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Earths magnetic field serves as ubiquitous source of directional and positional information for many marine animals. These animals use this information for their needs to survive like choosing correct migratory pathways, returning back home and to catch their prey to name a few. This has been confirmed by many researchers by behavioral experiments on many marine animals. Some examples are migration of Hatchling sea turtle in the North Atlantic Gyre, navigation of Spiny Lobsters and magnetic orientation behavior of Tritonia Diomedea. This paper reviews the magnetic navigation behavior of the Hatchling Sea Turtles and Spiny Lobsters and the behavioral experiments carried out to confirm the same by different researchers.
Very little is known about the neural mechanisms underlying magnetic navigation behavior of different marine animals due to complexity of neural network. Therefore, researchers have chosen Tritonia Diomedea, which has relatively simple neural network and studied its magnetic orientation behavior by employing electrophysiological methods to explore the underlying neural mechanisms and try to extract the bigger picture by analyzing and correlating the data. This paper reviews the research in this direction as well and discusses important findings and finally the author concludes that besides behavioral experiments little is known at least about the basic mechanisms and processes underlying the magnetic navigation behavior of marine animals and besides the other important aspects of the issue, work needs to be done on the role played by higher order processing in circuits involving the sensory information.
Sensory System and Navigation
Human beings are having a set of sensory systems that fullfils their needs of survival on this planet in their habitat. The important sensory systems are the vision system, the hearing system, contact sensing system, temperature sensing system etc. Any of the sensory systems can be looked upon to be made up of receptors, signal processing systems and actuation mechanisms. These sensory systems are known to adapt to the habitat and guides our actions, reactions whatever we call it. In different organisms different sensory systems are evolved different levels of sophistication according to the needs of their habitat.
Of these sensory systems, it is mainly the vision system that guides navigation behavior of most of the organisms, at least on the land mass of the Earth. The organisms that rely on their vision system identify and store various landmarks and thus make a map, in their mind, which they use to navigate to the desired destinations. Besides, they have visibly identifiable direction markers like rising and setting sun and different stars. So obsessed we are with our vision system, which is quite natural looking at the extent to which we rely on it for our own navigational and other needs, that apparently it looks as the only possible sensory system to support a sophisticated navigational behavior.
However, scientific finding contradict this notion and present before us strange observations where highly sophisticated navigational behavior of many species could exists with little support from the vision system. This is observed mainly in marine animals. These animals reside in different habitat, need to migrate on different length and time scales and finally they need to return home. Not only this, they need to be reasonably accurate and precise on their pathway during the course of migration as the deviation from the proper path could prove fatal. Surprisingly they do so under seawater, where sunlight can hardly reach to shine their habitat or pathways, visibility is very poor due to turbidity and more importantly their habitat lacks features or what we say without any landmark (so obsessed we are with landmarks!).
Migratory Behavior of Hatchling Sea Turtle
Hatchling sea turtles go on their specific migratory track, along the North Atlantic Gyre, since generations, in their life cycle. This migratory track originates from their birthplace at the east coast of Florida, USA. Starting from their nests at the east coast of Florida, USA, they cross the beach to the sea and migrate offshore to the Gulf Stream and North Atlantic Gyre. This is a circular system encircling the Sargasso Sea (Cain et al 2005). They remain in the Gyre system for several years and cross to the eastern side of the Atlantic Ocean. After that they return to the south eastern USA i.e. their birth place, to reside in the coastal feeding grounds. The primary reason for this migratory pathway is that the North Atlantic Gyre is food-rich environment for young turtles. However, the turtles can not afford to stray beyond the extremes of the gyre as it will be fatal for them.
Near Portugal, the east-flowing stream of the Gyre pides and the northern branch goes past Great Britain and temperature of the water falls rapidly. If at all the turtles are swept northwards they will soon die of cold. Similar is the situation at the southern extreme of the Gyre where failing to recognize the extremes of the Gyre will throw them in the vast sea and far away fro their home. So the question is how the young turtles with no experience of the Gyre, determine extremes of the Gyre so precisely? Do they have some kind of map built in them and some sensory system continuously sensing and telling them as where on this map they are? We will return to the answer but before, let us explore another equally important example.
Navigation of Spiny Lobsters
The Caribbean Spiny Lobster resides on hard bottom and coral reefs throughout the waters of Caribbean and south eastern US. They are nocturnal and spend their day protected within crevices and holes. They travel considerable distance during night and at the end comeback to either same den or another similar den nearby. Even when these lobsters are taken away and left at a location several kilometers away from their home, they come back home. Again the question is how they the direction and path to their home. Which sensory system they rely upon for such a perfect navigational capability?
Navigation Based on Magnetic Sensory System
These marine animals have developed a geomagnetic compass to determine direction (Lohmann et al 1995a). Behavioral experiments under replicated magnetic field conditions have stimulated similar navigational behavior in hatchling turtles and spiny lobster than that in their actual migratory path. But to return home with such an accuracy and precision requires having much more than just magnetic compass sense. They must be capable be constructing maps by detecting and combining different features of magnetic field. Several features of magnetic field like field intensity and the angle at which geomagnetic lines intersect Earth’s surface vary with latitude, predictably and these can be used in position finding (Lohmann 1999).
Johnsen et al (2005) have reviewed physics and neurobiology of magnetorecection. They have examined the possible mechanisms of magnetoreception. The mechanisms are – 1. Electromagnetic Induction, 2. Chemical Magnetoreception and 3. Magnetite sensors. The authors have evaluated the physics and / or chemistry underlying each of these mechanisms, evidence for these mechanisms and contradictions in accepting these mechanisms.
Mechanism Underlying Magnetic Orientation
Though, little understanding has been gained about the basic mechanisms underlying magnetoreception, based on behavioral experiments it can be said without any doubt that, there exists a fairly evolved magnetoreception and magnetic sensory system based on geomagnetism that helps them navigate so well in absence of any other sensory system to do the same. Not only the magnetoreception system, rather the entire magnetic sensory system is still a mystery to be solved.
Which part of the neural system receives the magnetic signal from the environment, which part of the neural system is then takes these signals for further processing, how the signals are processed, what signals are given to which part of the neural system to generate the actuation signal. The entire circuitry and not just the circuitry but the higher order processing aspects are all very little known. The reason being the complexity of the neural network of the vertebrate, which makes it difficult to isolate the magnetic effects from other effects for deriving useful information by employing the electrophysiological studies.
To overcome complexity of neural network of the vertebrates, a simple invertebrate – Tritonia Diomedea – was chosen for electrophysiological studies. This animal was chosen as it shows magnetic orientation under behavioral experiments and has very simple central nervous system – consisting of approximately 7000 neurons in six fused ganglia and many of these neurons can be identified by their color, size and location within central ganglia. Besides, the nervous system is readily accessible for electrophysiological studies.
Intracellular electrophysiological recordings show that three bilaterally symmetric pairs of identifiable neurons respond with altered electrical activity to changes in Earth’s magnetic field strength. Two of these pairs identified as Pd5 and Pd6 are excited by changes in direction of ambient magnetic field, while the third pair Pd7 neuron is inhibited by same magnetic stimuli that excites Pd5 and Pd6. Therefore, it can be concludes that in Tritonia Diomedea the three neuron pairs – Pd5, Pd6 and pd7, are magnetically responsive cells and function in the neural circuitry underlying magnetic orientation behavior.
Recent anatomical, electrophysiological and immunological analyses have provided some insight into likely roles of these neurons. Pd5 and Pd6 are large cells (~ 400 μm) and produce peptide neurotransmitters and therefore, these are likely to function in generating or modulating the motor output of the magnetic orientation circuitry. The function of Pd7 is not so obvious, however some possible functionality of the same has been offered. Like Pd7 possibly controls or modulates some subtle aspects of tuning or locomotion that occurs during magnetic orientation behavior. Another possibility is that it might be suppressing the behavior that otherwise impede magnetic orientation or locomotion.
Besides, neurobiological investigation of magnetic orientation in Tritonia Diomedea a few work has been done on the neural mechanisms underlying magnetic navigation behavior of vertebrates. However, not much success has been reported so far primarily due to complexity of their neural network. Also, hardly any work has been done on the role higher order signal processing associated with sensory systems. It should be noted that the original signals detected by sensors are considerably altered by the higher order signal processing and has hardly any resemblance with the original signals. Therefore, this aspect is very important and should find special focus in any serious research aiming to reveal the basic mechanisms underlying magnetic orientation behavior of different animals.
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