Sharks play an essential rol in the health of our oceans


A shark dissection dissected!

By Dorien Schröder, marine biologist and educator of Dutch Shark Society

Dorien Schröder dissects a small spotted cat shark!

Dorien Schröder dissects a small spotted cat shark!

Recently, I did a shark dissection and as I had such an interested audience there, I thought I should share it here too. When you look at the outside of a shark, you will obviously notice the fins. These help the shark with swimming and staying upright in the water. Another thing a lot of people notice are the teeth. As I was dissecting a small-spotted catshark, the teeth are quite small and made for eating crustaceans, worms and shellfish mostly. Shark skin is also made of small teeth, called denticles. This is why they feel smooth when you stroke them head to tail, but very rough the other way around. Many decades ago, people used shark skin as sand paper for that reason. In the mouth, you can also see the tongue, which is attached to the bottom of their mouth, so sharks cannot stick out their tongues. On the snout you can also spot tiny holes, called the Ampullae of Lorenzini, that help sharks detect small electrical fields that every living animal has. Another thing you can see on a sharks head are the holes just behind their eyes, the spiracles. These holes help the sharks pump water over their gills, which allows some species to be able to lie still on the bottom of the sea. Lastly, when you have a look at the pelvic fins, you can see a pair of claspers if the shark is a male, which was the case in the shark I dissected. That’s right, sharks have two penises!

When I dissect a shark, I usually make a rectangular cut in the belly, creating a sort of door through which you can see all the organs. The cut can be quite difficult to make, because of the thick skin of the shark. There is never a lot of blood, because the big blood vessels lie in the back of the shark.

The first thing you usually see is the liver. All sharks have big oily livers, that help them float, as sharks do not have a swim bladder like other fish do. The liver is made up of a big lobe on each side of the body and a small one in the center. In big sharks the liver can be up to a third of the body weight. Once I have removed the liver, you can see the oesophagus and the stomach. Sharks have the unique ability to be able to turn their stomach inside out through their mouths in case they have eaten something bad. They also do this when they are stressed, which is why you often see this when they are caught by fishermen. You can cut open the stomach to see if anything is in there, such as squid beaks or earbones from fish, but in my shark, the stomach was empty. Right next to the stomach, you can find the bright red spleen, which creates red blood cells and is part of the shark’s immune system. From the stomach, the food moves into the intestine. Rather than having really long intestines like humans have for taking up nutrients, sharks have a short intestine with spirals on the inside. These spirals make sure that the food still has to go a long distance through the intestine to take up enough nutrients. Sharks can also turn this spiral valve inside out through their anus to get rid of any bad foods they ate.

Once all of the digestive tract has been removed, you can have a look at the reproductive organs. All sharks have these organs double, which not only means that male sharks have two penises, but female sharks also have two uteri. The shark I dissected was a male, so we found two testes, with the sperm ducts flowing through the muscles in the back of the sharks towards the sperm sacs. Next to the sperm ducts, also in the muscle in the back, lie the kidneys that help the liver with getting rid of toxins and waste. The sperm sacs lie right before the claspers. This shark had sperm cells in its sperm sacs and the claspers were calcified (hard), which indicated that it was a mature male.

The last thing you can look at are the gills and the heart, but unfortunately in my shark, these had been eaten by sea gulls. A sharks heart is very small for its size and lies close to the gills, where the blood gets its oxygen from. When you cut open the skull, you can see the Y-shaped brain. The top of the Y lies next to the eyes and processes that information. The bottom of the Y lies close to the snout and processes information coming from there.

Dissecting sharks is very interesting, because you always find different things, that sometimes tell you the cause of death. It also teaches us a lot about how sharks develop and about the differences between different species, which can be very useful for research.

Dissection of small-spotted catshark, Scyliorhinus canicula from Dutch Shark Society on Vimeo.

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