ADAPTATION OF THE MAGNETORECEPTIVE MECHANISM OF MUD-SNAILS TO GEOMAGNETIC STRENGTH

¹ Marine Biological Laboratory, Woods Hole, Mass., and the Departments of Biology, Northwestern University, Evanston, Ill., and Goucher College, Towson 4, Md.

1. The maximum orientational response of the snail, Nassarius, to an abrupt experimental reversal of the horizontal vector of geomagnetism occurs when the reversed field deviates least in strength from the earth's horizontal 0.17-gauss one.

2. The responsiveness to field-direction change drops off very substantially even before the field strength has decreased to 0.1 gauss or increased to 0.4 gauss. It is already minimal at 0.8 gauss.

3. The receptor mechanism for compass orientation in response to geographic direction is, therefore, exquisitely adjusted to the strength of the horizontal vector of geomagnetism, even in contrast with the total strength of geomagnetism.

4. There is a persistent effect of experimental magnetic fields deviating in strength from the natural one which remains for at least three to five minutes following removal of the experimental field.

5. Following exposure to reversed horizontal fields stronger than the earth's, southbound snails respond in the earth's field by clockwise turning, and following exposure to reversed fields weaker than the earth's, by counterclockwise turning, relative to their path in the earth's field following exposure to an experimentally reversed horizontal field of the strength of the earth's natural one.

6. These results suggest that the receptor mechanism of the organism for very weak magnetic fields is highly specialized and adaptive, probably playing important roles yet to be disclosed.