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Mar 16

Magnetic Highways - Sharks on the beach!!


It is becoming more and more interesting the more we read about magnetic highways and pathways our forbearers travel (that is if you believe in evolution), So much knowledge and so little time to learn and live a life, make a living etc etc. We just learned strep bacteria lived on the moon for 2 1/2 years after being left there by an astronaut - so much knowledge, so little time to learn... So much in front of us we don’t understand or even see so much of the time – I mean – who knew birds and fish might follow magnetic pathways??? This next article is interesting - ENJOY!!

Inspired by recent posts on this list about strandings of white and whale sharks, I have been pondering the matter of shark strandings in general. Following is a synopsis of my thoughts on this topic, which I hope will be of interest to users of this list and will spark an exchange of ideas about this most intriguing topic:

Shark strandings or beaching events are something of a mystery. For fishes that are generally regarded as being negatively buoyant in seawater, these events occur with surprising frequency yet with little or no apparent regularity.

Classic studies by Bone & Roberts (1969) and Baldridge (1970, 1972) on tissue densities and buoyancy in sharks revealed that due largely to the accumulation of low-density oils in the large liver, sharks are only slightly heavier than the medium through which they swim. Baldridge (1974) noted that a 1 015-pound tiger shark (Galeocerdo cuvier) tested at the Mote Marine Laboratory had, when immersed in sea water, an apparent weight of only 7.3 pounds (that works out to about 0.72% of its weight in air). Thus, sharks must invest very little energy in order to prevent sinking.

In cool temperate zones of both Hemispheres, basking sharks (Cetorhinus maximus) wash ashore with surprising frequency, and - in various stages of decay - have frequently been misinterpreted as 'sea monsters' (see Heuvelmans 1965 for a discussion of this matter). Although phylogenetically allied with the lamnids (the family which includes the white, makos, porbeagle and salmon sharks) the basking shark shares many hydrostatic features with deep-sea squaloids and hexanchoids (the orders which embrace the dogfishes and the cow sharks, respectively), including an exceptionally long body cavity filled with an enormous liver (up to 25% of the total body weight in C. maximus; up to 35% in certain deep-sea squaloids) which is low in vitamin content but rich in low-density oils (870-880 kg/m3 compared with about 1028kg/m3 for seawater), including a high percentage (70-98%) of squalene. Since the gastrointestinal tract (rich in autochthonous bacteria and other microbes) is typically one of the first organ systems to break down upon death of a host animal, it is tempting to speculate that the gases liberated through the processes of decomposition may be sufficient to tip the hydrostatic balance, rendering the carcass as a whole positively buoyant. Time and tide may eventually carry the carcass to shore, where it may be reported by a terrestrial primate with limited swimming capability but boundless curiosity.

Compared with their elephantine cousin, the basking shark, lamnids have a relatively short body cavity and smaller liver (about 15% of the total body weight). Yet these sharks, too, occasionally wash ashore - sometimes in moribund or freshly expired yet apparently uninjured condition. Users of this list will no doubt recall recent reports from South Africa of large white sharks (Carcharodon carcharias) washed ashore - in one disgusting case, to be beaten and fairly torn asunder by irrational and unsympathetic 'beach apes'. From near my own base of operations, in British Columbia, Canada, no fewer than six white sharks have been found stranded or beached in the province since 1962, mostly from the western coasts of the Queen Charlotte Islands. The most recent of these was a 5.2-metre-long male beached at Long Inlet, Graham Island, BC, on 16 December 1987. List-user and frequent contributor Ian Fergusson can, no doubt, provide information on white shark strandings from elsewhere, particularly from the Mediterranean and off southern Africa. In recent years, researchers have noticed that each spring a small number of salmon sharks (Lamna ditropis) beach themselves in central and southern California. This phenomenon is poorly understood and is being studied. This species is most common in continental offshore and epipelagic waters, from the surface down to a depth of at least 152 metres, but it has been known to come inshore - sometimes just beyond the breaker zone - which may contribute to this phenomenon. List-user Sean Van Sommeran can perhaps favor us with more information about this intriguing mystery.

Some time ago, a list-user (who's name escapes me at the moment), asked whether shark strandings may be somehow similar to those of whales. At the time, I thought the notion was charmingly naive, but have since had time to reconsider.

Klinowska (1988) compared records of mass cetacean strandings in Britain and the United States against geomagnetic maps (which plot variations in the intensity of magnetic fields at the Earth's surface caused by differences in the underlying rock; these variations are represented as contour lines, so that areas of high magnetism appear as 'hills' and areas of low magnetism as 'valleys'). Klinowska's analysis revealed that most mass strandings and virtually all repeated strandings occurred where the magnetic valleys were oriented perpendicular to the shore. This sensational finding suggests that at least some whales navigate by following a magnetic map of the ocean floor. On land, magnetic variations are very irregular and there are many visual cues to guide navigation. There are no such landmarks in the vast, dark ocean. But there are regular magnetic variations. Magnetic hills and valleys stretch for huge distances across the ocean floor, and toothed whales seem to use the magnetic contour lines as invisible 'roads'. These magnetic freeways often follow continental margins, but not always. Klinowska theorizes that whales may strand when they follow these magnetic roads onto shore. Klinowska has also suggested that the daily pattern of variation in the total geomagnetic field may function as a biological 'travel clock' for whales; solar activity can affect this pattern, possibly causing irregular fluctuations which disturb the clock. Therefore, whale mass strandings may be the magnetic equivalent of traffic accidents.

It is not yet clear how whales sense Earth's magnetic field. Evidence is accumulating from studies carried out in Germany which suggest that cetacean retinas (which contain magnetite) are sensitive to magnetic fields of an..........Read on......http://www.elasmo-research.org/education/topics/b_strandings.htm

Posted by Jay Roberts at 05:26 PM | Permalink

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