CSB #3: Great White Sharks in the Mediterranean Sea Actually Come from Australia

A great white shark in the Mediterranean Sea.

Definitions:

• Mitochondrial DNA- DNA that is passed down to the offspring by the mother only.
• Molecular Dating- Using genetic information to approximate when an event or series of events in evolution took place.
• Pleistocene Epoch- A period in the history of the world that took place between two ice ages about 450,000 years ago. During this time, the climate changed and the earth experienced may changes in it's seas, like the rising of the sea levels.
• Agulhas Ring- an eddy on the tip of Africa, by South Africa.

Summary:
Scientists discovered that Mediterranean great white sharks, whose scientific name is carcharodon carcharias, originally came from Australia millions of years ago. They discovered this by examining the mitochondrial DNA from a Mediterranean shark and comparing it to the mitochondrial DNA of sharks in Australia and they found similarities. The sequenced part of the DNA that they used was the same in 75% of the Australian sharks. They concluded from this that the sharks most likely originated in the ocean area by Australia and then migrated to the Mediterranean sea. Using a technique known as molecular dating, it was found that the sharks migrated around 450,000 years back. Around this time, the Pleistocene epoch was taking place in the world. This is relevant because during this epoch, many changes in the oceans occurred and off the coast of Africa water in the Agulhas Ring began to flow extra fast. Great white sharks have been found to travel along the coast of Africa, so it would make sense that while they were swimming in that area, they got stuck in this eddy, were tricked, and were set off course up the west coast of Africa and found there way into the Mediterranean. Researchers believe that once the sharks arrived in the Mediterranean, they probably got stuck because of all the confusing land forms protruding into the sea, such as the Straits of Gibraltar. This small group of great whites decided to stay and reproduce in the Mediterranean, and they are still there today. Because there are so few great whites, scientists fear that the population won't be as resilient to diseases or other mutations because there are not too many new genes in the gene pool. Sharks from the Atlantic are not coming into the Mediterranean to breed with the sharks there, so scientists wonder where they can get more diverse genes from. They also worry that because there is so little diversity in the genes of these sharks, the population will see it's demise. As well, it would be very bad for the ecosystem of the Mediterranean if these important predators become extinct.

Discussion:

This is a map of Africa, Europe, Asia, and
Australia. Using a map, it is easier to visualize
the path that the sharks took from Australia,
around the tip of Africa to get to the Mediterranean.   

I concerns me that the population of great whites in the Mediterranean has a possibility of becoming extinct. I understand how bad that would be for the Mediterranean Sea. If this happened, the population of the marine animals that the shark had prayed on would increase drastically, and these type of increases in population are often bad for the environment. As well, I find it quite amazing that the scientists were able to find where the Mediterranean sharks had originally come from and around how long ago did they migrate by just studying the DNA of the sharks. I find it fascinating that DNA can provide so much information.

Questions:

• When other species experience this type of accidental migration, have they survived a long as these sharks?
• Why can't we introduce some new sharks from other areas that have more diverse genes into the Mediterranean? This would introduce new genes into the gene pool, and increase the population. It seems fairly simple to do this. It appears that one must just trap some sharks from outside that are at an age that they can reproduce and bring them into the Mediterranean to mate with those sharks.
• If the sharks were to die off, what would be the impact on the ecosystem of the Mediterranean? I understand that it would not be good, but would it become a major issue, or would it fix itself over time?
• What gave the scientists the idea that the great whites in the Mediterranean were decedents from the ones in Australia? Where did they get the idea for the whole experiment?
• Can scientists use Mitochondrial DNA to figure out where my ancestors were from?

Source:

Website: 

Milton, Joseph. "Mediterranean sharks are Australian immigrants." Scientific 

     American. N.p., n.d. Web. 18 Nov. 2010.
     <http://www.scientificamerican.com/article.cfm?id=mediterranean-sharks>.

Pictures:

http://miranda.hemscott.com/ir/mrs/images/EuropeAfricaMapColor.gif

http://uk.news.yahoo.com/21/20101117/img/psc-great-white-sharks-in-m-f20a74e64e53.html