Crabs in the river Thames are ingesting “shocking” amounts of plastic with in excess of 100 synthetic fibres in some crustaceans, according to researchers.

Almost all of the 55 shore crabs and 37 mitten crabs dissected by experts had plastic inside their stomachs, intestine or gills.

Speaking at New Scientist Live, Alexandra McGoran from London’s Natural History Museum said most of the plastic was inside the crab’s small stomach, called the gastric mill.

“What is particularly shocking is not only are they filling the stomach but they can be made up of over 100 fibres [in each crab], so they are very highly contaminated,” Dr McGoran said in her talk recorded by New Scientist.

“A lot of that plastic was so tightly wound together through this grinding mechanism (the gastric mill) that they completely filled the stomach with these big knots of fibres.”

Sanitary pads and a yellow balloon were among the plastics discovered in the creatures.

“Crabs are a very unusual sink for plastic. They seem to contain a lot of it and they retain it for a long time. We don’t know whether they’re predated on and whether that high dose is passed on to other animals,” said Dr McGoran.

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Plastic pollution in the Thames comes from many sources, including from household washing machines. Millions of plastic microfibres are shed every time we wash clothes that contain materials such as nylon, polyester and acrylic.

One of the biggest polluters in the Thames is wet wipes and sanitary products, which are often flushed down toilets. These are blocking sewage systems and are a huge issue both for the environment and also for water systems, Dr McGoran said.

Plastic has been found throughout the world’s seas, with pieces showing up everywhere from Arctic sea ice to the bottom of the Mariana Trench, the deepest trench in the world. Often its harmful effects are obvious, particularly when whales die after choking on plastic bags or turtles become tangled in discarded fishing nets.

Plastics also contain a range of potentially toxic chemicals, including additives used in their manufacture and substances that they absorb from the environment.

The impact of these chemicals leaching into the surrounding water is not well understood, although some previous work published in Biology Letters has suggested they can disrupt the growth of animal embryos.