MAGNETIC ORIENTATION OF SPINY LOBSTERS IN THE OCEAN - EXPERIMENTS WITH UNDERSEA COIL SYSTEMS

The western Atlantic spiny lobster Panulirus argus undergoes an annual migration and is also capable of homing to specific dens in its coral reef environment. Relatively little is known, however, about the orie ntation cues that lobsters use to guide their movements. To determine whether lobsters can orient to the earth's magnetic field, divers moni tored the orientation of lobsters tethered inside magnetic coil system s submerged offshore in the Florida Keys, USA. Each coil could be used to reverse either the horizontal or vertical component of the earth's field. Tethered lobsters walking inside the coils often established a nd maintained consistent courses towards specific directions. After a lobster had established a course, it was exposed to one of three condi tions: (1) a reversal of the horizontal component of the earth's field ; (2) a reversal of the vertical component of the earth's field; or (3 ) no change in the ambient field (controls), Lobsters subjected to the horizontal field reversal deviated significantly from their initial c ourses. In contrast, control lobsters and those subjected to the rever sed vertical field did not. These results demonstrate that spiny lobst ers possess a magnetic compass sense. Because inverting the vertical c omponent of the earth's field had no effect on orientation, the result s suggest that the lobster compass is based on field polarity and thus differs from the inclination compasses of birds and sea turtles. The magnetic compass of lobsters may function in homing behavior, in guidi ng the autumn migration or in both.