The barrel amphipod is a hitchhiker that devours its host

The midwater is a vast expanse of water between the surface and the seafloor. There are few places for rest or refuge here. The barrel amphipod (Phronima sedentaria) is a shrimp-like crustacean that adapts by living inside the repurposed bodies of gelatinous animals called salps. The amphipod uses its sharp claws to snag a salp, then rips out the soft tissues inside. But that salp is more than a meal—Phronima carries around the carved-out carcass as its home. It even “remodels” its shelter by reshaping the barrel and secreting chemicals to toughen the structure. Female barrel amphipods release their young inside the salp, and their hatchlings feast on the salp’s tissues.