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 of the olfactory lamellae contained iron-rich crystals that were similar in size and shape to magnetite crystals extracted from salmon1,2. Here we show 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 magnetic properties identifies the crystals as single-domain magnetite. In addition, three-dimensional reconstruction of the candidate receptors using confocal and atomic force microscopy imaging confirm that several magnetic crystals are arranged in a chain of about 1 µm within the receptor, and that the receptor is a multi-lobed single cell. These results are consistent with a magnetite-based detection mechanism2,3, as 1-µm chains of single-domain magnetite crystals are highly suitable for the behavioural and physiological responses to magnetic intensity previously reported in the trout.
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We thank K. Babcock at Digital Imaging for the generous use of the AFM/MFM. The Biological Imaging Research Unit at the School of Medicine, University of Auckland, provided the CLSM and imaging facilities. In addition, we thank S. Edgar, A. Turner, H. Holloway and especially B. Beaumont for their assistance in preparation and viewing samples on the CLSM and transmission electron microscopy. Financial support came from the Marsden Fund and the School of Biological Sciences.
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