A Discarded Tire in the Ocean Is a Hermit Crab Death Machine

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In June 2012, when surveying the seabed at a depth of 8 meters in Mutsu Bay, Japan, Hirosaki University biologists Atsushi Sogabe and Kiichi Takatsuji, came across a macabre sight (pictured top): a discarded car tire filled with hundreds of hermit crab shells, some occupied by living inhabitants, but many more damaged and derelict, their occupants apparently long deceased.

The duo hypothesized that hermit crabs could climb up the sides of the tire and enter the inner circle, but once there, they could not escape on account of the tire's concave inner wall. Sogabe and Takatsuji tested their prediction between October 2015 and September 2016. They set six passenger car tires on the seabed at various locations in Mutsu Bay and checked them once a month. Any hermit crabs inside were counted, collected, examined, then released at least fifty meters away from any of the study tires.

Over the year-long experiment, 1,278 hermit crabs were trapped within the six tires in Mutsu Bay. If not collected and released at monthly intervals, they would have undoubtedly died of starvation or cannibalism, the researchers say.

Atsushi Sogabe and Kiichi Takatsuji

Later on, Sogabe and Takatsuji conducted a controlled experiment in an aquarium to confirm whether or not crabs could enter and escape a tire. They placed a car tire in a large tank with a sandy bottom, then added hermit crabs from the most common species they observed during the field experiment. In some trials, hermit crabs were placed outside the tire. In others, they were placed inside. Over six trials, each lasting 18 hours, they confirmed that hermit crabs could climb into the tire, but not get out.

Taken together, the two experiments clearly show that discarded tires on the ocean floor can be a scourge to hermit crabs. The researchers describe their devastating effects as analogous to 'ghost fishing', in which abandoned fishing gear can continue to catch and entangle aquatic animals without human control. But tires could be worse, Sogabe and Takatsuji say.

"The length of time that fishing gear can ghost fish ranges from a few months to 3 years, depending on gear type, what it is made of, and the environment into which it was discarded. Because of their simple structure, temporal persistence and robustness, tires may ghost fish hermit crabs for considerably longer."

"Hermit crabs are important as both prey and scavengers and ecosystem engineers..." the authors note. "The effects of ghost fishing on their populations, and cascading effects these may have on coastal communities and ecosystems, are unknown."

Source: Sogabe Atsushi and Takatsuji Kiichi 2021 "Marine-dumped waste tyres cause the ghost fishing of hermit crabs." R. Soc. open sci. 8210166210166 http://doi.org/10.1098/rsos.210166

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