Y: Don, why is your backpack all wet? It’s not raining outside.
D: I drenched it in bug repellent.
Y: Why? Do mosquitoes like your backpack?
D: No, they like me, but I don’t like bug repellent. It makes my skin itch. So I figured I could just wear a bug repellent-drenched backpack like a shield, and mosquitos would stay away from me without a speck of repellent having to touch my skin. The jury’s still out on whether it works or not.
Y: Keep me updated. You know, you’re not the first to use that technique. Amphipods, a shrimp-like crustacean living in the Southern Ocean, hold another animal called pteropods—a planktonic mollusk also known as sea snails, sea angels, or sea butterflies—on their backs like backpacks and use them as a living shield. The sea snails secrete a poisonous chemical that deters predators like cod icefishes. Amphipods are unaffected by the chemical, and when they carry the sea snails on their backs, they’re also protected from predators. Icefishes quickly realize the amphipods carrying pteropods piggyback don’t make a great snack, so they learn to avoid them.
D: That’s taking my backpack shield idea to a whole new level.
Y: And it gets worse. Once the pteropods are in the clutches of the amphipods, they can’t hunt to feed, so they starve to death. As far as researchers can tell so far, the sea snails don’t benefit at all from the association.
D: You know, my backpack doesn’t really benefit from our association.Y: No, but I don’t know if you’ll get much benefit either.