In Invertebrates
The mollusc Tochuina tetraquetra (formerly Tritonia diomedea or Tritonia gigantea) has been studied for clues as to the neural mechanism behind magnetoreception in a species. Some of the earliest work with Tochuina showed that prior to a full moon Tochuina would orient their bodies between magnetic north and east. A Y-maze was established with a right turn equal to geomagnetic south and a left turn equal to geomagnetic east. Within this geomagnetic field 80% of Tochuina made a turn to the left or magnetic east. However, when a reversed magnetic field was applied that rotated magnetic north 180° there was no significant preference for either turn, which now corresponded with magnetic north and magnetic west. These results, though interesting, do not conclusively establish that Tochuina uses magnetic fields in magnetoreception. These experiments do not include a control for the activation of the Rubens’ coil in the reversed magnetic field experiments. Therefore, it is possible that heat or noise generated by the coil was responsible for the loss of choice preference. Further work with Tochuina was unable to identify any neurons that showed rapid changes in firing as a result of magnetic fields. However, pedal 5 neurons, two bisymmetric neurons located within the Tochuina pedal ganglion, exhibited gradual changes in firing over time following 30 minutes of magnetic stimulation provided by a Rubens’ coil. Further studies showed that pedal 7 neurons in the pedal ganglion were inhibited when exposed to magnetic fields over the course of 30 minutes. The function of both pedal 5 neurons and pedal 7 neurons is currently unknown.
Drosophila melanogaster is another invertebrate which may be able to orientate to magnetic fields. Recently, precise experimental techniques such as gene knockouts have allowed a closer examination of possible magnetoreception in these fruit flies. Various Drosophila strains have been trained to respond to magnetic fields. In a choice test flies were loaded into an apparatus with two arms that were surrounded by electric coils. Current was run through each of the coils, but only one would a 5 Gauss magnetic field at a time. The flies in this T-maze were tested on their native ability to recognize the presence of the magnetic field in an arm and on their response following training where the magnetic field was paired with a sucrose reward. Many of the strains of flies showed a learned preference for the magnetic field following training. However, when the only cryptochrome found in Drosophila, type 1 Cry, is altered, either through a missense mutation or replacement of the Cry gene, the flies exhibit a loss of magnetosensitivity. Furthermore, when light is filtered to only allow wavelengths greater than 420 nm through, Drosophila loses its trained response to magnetic fields. This response to filtered light is likely linked to the action spectrum of fly-cryptochrome which has a range from 350 nm – 400 nm and plateaus from 430-450 nm. Although researchers had believed that a tryptophan triad in cryptochrome was responsible for the free radicals on which magnetic fields could act, recent work with Drosophila has shown that tryptophan might not be behind cryptochrome dependent magnetoreception. Alteration of the tryptophan protein does not result in the loss of magnetosensitivity of a fly expressing either type 1 Cry or the cryptochrome found in vertebrates, type 2 Cry. Therefore it remains unclear exactly how cryptochrome mediates magnetoreception. These experiments used a 5 gauss magnetic field, 10 times the strength of the Earth's magnetic field). Drosophila has not yet been shown to be able to respond to the Earth’s weaker magnetic field.
Read more about this topic: Magnetoception