Wednesday, May 20, 2015

Fish and Sensing Geomagnetic Fields by Sasha Doss

Long before the compass was invented, humans used landmarks and celestial bodies to navigate the Earth’s oceans. Fortunately, for many fish, neither the invention of the compass nor the use of celestial bodies has been necessary for navigation. Many fish have a navigational “sense” all their own—magnetoreception. It is the magnetic sensitivity of fish that allows them to detect Earth’s weak electromagnetic field and to use that information to orient, migrate, and/or home to a specific area.
The convection of molten metal deep in the Earth’s core creates electric currents, which in turn create magnetic fields. The spin of the Earth (the Coriolis effect) aligns these separate magnetic fields to form Earth’s one large magnetic field. (Hong Kong Observatory 2015, Institute of Physics 2015). Scientists describe the Earth’s magnetic field in two parts: inclination angle and field intensity. The former changes with latitude (see Figure 1). The inclination angle is the angle at which the magnetic field intersects the Earth’s surface. It is horizontal at the magnetic equator (represented by the curved line in the figure), vertical at the poles, and sloped in between. Field intensity also varies around the world, and the two parts essentially create a map of unique magnetic locations. Figures 2 and 3 show isolines of inclination angle and field intensity, lines along which the inclination angle or field intensity are the same. The isolines substitute for latitude and longitude coordinates, and by detecting the isolines along a coast, animals can deduce where they are relative to a specific site. 
This figure from the Lohmann Lab (2008) demonstrates how field lines (represented by arrows) intersect the Earth's surface and how the angle formed between the Earth's field and the Earth varies with latitude.

The ability to detect this information, magnetoreception, was first recognized in birds. Wolfgang Wiltschko documented the phenomenon in 1966 (Ritz and Schulten 2007). He found that some birds during migratory unrest would orient themselves in cages for migration. Their orientation changed as he simulated different magnetic fields around them. Since Wiltschko’s discovery, many scientists have documented magnetoreception in animals, including salmon (Moore, et al. 1990, Ogura et al. 1992, Taylor 2006), spiny lobsters (Lohmann et al. 1995, Boles and Lohmann 2003), sea turtles (Light et al. 1993, Lohmann et al. 2008), bats (Holland et al. 2006, Wang 2007), and even mole rats (Marhold et al. 1997). Unfortunately, the pathways by which the animals process electromagnetic information are only partially understood.
Figure 2 and 3 (Lohmann 2008) show isolines of inclination angle and field intensity, respectively.

Scientists have hypothesized three ways in which animals may process magnetic information: mechanical reception, electric induction, and chemical reception. The first, mechanical reception, happens when a magnetic field exerts torque on a ferromagnetic material. One such ferromagnetic material is magnetite. The heads and bodies of some animals contain magnetite in crystalline form, and the crystals act like “microscopic compass needles” directing the animals (Klappenbach 2015). Magnetite crystals have been discovered in the bodies of many fish, including Rainbow Trout, Yellowfin Tuna, and some species of salmon (Walker et al. 1997, Walker et al. 1984; Kirschvink et al. 1985; Ogura et al. 1992). Salmon have one of the strongest cases for mechanical geomagnetic reception in fish. Scientists believe that when salmon enter the ocean as juveniles, they undergo geomagnetic imprinting (Lohmann et al. 2008, Putin et al. 2013). The geomagnetic field stored is then compared to the local magnetic field to navigate back to the same site years later (Bracis and Anderson 2012). For example, Putman et al. (2013) found that Sockeye Salmon heading to Fraser River in Washington selected a northern or southern passage around Vancouver Island using a combination of geomagnetic cues and temperature. Mechanical reception cannot explain all magnetoreception, however, because not all animals or fish that demonstrate magnetoreception have magnetite crystals.
For instance, sharks lack magnetite crystals and are thought to use the second hypothesized pathway, electric induction. During electric induction, a shark moves through the Earth’s magnetic field generating an electric field. The shark can detect the electric field created using its ampullae of Lorenzini. The ampullae of Lorenzini are small sensing organs that allow sharks to perceive electricity (Helfman et al 2010). The ampullae are typically located on the underside of the shark, beneath the head and snout. The ampullae permit sharks to detect the smallest electrical fields—5 nano-Volts per cm (SharkProject). Shark’s acute electroreception could feasibly be used to sense the Earth’s magnetic field (Kato 2006), but the use of geomagnetic cues in this way has not been widely confirmed (Walker et al. 2007).
Some animals have neither magnetite crystals nor the ability to detect weak electric signals, and so their ability to sense magnetic fields must come from an alternative pathway. The last pathway, chemical reception, involves chemical reactions with transitions between different spin states (Ritz and Schulten 2007). Scientists hypothesize that the spin states are influenced by magnetic fields, and that one product is favored depending on the ambient magnetic field (Ritz et al. 2009). This process, however, has not been documented in fish.
If fish do have the ability to detect Earth’s magnetic field, how do they use this sensory process? Scientists believe geomagnetic information is used in two ways: simple orientation and/or migration to a specific location. Kalmijn (1978) found that while Round Stingrays searched for food, they oriented themselves in induced magnetic fields and changed their orientation as the magnetic field changed. This use of geomagnetic cues in simple orientation means that some fish may use the Earth’s magnetic field for daily activities, like feeding. Another example of using geomagnetic clues for simple orientation is that of the Scalloped Hammerhead sharks in Baja, California (Kimley 1993). The hammerhead sharks made one repeated daily journey from El Bajo Espiritu Santu seamount to the surrounding pelagic area and back using the residual magnetic field.
The second use of geomagnetic reception is homing and migration. The use of geomagnetic information for migration has been documented in salmonids, eels, and Yellowfin Tuna (Helfman et al. 2010). Walker (1984) found that Yellowfin Tuna could distinguish different magnetic fields and later discovered the presence of magnetite in the dermethmoid bone in the skull (Walker et al. 1984). He hypothesized the use of mechanical induction as a means by which the tuna navigated long distances. Furthermore, Nishi et al. (2004) found that Japanese eels exhibited magnetosensitivity at sea and in freshwater alluding to the feasible use of geomagnetic cues in migration. Directly linking migration accuracy and precision to magnetoreception has been an issue of controversy, and one of the main gaps in the field has been determining methodological frameworks against which to test hypotheses and evidence on how fish use the Earth’s magnetic field (Walker et al. 2007).
Although scientists have decrypted much about magnetoreception in fish, there are still many details that need to be explored. Distinguishing between electrical and mechanical induction demands more behavioral experiments. Additionally, each sensory pathway must be described in more detail—which neurons are involved, where might magnetite crystals be located in different fish, is magnetic sensory information used in conjunction with another sensory process, etc. Magnetoreception is a promising “field” in fish biology, and the advances in knowledge about this sensory process will surely answer interesting questions about fish behavior. 

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