This hitchhiker 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.
Smithsonian’s National Museum of Natural History and MBARI scientists are examining the extraordinary “double” eyes of Phronima that have perplexed scientists for decades. The barrel amphipod has a small pair of eyes that look to the side and a second, giant pair that looks up. Scientists have now shown that Phronima’s unique eyes evolved to help it see from within its gelatinous barrel. Understanding how deep-sea animals see the world around them has inspired the development of new artificial visual systems used on robots like the Mars Rover and for medical equipment.
Maximum size: 4 centimeters (about 1.5 inches)
Depth: surface to 600 meters (2,000 feet)
Habitat: surface zone (epipelagic zone) to twilight zone (mesopelagic zone)
Range: worldwide in tropical and temperate waters
Diet: zooplankton, including crustaceans, cnidarians, pelagic tunicates, and other gelatinous animals
Baldwin Fergus, J., S. Johnsen, and K.J. Osborn (2015). A unique apposition compound eye in the mesopelagic hyperiid amphipod Paraphronima gracilis. Current Biology, 25(4): 473-478. doi.org/10.1016/j.cub.2014.12.010
Stenvers, V.I., B.C. Gonzalez, F.E. Goetz, J.M. Hemmi, A.-L. Jessop, C. Lin, H.-J.T. Hoving, and K.J. Osborn (2021). Extraordinary eyes reveal hidden diversity within the holopelagic genus Paraphronima (Amphipoda: Hyperiidea). Deep-Sea Research I, 177: 103610. doi.org/10.1016/j.dsr.2021.103610