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.