The amphipod places the captured pteropod on its own back in full view, still holding it with the pincers, and goes about its business. Presumably, amphipods capture pteropods as a chemical weapon against fish predation. The scientists ran laboratory tests to find out for sure. The experiments revealed that amphipods carrying a pteropod were immune to fish attack. Those without pteropods were readily consumed by fish.
Many animals use chemical defense against predators.
Skunks, scorpions, and stingrays are but a few. But all these as well as some other species produce their own chemical warfare. The amphipod-pteropod relationship is a rare example of how a prey species actively exploits another for chemical protection. The amphipod pays a price for carrying around its kidnaped bundle of protection. With the added weight, swimming through the Arctic sea is a bit more difficult.