Magnetotactic bacteria carry chains of magnetite crystals that align their bodies with Earth's magnetic field. When Richard Blakemore discovered them in 1975, he named the behavior for what he observed: the bacteria swim in a magnetic direction. North-seeking populations swim north. South-seeking populations swim south.
This description is accurate and misleading. The bacteria are not navigating toward a cardinal direction — they are using the field's inclination angle (the degree to which it tilts below horizontal) as a proxy for gravitational down. In both hemispheres, "align with field, swim toward the pole-end" points the bacteria toward the sediment where oxygen gradients are favorable. The hemisphere paradox makes this visible: north-seeking and south-seeking bacteria do opposite things magnetically but the same thing functionally.
Switch hemispheres to see the paradox. Then transplant a population across the equator to see what happens when the mechanism is correct but the context is wrong.
What the simulation shows: in either hemisphere, the native population reaches the sediment. The mechanism (align with field, swim toward the steeper-inclination end) is identical. The result (descend toward sediment) is identical. Only the cardinal direction label — north-seeking, south-seeking — differs.
What it hides: the bacteria don't experience "hemisphere" as a category. There is no conditional logic. The mechanism runs the same way everywhere; the hemisphere is a property of the environment, not a parameter the cell reads. The transplant failure is not a bug in the bacterium — it is the same correct mechanism running on a different field geometry.
The naming problem: "magnetotaxis" describes the mechanism — field alignment — before the function — aerotaxis with a geometric shortcut — was understood. From inside a single hemisphere, both descriptions are consistent with the data. The hemisphere experiment is the natural test that separates them.