The ability of animals to sense weak magnetic fields is a fascinating ability that as of yet does not have a fully elucidated mechanism. One model has been proposed based on findings from the glass catfish, by Kolomytkin et al. In their model, glycoprotein molecules (which may consist of negatively charged oligosaccharide side chains) are tethered to an ion channel gate in electrorecepting cells. The glycoprotein molecules each contain many negative charges, and the applied electrical and/or magnetic field would exert a force on these glycoprotein molecules. If substantial enough (i.e., if it is greater than the thermal energy associated with one degree of freedom), it would mechanically cause the ion channel to open because they are covalenty bonded to the glycoproteins. If enough of these channels opened, they could sum to an action potential in the sensory neuron.
Using EEG, Marino et al evaluated the likelihood of a magnetic stimuli evoking a post-transduction action potential in ten human subjects. Even with only a 10 ms rise time and a 0.2 ms fall time, field potentials during the onset of the magnetic field occured 100% of the time, and field potentials in the offest of the magnetic field occured 60% of the time. They were able to rule out the possibility that the field potentials resulted from interactions between the field and the scalp electrodes based on other preliminary tests. The fact that the receptor was able to detect such a rapid change (0.2 ms) suggests that the signal transduction process may indeed be initiated by a mechanical force. So although their location has not been pinpointed, it appears that humans also have electromagnetic receptors and that they act through a phylogenetically conserved mechanism.
Kolomytkin OM, et al. 2007 Glycoproteins bound to ion channels mediate detection of electric fields: A proposed mechanism and supporting evidence. Bioelectromagnetics 28:379-385.
Marino AA, et al. 2009 Evidence that transduction of electromagnetic field is mediated by a force receptor. Neuroscience Letters. doi:10.1016/j.neulet.2009.01.051.