Lunar-modulated geomagnetic orientation by a marine mollusk.

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 nudibranchs 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 a circa-lunar rhythm. The discovery of a magnetic sense in a mollusk with giant, reidentifiable neurons provides a unique opportunity to study the cellular mechanisms underlying magnetic field detection.

Regional magnetic fields as navigational markers for sea turtles.

Young loggerhead sea turtles (Caretta caretta) from eastern Florida undertake a transoceanic migration in which they gradually circle the north Atlantic Ocean before returning to the North American coast. Here we report that hatchling loggerheads, when exposed to magnetic fields replicating those found in three widely separated oceanic regions, responded by swimming in directions that would, in each case, help keep turtles within the currents of the North Atlantic gyre and facilitate movement along the migratory pathway. These results imply that young loggerheads have a guidance system in which regional magnetic fields function as navigational markers and elicit changes in swimming direction at crucial geographic boundaries.

Perception of airborne odors by loggerhead sea turtles.

Sea turtles are known to detect chemical cues, but in contrast to most marine animals, turtles surface to breathe and thus potentially have access to olfactory cues both in air and in water. To determine whether sea turtles can detect airborne chemical cues, captive loggerhead turtles (Caretta caretta) were placed into a circular, water-filled arena in which odorants could be introduced to the air above the water surface. Air that had passed across the surface of a cup containing food elicited increased activity, diving and other behavior normally associated with feeding. By contrast, air that had passed across the surface of an identical cup containing distilled water elicited no response. Increases in activity during food odor trials occurred only after turtles surfaced to breathe and peaked in the first post-breath minute, implying that the chemical cues eliciting the responses were unlikely to have been detected while the turtles were under water. These results provide the first direct evidence that sea turtles can detect airborne odors. Under natural conditions, this sensory ability might function in foraging, navigation or both.

The neurobiology of magnetoreception in vertebrate animals.

Diverse vertebrate animals can sense the earth's magnetic field, but little is known about the physiological mechanisms that underlie this sensory ability. Three major hypotheses of magnetic-field detection have been proposed. Electrosensitive marine fish might sense the geomagnetic field through electromagnetic induction, although definitive evidence that such fish actually do so has not yet been obtained. Studies with other vertebrates have provided evidence consistent with two different mechanisms: biogenic magnetite and chemical reactions that are modulated by magnetic fields. Despite recent progress, however, primary magnetoreceptors have not yet been identified unambiguously in any animal.

Magnetic orientation by hatchling loggerhead sea turtles (Caretta caretta).

Laboratory experiments were conducted to test the ability of loggerhead sea turtle hatchlings (Caretta caretta L.) to orient using the magnetic field of the earth. Hatchlings were tethered to a rotatable lever-arm apparatus which tracked swimming orientation in complete darkness. Hatchlings tested in the earth's magnetic field were nonrandomly oriented with a mean angle of 42 degrees; those tested under an earth-strength field with a reversed horizontal component were also nonrandomly oriented, but with a mean angle of 196 degrees. The distributions under the two magnetic field conditions were significantly different, indicating that loggerhead sea turtle hatchlings can detect the magnetic field of the earth and use it as a cue in orientation.

A Light-Independent Magnetic Compass in the Leatherback Sea Turtle.

Diverse animals can orient to the earth's magnetic field (1-6), but the mechanism or mechanisms undrlying magnetic field detection have not been determined. Behavioral (7-9) amd neurophysiological (10-12) results suggest that the transduction process underlying magnetic compass orientation in vertebrates is light-dependent, a finding consistent with theoretical models proposing that magnetoreception involves a modulation of the response of retinal photoreceptors to light (13, 14). We report, however, that leatherback sea turtle (Dermochelys coriacea) hatchlings orient to the geomagnetic field in complete darkness. Thus, light-dependence is not a universal feature of vertebrate magnetic compasses.

An identifiable molluscan neuron responds to changes in earth-strength magnetic fields.

Diverse animals can orient using geomagnetic cues, but little is known about the neurophysiological mechanisms that underlie magnetic field detection. The marine mollusc Tritonia diomedea (Bergh) has a magnetic sense and its nervous system is amenable to cellular-level electrophysiological analysis. In a semi-intact whole-animal preparation, intracellular recordings from the large, visually identifiable neurons left pedal 5 (LPe5) and right pedal 5 (RPe5) in the brain of Tritonia revealed enhanced electrical activity in response to changes in ambient earth-strength magnetic fields. No such changes in activity were observed in approximately 50 other neurons subjected to identical magnetic stimuli. The responses of LPe5 were characterized by increases in spiking frequency occurring about 6-16 min after the ambient magnetic field had been rotated to a new position. The response was abolished when the brain had been isolated from the periphery of the animal by severing nerves, a procedure that also transected prominent neurites of LPe5. We hypothesize that LPe5 is one component of a neural circuit mediating detection of the earth's magnetic field or orientation to it.