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I wonder if it’s actually a form of touch sensation. Some parts of the body might be minutely susceptible to magnetic fields and the feeling is distributed across areas of the turtle. The human finger can sense things only a micron in size.

Something like miniscule tugging of the shell in certain directions being felt?

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In some living beings it is known that they sense the magnetic field with microscopic magnetite crystals (i.e. iron oxide, Fe3O4), that are inside some cells.

The magnetic field is sensed by the magnetic forces that act on the magnetite crystals, much in the same way as we sense gravity and acceleration by the forces that act on the otoliths in our inner ear.

While in turtles it has not been identified how they sense the magnetic field, one of the possibilities is that they also use magnetite crystals located somewhere, at least for one of the two kinds of magnetic senses identified in them.

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