Today’s factismal: Bull sharks have been seen in Lake Michigan, more than 1,600 miles from the ocean where they live.
Imagine, if you will, that you are in Chicago trying to out-do Ferris Beuller. You’ve gone to a Cubs game. You’ve eaten in the fanciest restaurant you could find. You’ve even headed for the top of the Sears Tower (excuse me – the “Willis Tower”). And you decide to round off the day by heading out to the lake for a nice, relaxing afternoon by the lake. But what do you see when you get there but the fin of a large shark, cruising up and down the coast! Sound like another fake Discovery Channel documentary? Believe it or not, it really happened!
The reason that it happened is that not all sharks live in the ocean. Though the vast majority of sharks are dedicated sea dwellers, there are about five species that live exclusively in fresh water and another ten or so that can live in both fresh and salt water. And that turns out to be a much harder trick than it sounds. The reason that it is difficult is because salt water has a lot of salt and fresh water doesn’t. And the reason that is important is because an animal, such as a shark, can only survive if it has the right amount of salt in its blood; too much or too little and it will die.
Fortunately, sharks (and people) have kidneys that have evolved to remove just the right amount of water and keep the blood at exactly the right level of saltiness. If it is a freshwater shark, then water percolates into the shark via osmosis (the movement of water through a membrane in order to balance solution strength) and the kidneys remove a lot of water to keep the shark’s blood salty enough. If you’ve got a saltwater shark, then water leaves the shark through osmosis and the kidneys remove just enough water to move wastes out. But if you put a freshwater shark into salt water, the kidneys won’t know that it is in salt water and will keep removing water until the shark dies of dehydration in the ocean. And if you put a saltwater shark into fresh water, the kidneys won’t remove enough water and the shark will die of bloating.
The bull shark and its other fishy friends who move from salt to fresh water and back have kidneys that are capable of adjusting the amount of water that they remove from the shark’s blood. That allows them to live in both the salty ocean and the fresh lakes, which means that they can search for food in more places. (And food makes sharks very happy.) And that’s probably what happened in Chicago; a bull shark headed up the Mississippi River chasing after lunch, then followed the fish through the Illinois River and ended up in Lake Michigan. Though it is fairly rare to see a bull shark that far up a river, they are fairly common in the estuaries and river mouths near oceans all over the world.
But we’re still learning about the bull shark and other fish. We still don;’t know for sure how common the bull shark is in shallow freshwater or how many rivers it swims up or what it likes to eat on these excursions into freshwater. If you see a bull shark (or other fish) and would like to help scientists learn more about them, why not add your information to the pile already stored over at Fish Base?