No. 50: Mar-Apr 1987
From the abstract of a paper in Science:
"Behavioral experiments indicated that the marine opisthobranch mollusk Tritonia diomedea can derive directional cues from the magnetic field of the earth. The magnetic direction toward which nudibrachs spontaneously oriented in the geomagnetic field showed recurring patterns of variation correlated with lunar phase, suggesting that the behavioral response to magnetism is modulated by circa-lunar rhythm."
The magnetic and lunar-phase detectors of this mollusc are not known. In fact, the authors remark in their introductory paragraph that, even in organisms possessing ferromagnetic materials in their systems, there exists no "direct neurophysiological evidence implicating ferromagntic particles in the the detection of magnetic fields."
(Lohmann, Kenneth J., and Willows, A.O. Dennis; "LunarModulated Geomagnetic Orientation by a Marine Mollusk," Science, 235:331, 1987.)