New research has found that Fiddler crabs are playing an unheralded role when it comes to hoovering up microplastics found in the world's mangrove forests and salt marshes.
Scientists studying a thriving population of Fiddler crabs in a polluted mangrove forest in Colombia have found that they can ingest and break down large quantities of small plastic particles in the sediment.
The ability to mobilise large amounts of sediment for feeding and sheltering, and the creature's specialised digestive processes, which earn it the reputation of being an 'ecosystem engineer', can break down plastics within days, much faster than sunlight and waves.
However, scientists say that this litter recycling service may come at a cost, with the potential release of harmful nanoplastics into their tissues and, consequently, the food chain.
The research was led by Professor José M. Riascos, with a team drawn from several institutions, including Universidad de Antioquia in Turbo and Medellin, the University of Exeter, and the Corporation Center of Excellence in Marine Sciences (CEMarin), in Bogotá. It's been published today (Wednesday 17 December) in a new paper in the journal Global Change Biology and challenges the theory that bivalves such as scallops are the most vulnerable to microplastic ingestion.
"There are many stories about the detrimental impacts of plastic waste on animals and plants," says Tamara Galloway, Professor of Ecotoxicology in Exeter's Department of Biosciences. "But the converse is less often studied: how do animals interact with and affect plastic litter, especially in pollution hotspots?
"We know that Fiddler crabs eat a wide range of food and will ingest plastic in laboratory settings. But until now, we did not know whether they avoided plastic in the natural environment or would adapt to its presence."
The researchers conducted the study in the highly polluted mangrove forests of Turbo, on the North coast of Colombia. Here, in the Gulf of Urabá, years of urban and agricultural expansion have degraded the Mangrove systems, leading to some of the highest levels of plastic contamination reported anywhere in the world. Despite this, Fiddler crab populations are thriving in the area, raising the question of how these creatures tolerate such high levels of plastic waste.
Five one-metre-square plots of urban mangrove area were selected, and in each, the researchers sprayed 100ml solutions containing red and green fluorescently labelled polyethelene microspheres. They repeated the process over the course of the 66-day experiment, before sampling the soil and a total of 95 crabs.
Professor Riascos, of the Instituto de Ciencias del Mar, at the Universidad de Antioquia, and CEMarin, said: "We set out to answer three specific questions. The first question was: What is the uptake of microspheres by the crab under natural feeding conditions? Secondly, we wanted to learn how the microspheres are distributed among the major organs. And finally, we sought to find out whether the crab's interaction with microspheres resulted in their physical fragmentation into smaller particles."
The results revealed that the crabs had accumulated microplastics at 13 times the concentration found in the sediments. They also showed how the plastics were not evenly distributed in the crabs' organs, but found in the greatest quantity in the hindguts, followed by the hepatopancreas and then the gills.
Many of the microplastics were fragmented further in the process, with the team concluding that the animal's specialised grinding gut, together with plastic-degrading bacteria, could be aiding this process. There was also a greater chance of finding fragmented particles in females than in males.
"The results emphasise that living creatures are not just passive components of the marine ecosystem but may be finding ways to cope with chronic anthropogenic pressures according to their evolutionary histories," adds Daniela Díaz , of the Universidad de Antioquia. "The results could lead to a better understanding of how animals adapt to pollution and the fate of plastics in the environment."
