Creature Feature: Bull Shark

Photo by Nick Hobgood

Photo by Nick Hobgood

You will recall that at MSI, we don’t always like to use the word “dangerous” to describe sharks.  That makes it sound like sharks are cruising through the ocean looking for humans to eat, WHICH THEY’RE NOT!  However, there are some shark species that are known to attack humans more often.  In 2014, 72 people worldwide were attacked by sharks.  Of that number, only 3 people died.  It is presumed that the attacks were the result of young sharks mistaking humans for prey.  This goes to show that, given the chance, sharks don’t really want to eat humans and that areas of the ocean typically inhabited by sharks might not be the best places to practice your seal impression.

That being said, the moment has arrived to reveal the NUMBER ONE MOST “DANGEROUS” SHARK IN THE WORLD… (drumroll please)…

THE BULL SHARK!!!

If you guessed it right, well done! If you didn’t guess but looked it up on the internet, well done! That’s research, which scientists do all the time!

The Bull shark (Carcharhinus leucas) is highly aggressive and is the only shark that can survive in salt water and fresh water.  The word for fish that can swim in both fresh and salt water is anadromous. Bull sharks typically inhabit highly populated tropical shorelines, swimming in the salty, brackish, and fresh waters included bays and estuaries. Still, bull sharks have been discovered swimming in the fresh waters of the Mississippi River, as far up north as Illinois.  Their ability to travel into fresh water is partly why they are considered to be the most dangerous.  They also eat pretty much anything and are a very curious species. Bull sharks are named for their blunt, somewhat flattened snout that they use to ram into their prey, much like a charging bull.

How are these sharks able to survive in fresh and salt water? you may ask. To answer that, we must osmoregulation.  In this case, osmoregulation means the process that an animal uses to maintain a balance of the water concentration in its body.  For a fish, having too much salty water inside its body will cause dehydration and even death (and that’s no good!).  Not having enough salt though can cause their cells to fill with too much water and burst (which is ALSO no good!)  Since they breathe in salt water, marine fish have to have some way to flush it out. Fish have various adaptations that allow them to get rid of the extra salt that would be otherwise harmful.  Sharks are able to process the salt into urea in their kidneys, which is then flushed out of their bodies through their rectal glands, maintaining healthy levels of salt and other solutes.

For bull sharks, their rectal gland dispenses the excess salt, just as any other sharks would.  However, bull sharks can do something in fresh water that other sharks cannot.  Since they are not taking in the same amount of salt water as they would in the ocean, they don’t want to get rid of the salts that keep their bodies healthy. Bull sharks are able to detect salinity levels through their skin and their rectal gland will restrict the amount of urea that is released.  Check out this video to learn more about bull shark osmoregulation! 

Join us next week as we travel to the East Coast of North America to learn about more fantastic creatures!

References:
Edited by KC O’Shea
Photography: Nick Hobgood
http://animals.nationalgeographic.com/animals/fish/bull-shark/
http://news.nationalgeographic.com/news/2005/07/0719_050719_bullsharks_2.html
http://www.amnh.org/exhibitions/past-exhibitions/water-h2o-life/life-in-water/surviving-in-salt-water
http://kids.nationalgeographic.com/animals/bull-shark/
http://www.oceanconservationscience.org/publications/files/papers/HammerschlagReview.pdf
http://www.businessinsider.com/r-fewer-shark-bites-in-2014-but-florida-still-tops-for-attacks-2015-2

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