplastics have been found in virtually all aquatic ecosystems, from the deep oceans to the Antarctic ice sheet and new sampling techniques show that the abundance of small microplastics might be far higher than previously thought. Concentrations of small microplastics in the world’s oceans are likely to rise even further because global plastic pollution is still increasing. In fact, some projections predict a doubling of microplastic concentrations in the world’s oceans over the coming decade.

“The natural habitat of hermit crabs also makes them particularly vulnerable to high microplastic exposure,” added Dr Nanninga from Essex’s School of Life Sciences. “Many species inhabit estuarine ecosystems, which, particularly in urban areas, are prone to high levels of microplastic contamination.”

The research was undertaken in collaboration with researchers from the University of Plymouth. Before behavioural testing, the crabs were kept in individual tanks with different levels of microplastic pollution: a control, a low concentration treatment (representative of environmental concentrations currently measured in some areas) and a high concentration treatment (representing potential future levels of exposure over coming decades). A startle response was induced by lifting individual crabs out of the tank by hand and inverting it for five seconds – which caused the crab to withdraw into its shell – before gently placing it back in the tank in the inverted position. Startle response duration was then measured as the time it took the crab to first touch the tank floor with one of its legs.

One possible theory behind this change in behaviour is that microplastic exposure may lead to an energy imbalance. The resulting increased need for ventilation and/or food could make the crabs leave their shells more quickly than they would normally do, thereby exposing themselves to greater risk from predators.