Science for All newsletter: The marine crab that grew too big for its plastic shell

In July 2022, a group of scientists recovered a seemingly unremarkable barnacled, opened plastic bottle bobbing off the coast off Okinawa in Japan; but something piqued their imagination

This bottle, investigated by Hiroshima University researchers indeed contained a story: living inside it was a large female crab, missing a leg, its body larger than the mouth of the bottle, leaving scientists puzzled about how it got trapped in there, and survived.

The opening of the bottle was 24 mm in diameter, while the crab inside was 40.31 mm long, 88.23 mm wide, and weighed 42.06 gm. Interestingly this individual was found to have a higher weight compared to wild populations. The paper was published in the journal Ecosphere earlier this year.

The crab, the scientists deduced through dissection, had entered the bottle as a juvenile, fed on small fishes and algae that wafted into the opened mouth of the bottle and survived there for about two months; it then grew too big to make its way out. The scientists analysed the crab’s stomach content and found it had eaten juvenile fishes such as the rough triggerfish and algae that had probably grown inside the bottle.

A study of the barnacles on the bottle helped the scientists estimate that the it was drifting for 62 days. “These results suggest that the crab entered the bottle at the larval or juvenile stage, survived with sufficient nutritional conditions, and continued to grow inside the bottle while drifting for approximately [two] months,” the paper said. It warned that anthropogenic debris such as this gives their victim no opportunity to reproduce or enhance their fitness.

The bottle was made of high-density polyethylene, which is buoyant in seawater and can remain in the environment for several decades while retaining its shape. “Given its narrow-neck design, we hypothesize that such bottles may have similar impacts on other crustaceans and fishes associated with drifting objects,” the paper cautioned.

The authors likened the crab’s short life to a work of fiction by Japanese novelist Masuji Ibuse, titled Salamander: “The salamander felt sad. He had tried to leave the cave that was his home, but his head stuck in the entrance and prevented him from doing so,” Ibuse wrote in 1929, about the despondent salamander that spent two years continuously eating in its burrow, until it became to big to escape.

Plastic waste, we know well, poses a significant danger to marine life, killing sea creatures across taxa, including birds, marine mammals, reptiles, fishes, and invertebrates. Seabirds routinely get entangled in fishing nets; animals ingesting plastic is documented; and the iconic image of a plastic straw found embedded in a turtle’s nostril is again emblematic of this malaise. And of course, the Great Pacific Garbage Patch, the world’s largest accumulation of ocean plastic in the North Pacific, is growing bigger everyday.