Swimming crab in a bottle: A two‐month drift on the ocean surface while entrapped

Abstract Marine debris negatively impacts coastal marine ecosystems through ghost fishing gear that traps marine life, entanglement, and the transport of biofouling communities outside their native habitats. During a boat survey on July 15, 2022, a female swimming crab ( Portunus sanguinolentus ) entrapped inside a high‐density polyethylene bottle was found floating on the surface of offshore tropical waters along the western coast of Okinawa, Japan. The crab was larger than the mouth of the bottle (24.0 mm) in both carapace width (CW = 88.23 mm) and carapace length (CL = 40.31 mm), and appeared unable to escape on its own. Four juvenile fish species ( Canthidermis maculata , Elagatis bipinnulata , Abudefduf vaigiensis , and Psenes cyanophrys ) were associated with the bottle. We hypothesized that the crab survived by hunting juvenile fish that entered the bottle. DNA metabarcoding of the stomach contents revealed that the crab consumed the same fish species ( C. maculata and A. vaigiensis ) observed near the bottle. Green and brown algae ( Ulva compressa and Myrionema strangulans ), most likely growing inside the bottle, were also identified in the diet. Based on the growth rate of the pedunculate barnacle Lepas anserifera (Lepadidae) attached to the bottle, the drifting period was biologically estimated to be 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 2 months. In conclusion, this study documents previously unreported negative impacts of marine floating debris on small crustaceans. Although this crab was able to feed and survive, individuals entrapped in such anthropogenic debris likely have no opportunity to reproduce and thus have no prospect of enhancing their fitness.