Hermit crabs in a bottle on a remote island.

The smooth surface of plastic means that the hermit crabs cannot escape once they have fallen in © Lavers et al. 2019

Over 600,000 hermit crabs die in plastic bottles on two remote islands

Hundreds of thousands of hermit crabs are trapped and killed by discarded plastic on the beaches of two remote tropical islands, causing unknown damage to the animals' populations. 

This study is thought to be the first time anyone has measured the population-level impacts caused by plastics on any species.

The Cocos (Keeling) Islands and Henderson Island are remote outcrops found in the Indian and Pacific Oceans. Despite their isolation, the islands are covered with rubbish. Recent work shows that the uninhabited Henderson Island has some of the most polluted beaches in the world

The steady tide of plastics that enter the world's oceans show no signs of stopping. 

Over the last few years, this continual outpouring of debris that can remain in the environment for decades has been shown to have terrible impacts on the planet's wildlife, from seabirds that are feeding their hungry chicks plastic fragments to whales washing up with flip-flops in their stomach. Yet quantifying the deaths of animals caused by plastic is incredibly difficult. 

Despite the remoteness of the Cocos (Keeling) Islands and Henderson Island, both are covered in plastic trash © David Stanley/Flickr CC BY 2.0

While working on these two islands Dr Alex Bond, the Museum's senior curator in charge of birds, and his colleague Dr Jennifer Lavers noticed something worrying. 

'Over the years we had noticed a lot of plastic containers lying on the beach with dead hermit crabs in them,' recounts Alex. 'Initially we only saw one or two dead crabs but the more places we went, the more we saw.

'We conducted surveys on the Cocos (Keeling) Islands and Henderson Island. Using the same statistical analysis that is used for bycatch in fisheries, we then estimated how many hermit crabs were trapped in bottles at these two sites. 

'The results were quite staggering.'

On the Cocos (Keeling) Islands, they've estimated that over 500,000 hermit crabs are trapped and killed by plastic debris that has washed up on the beaches, while on Henderson this number is thought to be around 60,000.   

The work as been published in the Journal of Hazardous Materials.

A deadly feedback loop

Recent studies have found that remote islands often act as sinks for the plastic floating around the world's oceans. Currents that swirl around these outcrops carrying and distributing much-needed nutrients are now also loaded with plastic and debris.

Plastic covering the beaches of Henderson Island

Henderson Island is one of the most remote places in the world, and yet has some of the highest concentrations of plastic © Melva Evans

This debris can include food wrappers, bottles, fishing gear and household trash. As it makes its way to the beaches of these islands, the garbage begins to break up and degrade. 

When this plastic lands on the beaches, it creates the perfect trap for unsuspecting hermit crabs that are wandering the islands looking for bits of food or fresh water that may be found in the bottom of these containers.

Anything that has an opening can form a trap for the crabs - drink bottles with just a small hole, to giant fishing buoys, five-gallon fuel drums. As plastic is incredibly slippery, once the crabs fall in they simply cannot climb out.

But the situation is far more sinister than that. One plastic container was found to contain the shells of over 520 dead crabs.

'It's quite insidious really, because it only takes one crab,' says Alex. 'We're talking about hermit crabs, which means that they don't have a shell of their own. This means that when they die in one of these containers, they emit a chemical signal that basically says there's a shell available. 

Hermit crab shells removed from a bottle

In a single plastic container, the team found over 250 hermit crab shells likely representing the number that died © Mandy Barker

'This attracts more crabs that then fall into the containers and die, which then sends out more signals that say there are more shells available. 

'Essentially, it's a gruesome chain reaction.' 

Stemming the tide

The strawberry hermit crab (Coenobita perlatus) is found across the Indian and Pacific Oceans. Living up to 30 years in the wild they are efficient scavengers, cleaning up the beaches and forests of islands, and in doing so they help the turnover of nutrients.

'They perform essential ecosystem services,' explains Alex. 'These crabs are the ones cycling nutrients on the sandy beaches, as well providing prey for predators. They are an essential part of the food chain.

'But it is tricky to saying how these deaths are impacting hermit crab populations as we don't really know how many hermit crabs are living on these islands. But the simple thing is that on Cocos there are now half a million fewer hermit crabs than there would be otherwise.'

Hermit crab on a beach

Hermit crabs are an essential part of an islands ecosystem, meaning that the huge number of deaths could be having an unknown impact © Melva Evans

The one certainty is that if an island has populations of hermit crabs, and there are bottles and plastic containers on the beach, then there will be trapped crabs.

The solution to this situation is, as is often the case, complicated. While there are already hundreds of millions of tonnes of plastic in the world's oceans, more and more are being added every single day.

'The solution is twofold,' explains Alex. 'The first is reducing our reliance on plastic in general. We can do some of that through, for example, using reusable drinking bottles.

'Equally a lot of these bottles that are already out in the ocean are a result of leaks in waste management systems. Secondly, we need to find those leaks and prevent that escape from proper waste management getting into the ocean.'

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