Magnetite defines a vertebrate magnetoreceptor

The key behavioural, physiological and anatomical components of a magnetite -based magnetic sense have been demonstrated in rainbow trout (Oncorhynchus mykiss)(1). Candidate receptor cells located within a discrete sub-layer o f the olfactory lamellae contained iron-rich crystals that were similar in size and shape to magnetite crystals extracted from salmon(1,2). Here we sh ow that these crystals, which mapped to individual receptors using confocal and atomic force microscopy, are magnetic, as they are uniquely associated with dipoles detected by magnetic force microscopy. Analysis of their magn etic properties identifies the crystals as single-domain magnetite. In addi tion, three-dimensional reconstruction of the candidate receptors using con focal and atomic force microscopy imaging confirm that several magnetic cry stals are arranged in a chain of about 1 mm within the receptor, and that t he receptor is a multi-lobed single cell. These results are consistent with a magnetite-based detection mechanism(2,3), as 1-mu m chains of single-dom ain magnetite crystals are highly suitable for the behavioural and physiolo gical responses to magnetic intensity previously reported in the trout.