THIS study documents an example of an invertebrate that cannot defend itself chemically (an amphipod) increasing its chances of survival by capturing and carrying a species that can (a pteropod). Although chemical defences are found in a wide variety of marine invertebrates1-4, few studies have established the extent to which these chemicals will deter predators5-7, and even fewer have investigated how one organism might exploit another's chemical defence to protect itself8-10. These chemicals are usually ingested or sequestered in the host's tissue8,9. Several species indiscriminately decorate themselves with potentially toxic organisms or are passively fouled with chemically defended organisms11,12. These seem to be commensalisms or sometimes mutualisms. Our example has benefits and costs for the carrier, but only costs to the captive. The antarctic marine food web not only has a variety of chemically defended organisms13-16, contrary to earlier predictions17-19, but has at least one unusual symbiosis that makes use of noxiousness. © 1990 Nature Publishing Group.