Perceived through human senses, the environment beneath the ocean's surface is often a murky, disorienting, and nearly featureless realm. Relative to air, visibility in the sea is greatly diminished by turbidity and the increased scattering of light. Visual landmarks are seldom present, and except under unusually clear conditions in shallow areas, celestial cues such as skylight polarization patterns, or the position of the sun and stars, cannot be perceived (Shashar et al., 2004
). To humans, the idea of holding a consistent heading under such conditions, let alone navigating with precision from one distant location to another, appears impossible without specialized equipment. Yet more than 70 percent of the Earth's surface is covered by ocean, and numerous marine animals move about from place to place during both day and night, often with remarkable precision and over long distances.
For anyone who has been diving in deep waters, it is tempting to conclude that there are fewer orientation cues available in the ocean than on land. However, although our own senses are not optimized to detect stimuli in the marine environment, it does not follow that animals that have evolved in the sea have the same limitations, or that environmental cues imperceptible to us are not readily detected by other species. Studies have revealed diverse and sometimes unexpected sensory abilities in marine animals. For example, hatchling sea turtles exploit ocean waves as an orientation cue and determine wave propagation direction by detecting sequences of accelerations (Lohmann and Lohmann, 1992; Lohmann et al., 1995b). Sharks have an acute electric sense with which they can detect prey (Kalmijn, 1971), dolphins can detect characteristics of objects from a distance using ultrasonic echolocation (Harley et al., 2003), and crabs perceive water depth by sensing water pressure (Fraser and Macdonald, 1994). Thus, the challenge of investigating orientation and navigation in the ocean is to attempt to perceive the marine environment not through our own sensory systems, but through the sensory modalities of the animals that live there.
Among a number of sensory cues potentially available in the ocean, the Earth's magnetic field is a particularly pervasive environmental feature (Skiles, 1985). In contrast with most other cues, the field is present night and day, is largely unaffected by weather and season, and exists in all parts of the ocean, from shallowest to deepest. Thus, it is perhaps not surprising that a number of ocean animals have evolved the ability to derive useful directional and/or positional information from the Earth's magnetic field (Wiltschko and Wiltschko, 1995a). Here we highlight recent findings about the magnetic orientation, navigation, and neuroethology of three well-studied but phylogenetically diverse marine animals: the loggerhead sea turtle Caretta caretta, the spiny lobster Panulirus argus, and the sea slug Tritonia diomedea. Despite vast differences in behavior, habitat, and the distances over which each travels, all of these animals depend at least partly on the Earth's magnetic field to guide their movements.
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