The Great White Shark and the Mediterranean: a 3.2 mln years long history

The Great White Shark has been in the Mediterranean for 3.2 mln years, way longer than researchers have hypothesized until now. The white sharks currently living in the Mediterranean are genetically closer to those of the Pacific Ocean than to their neighbours inhabiting the Atlantic.

Starting from the analysis of artifacts and ancient trophies kept in museums, a research group led by the University of Bologna managed to sequence the DNA of Mediterranean white sharks. Their approach combines genetics and mathematical models and proved to be effective in tracing back their peculiar evolutionary history. However, researchers also warn that they might become extinct. Their study was published on the Journal of Biogeography.

"White sharks have a complex evolutionary history, they are unusual. They evolved into sedentary populations scattered around the globe. Among these, there are the Mare Nostrum white sharks which are really unique" explains Agostino Leone, researcher at the University of Bologna and first author of the study. "White sharks in the Mediterranean have a very low genetic variability, which may hint at a very small and endangered group of sharks".

DNA and historical artifacts

The Great White Shark (Carcharodon carcharias) is the largest predatory fish on earth, with larger individuals growing over 6m in length and one ton in weight. We can find them on the shores of South Africa, Australia, New Zealand, Japan and North and South America, and, of course, of the Mediterranean. Despite being VIPs in the animal realm, starring in blockbuster movies like Spielberg's "Jaws" and in several documentaries, we know little about their history.

In particular, Mediterranean white sharks have been under-researched because, over the last century, their population has decreased, making it hard for researchers to analyse them. The researchers of this study tried to overcome this issue by turning to museums and private collections which keep historical artifacts made of white sharks, such as teeth, jaws and vertebrae from the past two centuries. Thanks to the most recent technologies able to analyse ancient genome, researchers were able to sequence the mitochondrial DNA of a number of Mediterranean white sharks and then compare it with other populations of sharks living elsewhere.

"This new data allowed us to observe the biological diversity of white sharks living in the Mediterranean" said Agostino Leone. "By analysing and comparing different specimens, we were able to estimate that the white shark population in the Mediterranean started to evolve differently from other cognate populations around 3.2. mln years ago. This essentially proves that those theories about sharks colonizing the Mediterranean around 450 thousand years ago are wrong".

From the Pacific Ocean to the Mediterranean Sea

The origins of Mediterranean white sharks go further in the past than previously thought. This allowed to confirm that these white sharks are more related to those inhabiting the Pacific Ocean than those living in the Atlantic Ocean. This similarity can only be explained by tracing back the predator's colonization path through the oceans.

According to the researchers, Mediterranean white sharks originated from those coming from the Pacific Ocean. The latter travelled across the Atlantic through the Central American waterway, before the Isthmus of Panama was formed, and they eventually reached the Mediterranean Sea. 3.5 mln years ago the formation of the Isthmus of Panama blocked the waterway between North and South America. This brought a series of drastic changes in the climate of the Atlantic Ocean and, as a result, many fish species became extinct. The white shark might have been among them. Therefore the Atlantic Ocean underwent a relatively recent re-population of white sharks, perhaps because of white sharks' migration waves from South Africa, hence the genetic difference between them and the Mediterranean white sharks.

Finally, the study on Mediterranean white sharks DNA brings forth a worrying result: the low degree of genetic variability between different individuals of the species. This suggests that the white shark population is quite small, and thus at endangerment risk. "The Mediterranean population of white sharks is probably a small endangered community", confirms Agostino Leone. "To save them, it is fundamental to act quickly: their extinction would be detrimental to the ecological balance of the Mediterranean Sea as well as to the already highly unstable global situation of these majestic sea predators".

The authors of the study

The study is on the  Journal of Biogeography under the title “Pliocene colonization of the Mediterranean by Great White Shark inferred from fossil records, historical jaws, phylogeographic and divergence time analyses”.

This research was carried out by an international group of scholars led by Fausto Tinti, Alessia Cariani and Agostino Leone from the GenoDREAM Lab (Laboratory of Genetics and Genome of Sea Resources and Environment) at the Department of Biological, Geological and Environmental Sciences of the University of Bologna. Moreover, scholars from the Museum of Comparative Anatomy and from the Department of Cultural Heritage also participated in the study.