An oceanic mystery has unfolded with the discovery of a large crab inside a plastic bottle, with an opening far too small for the crustacean to have fit through.
The seemingly impossible discovery, made by researchers with Hiroshima University, has now revealed a new dimension to the effects of pollution in our environment, and how plastic objects can have unexpected impacts on ocean creatures.
The researchers made the curious discovery off the coast of Okinawa, Japan. Upon discovering a polyethylene Shaoxing wine bottle drifting along on the ocean surface, they soon realized that the discarded plastic container was thriving with signs of life.
Most peculiar of all was the presence of a large swimming crab within the bottle, which was obviously too have made its way inside the container through its narrow opening.
“During juvenile fish surveys in offshore waters, we happened to encounter a floating plastic bottle approximately 500 meters (about 550 yards) off Sesoko Island, Okinawa, Japan, with many juvenile fish associated with it,” said Hajime Sato and Yoichi Sakai, the authors of a new study describing the discovery.
According to Sato, “a large live swimming crab, Portunus sanguinolentus, was trapped inside the bottle.”
“The crab was clearly larger than the opening of the bottle!” Sato said.
The Marine Plastic Problem
It is no secret that plastic has been a longtime contributor to pollution in Earth’s oceans, with wide-ranging impacts on oceanic environments that include ocean creatures becoming ensnared in discarded plastic bags, dishing nets, or other plastic refuse.
Sato and Sakai say the plastic bottle they collected in July 2022 had been manufactured the previous November, and had been left uncapped, allowing seawater to enter and leave. However, it was clear that the crab they discovered within it could not have made its way in as easily as the ocean water it contained.
Unraveling this oceanic mystery required an array of biological analyses, which ultimately led them to a striking conclusion: the crab had entered the bottle when it was far smaller, and remained there, subsisting on organisms and other food sources making their way into the small plastic vessel, allowing it to grow as the bottle drifted along.
Based on DNA analysis of the crab’s stomach contents, the crustacean had recently consumed juvenile fish that entered the bottle, including specimens of the rough triggerfish Canthidermis maculate. Additional analysis also revealed the presence of the sergeant major Abudefduf vaigiensis.
Evidence of algae within the bottle indicated another likely food source for the crab, while on the bottle’s exterior, goose barnacle growth revealed the likely time the bottle must have spent drifting for these oceanic hitchhikers to become attached and grow.
The most likely scenario, according to Sato and Sakai, is that the young crab entered the bottle, where it fed on juvenile fish and algae over the course of two months, eventually growing large enough that it could no longer escape.
“Plastic bottles discarded by humans can trap crabs and prevent their escape,” the authors conclude, adding that “Similar cases have already been reported from waters around Japan, suggesting that this was not an isolated accident.”
“Through this striking example, we would like readers to recognize that objects that make our lives more convenient can sometimes have unexpected effects on small marine animals,” Sato and Sakai add, “while also appreciating the remarkable vitality of the swimming crab,” the authors conclude.
The unusual discovery was detailed in a recent study, “Swimming crab in a bottle: A two‐month drift on the ocean surface while entrapped,” which appeared in the journal Ecosphere.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.