Wednesday, February 22, 2012

The Porbeagle Shark, Lamna nasus

Courtesy University of Denver
The Porbeagle (Lamna nasus) appears to be, at first glance, the very stereotype of the shark.  Its streamlined body and strong jaw give it away as a fast and efficient predator.  But there is more to this shark then one would expect.  In fact, it can even be described as adorable.

Other than the vaguely smile like appearance of the mouth, you might be asking yourself what one could find cute about this shark.  The reason can be found in their behavior.  Porbeagles have become well known for actions that can be described only as playing.  Young sharks have been seen rolling around in kelp until they are covered in it and then speed off, while other Porbeagles will then give chase trying to grab bits of the kept off, but never harming the entangled shark.  Soon, another shark will start rolling around until tangled as well and the group then begins chasing this individual.

Some originally thought that the sharks were trying to find small prey items hidden within the kelp, but they were never observed eating anything.  Others suggested that the action was done to remove parasites, but this idea was also shown to be lacking.  The only explanation that has been provided that makes any sense is that the behavior is done for play.

Another play like behavior can be seen when these sharks come across a floating object.  They will grab at the object in a way that goes beyond mere investigation and will even go so far as to throw their 'toy' out of the water repeatedly.  The idea that it is more then just curiosity at a novel object can be seen when multiple Porbeagles gather around a single object.  They will each attempt to play with it and have even been seen passing the object from one to another!

The leading hypothesis as to why Porbeagles engage in play is related to why most other organisms are thought to play.  It is a learning process, helping to develop and maintain their hunting techniques.  If this is accurate, then such playfulness likely gave these sharks an evolutionary advantage over those species that never developed this unusual behavior.

Porbeagles can be either solitary or gregarious, especially in the case of juveniles, but adults have been witnessed in groups for reasons other than mating as well.  Though these groups are often transitory.

They primarily reside in colder waters though the females will migrate to warmer climates to birth their pups.  To combat the cold, they are able to thermoregulate, making them endothermic.  They conserve heat produced by their muscles through a series of specialized blood vessels known as the retia mirabilia, or wonderful net.  This allows them to maintain an active lifestyle despite living primarily in cold waters.  Their ability to raise their body temperate is among the best of all sharks, second only to its close relative the Salmon Shark (Lamna ditropis).

A member of the order Lamniformes, the Mackerel Sharks, they are closely related to other more well known fast swimming predatory sharks such as the Great White and the Makos.  The Porbeagle is, like its relatives, aplacental viviparous, which means that the eggs are internally fertilized and the young will hatch before being fully developed and continue their gestation internally.  The Porbeagle also displays what is known as oophagy.  When the pups, still within the uterus, have exhausted their yolk supply, the female will begin to release unfertilized, yolk rich eggs that the young can feed on.  Once fully developed, the female Porbeagle can give birth to, on average, a litter of four, two from each uterus.

These sharks are harmless to humans with only three attacks being recorded.  One was a provoked non-lethal attack on a person, the other two were on boats.  Considering that they are common game fish, this is not surprising.  In fact, the over fishing of these sharks has led to them being listed as vulnerable.

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1 comment:

Ahab said...

Sharks playing seaweed tag! Who knew?