What makes a shark a shark?

I’d like you to pause and ponder the following question: how do we tell different animal species apart? At first glance you might think that is an easy question to answer, different species don’t look the same, for example a great white shark and a bull shark look different, so that’s how you tell them apart, right?

Unfortunately, it’s not always as easy as that. There are many different species that look almost exactly the same and are very difficult to tell apart based on appearance. These are called cryptic species. Take for example two species of stingray in South Africa, the reticulate whipray and the leopard whipray. Both have a ‘honeycomb-like’ pattern that look incredibly similar, and it wasn’t until 2008 that scientists actually realized these were two different species, and not one species.

One of the tools scientists rely upon to tell these cryptic species apart is that of genetics. Different species should be genetically distinct from each other. However, this brings its own set of problems. The genes we carry are different on so many levels – they’re different between individuals (no two individuals will carry the exact same set genes), populations (groups of the same species that may live in different locations) and then finally between species.

So where do you draw the line – how different does an individual’s genes have to be from another individual to make it a different species? This is difficult to answer but the general rule of thumb is that the amount of genetic difference between groups of animals needs to be more than the genetic difference within the groups of animals for it to be considered a separate species.

Now in southern Africa we have a special group of sharks that only occur here and nowhere else in the world – the Haploblepharus sharks, more commonly known as the shysharks. There are four species in total, but three of them live together in the Western Cape region – the puffadder, brown and dark shysharks. Unfortunately, these species fall into the ‘cryptic’ category, they can be quite difficult to tell apart because they often look very similar.

So a team of scientists, including lead author and PhD candidate Michaela van Staden, wanted to investigate the genetics of these three co-occurring shyshark species to see if that could help with telling them apart. However, the results were not as expected!

While there was some genetic differentiation between the 3 shyshark species, it was pretty low, particularly between the dark and and brown shysharks. In fact, most of the genetic difference was found within the species rather than between these two species. Also, a computer model was only able to accurately identify about half of the individuals to the correct species based on their genes.

You may now be thinking, how is that possible? Are they even different species then? Well there are two reasons that may explain the lack of genetic difference between these species. The first is that they may have only become two separate species in recent evolutionary times, and their genes have not had time to evolve and become significantly different. The second is that their may hybridization between these sharks, which means that individuals from these different species may sometimes be mating with each other, mixing up the genes between the species.

It’s hard to say for sure what these sharks are getting up to below the surface of the ocean and which explanation may be correct. But we do know for sure that it’s very difficult to tell these species apart, based on both appearance and genetics. As such, the scientists of this research suggest that any conservation plans for these sharks should consider all of them together, rather than focussing on only one of the three species, because you may not know which of the three species you will be protecting at the end of the day!

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van Staden, Michaela, Katie Samantha Gledhill, Enrico Gennari, Meaghen Erica McCord, Matthew Parkinson, Ralph Gareth Andrew Watson, Clint Rhode, and Aletta Elizabeth Bester‐van der Merwe. "Microsatellite development and detection of admixture among three sympatric Haploblepharus species (Carcharhiniformes: Scyliorhinidae)." Aquatic Conservation: Marine and Freshwater Ecosystems (2020).

(https://onlinelibrary.wiley.com/doi/abs/10.1002/aqc.3406)