Sharks are one of the most successful animal groups on the planet. Their lineage dates back to the age of the dinosaurs, much like crocodiles. We have all seen the fossils and programs about Megalodon, the enormous shark which became extinct 3.6 million years ago. But what has caused sharks to become one, if not the, most successful predators?
Well, sharks have evolved a number of adaptations which make them near enough perfect as underwater predators. It’s little wonder that they have become both feared and revered in equal measure. In this article we will look at shark anatomy, and look closely at these adaptations which make sharks so successful.
Shark’s skin is pretty amazing. It isn’t covered in scales like fish; instead it is covered in modified teeth, called dermal denticles. Dermal denticles are very similar to scales, and act in much the same way, working to reduce drag. They are covered in enamel, the same as human teeth have a layer of enamel.
Dermal denticles grow facing backwards, so shark skin has a smooth feel if you run your hand from head to tail, and a rough feel if you run your hand from tail to head.
These ‘scales’ have a dual function. They make the shark more streamlined and work to reduce surface drag so the shark can swim through the water faster and more efficiently. Not only this though, as the denticles are very tough they also act as armor plating.
Unlike fish scales, the denticles don’t grow with the shark. As shark’s get older they replace old denticles and grow more as the shark gets larger in size.
Shark’s dermal denticles are so effective at reducing drag and making sharks that swimwear manufacturers have copied it, allowing swimmers to swim ever faster.
Also, the skin of sharks acts as an external skeleton, providing sites for muscle attachment. The skin is a very thick and complex array of crisscrossed collagen fibers. Having this act as an external skeleton has numerous benefits, such as being very energy saving and efficient. We will discuss this more under the skeleton and muscle sections.
The skeleton of a shark isn’t made of bone like a humans, instead it is made of cartilage. Cartilage is very strong and tough, but most importantly, it is very light and flexible. The cartilaginous skeleton isn’t used for muscle attachment; instead this role falls to the skin, as we have mentioned.
The flexibility of cartilage allows sharks to turn quickly and rapidly, making them very agile. This agility is one of the reasons why sharks are such successful hunters.
Cartilage is around half the density of normal bone; this allows the shark to save a lot of weight. Sharks don’t have ribcages which also allows for huge weight savings. As sharks live in water they don’t need ribcages to help support their weight or protect their organs. Instead protection is provided by the dermal denticles on the skin and a thick layer of muscle.
Weight saving is a huge deal for sharks. Unlike bony fish, sharks lack a swim bladder, which bony fish use to achieve neutral buoyancy. This means sharks rely on the lift generated by their pectoral fins to keep them in the same level of the water column, called dynamic lift. Therefore any weight saving means less energy required to keep themselves balanced.
The jaw of a shark isn’t attached to the cranium, or the skull. This allows the jaw to protrude from the mouth to grab and latch onto prey.
There are also some other fascinating adaptations found in the jaws of sharks.
As the jaw is used for such physically stressful activities like biting, gripping, and tearing flesh from prey, it needs to have additional strength compared to the rest of the skeleton.
The jaw is still composed of cartilage, but it is reinforced with hexagonal crystal plates of calcium salts arranged in a mosaic pattern, called tesserae. This layer of tesserae makes the cartilage of the jaw as strong as bone.
In general, most sharks have one layer of tesserae. Larger shark species have more tesserae layers for increased jaw strength as it will be under even more pressure. Bull sharks have two or three layers. Large Great White sharks can have five layers.
Also fascinating is that in the nose the cartilage is very spongy, which allows it to take impacts without sustaining damage.
Shark teeth are incredibly numerous in the fossil record, and indeed are some of the only surviving records of some ancient prehistoric sharks. The oldest known shark teeth date back to 450 million years ago, in the late Ordovician period. Many of the sharks from this period are known only by their teeth; as a shark’s skeleton is cartilage it is often only the teeth that are fossilized.
Shark teeth continuously fall out and are replaced. Some sharks in the order Carcharhiniformes such as the Tiger shark, Bull shark, and Hammerhead sharks, can shed up to 35000 teeth during their life.
The type of tooth that a shark possesses depends on what their diet is. For complete information read our article what do sharks eat? There are four basic types of shark tooth; dense flattened, needle-like, pointed lower with triangular upper, and non-functional.
Dense flattened teeth are seen in sharks such as the Nurse shark, which uses these large hard teeth to crush crustaceans and bivalves. Sharks which possess dense flattened teeth are often found on the ocean floor, searching for this prey.
Needle-like teeth are found in sharks such as the Bull and Blue sharks. These sharp, pointy teeth are perfect for a diet of fish, squid, stingrays, and even small sharks. Needle-like teeth can easily grip and capture these slippery prey items.
Pointed lower teeth with triangular upper teeth are perhaps the most famous example of shark teeth, and the one that people picture when told to think of a shark tooth. The Great White shark possesses this type of tooth. The combination of pointed lower teeth and triangular upper teeth make it easy to rip and cut large prey items such as marine mammals like sea lions, seals, whales, dolphins, and other sharks as well. Serrated edges make cutting flesh into smaller bite-sized chunks easier in order to swallow.
Non-functional teeth are found in sharks which filter-feed. Examples of these sharks are the Basking shark and Whale shark. The teeth of these sharks are, as the name suggests, non-functional. They are greatly reduced, and aren’t involved in feeding at all. Instead these sharks filter out food as water passes over their gills.
Sharks have two types of muscle present in their bodies; red and white. Red muscle enables sharks to swim for long continuous periods of time while white muscle provides power for short bursts of speed.
This muscle uses fats as the fuel, and are what are used by sharks in their general swimming and wandering through the oceans. Red muscle is aerobic, it needs oxygen to function, and contains myoglobin. Myoglobin is very similar to haemoglobin found in the blood of humans.
White muscle uses sugars, namely glycogen, as the fuel. White muscle is used to generate bursts of power and speed, as needed to catch prey or escape predators.
The muscles attach to the skin rather than the skeleton, which helps to increase the efficiency of the muscles contractions.
Sharks are known as stiff body swimmers. They swim by arching their bodies and swinging their head from side to side, which causes high and low pressure zones in their bodies. They then use these pressure differences to help them flick their tails to propel themselves forward.
Tail and Fins
A shark’s fins provide balance and stabilize the animal as it swims. Sharks have a large and rigid dorsal fin to balance and stop them rolling in the water, and often have a second smaller dorsal fin as well. The pectoral fins are also rigid and provide lift and are used to change direction and steer.
The tail, or caudal fin, of sharks varies hugely amongst different species of shark depending on their life history. All sharks however have a heterocercal caudal fin. This means that the spinal column and vertebrae extend into the dorsal (top) portion of the fin.
Due to the spinal column extending into the dorsal portion of the caudal fin, the dorsal portion is usually much larger than the ventral (lower) portion.
The spine extending into the fin allows for a much greater surface area for muscle attachment, allowing for greater power to be generated. This allows for forward movement to be much more efficient.
Many shark species have different shapes to their tail for different purposes.
For instance the Porbeagle shark has a large lower ventral portion of the tail. This is so it can keep speed with and hunt it’s quick moving mackerel and herring prey.
Conversely, Tiger sharks have a very varied diet and so must be able to hunt many different prey items. They have a large dorsal lobe which allows these sharks to slowly cruise the seas before performing sudden energetic bursts of speed, and also allowing them to twist quickly and easily.
There are some stunning tail adaptations, such as that seen in the Thresher shark, which has a very elongated portion to the dorsal lobe of its tail to use as a whip to stun prey such as squid and fish.
Sharks breathe by drawing oxygen from the water as it passes over their gills. Water enters the gill chamber through the mouth, and exits from the gill slits on the side of the head.
Sharks have gill rockets which protect the gill filaments from debris in the water. Blood vessels draw oxygen from the water as it passes through the gills.
The spiracle is a vestigial first gill slit, which is reduced or not present at all in fast swimming sharks and is used in bottom dwelling sharks to provide oxygenated blood to the eye and brain. In rays, a close relative of sharks, the spiracle is used to pump water through the gill slits so they can breathe when hidden under sand.
There is a well known myth that all sharks have to keep moving in order to breathe. This actually is true of some sharks, especially larger sharks, as they have low blood pressure and need the action of muscle to help circulate the blood. However many sharks, such as Nurse sharks, can breathe by buccal pumping.
Buccal pumping is named after the mouth, or buccal, muscles. These sharks can use their mouth to draw water in and over their gills, allowing them to breathe when stationary. In videos of sharks laying on the seafloor and opening and closing their mouths they are actually breathing!
Ram ventilation is when sharks breathe by swimming with their mouths open, forcing water over their gills as they move through the water. Some sharks can switch between ram ventilation and buccal pumping depending on the situation. However some sharks are known as obligate ram ventilators. These sharks can’t breathe by buccal pumping, and so have to keep swimming in order to breathe.
As sharks don’t have a swim bladder like bony fish, they need to find ways of achieving buoyancy, or not sinking.
Adaptations to this end can be seen throughout the anatomy of a shark. Their skeleton is cartilaginous and is very light, they lack a ribcage, and their liver is huge!
The liver of a shark constitutes up to 30% of its total body mass. In other animals the liver makes around 5% of the total body weight, so you can see the difference and the adaptation in sharks.
It is also filled with oils and squalene. If you’ve tried to mix oil and water, you’ll know that oil floats on top of the water as it is less dense. Having a huge oily liver therefore makes the shark much more buoyant.
Not having a swim bladder isn’t a detrimental adaptation though. In fact it is one of the more extraordinary adaptations, as it allows sharks to move freely up and down in the water column as they are incompressible. Bony fish tend to be restricted to a certain depth range, otherwise they would be at risk of pressure changes exerting their effect on the gasses of the swim bladder, compressing or expanding the fishes body.
Buoyancy is instead maintained through dynamic lift created by the pectoral fins, much like the wings of a plane. As such they need to keep swimming or they will sink. This has given rise to the myth that sharks need to keep swimming or they will die. Although, as is the norm, some shark species are called obligate ram ventilators and have lost the ability to pump water over their gills and do need to keep moving to breathe!
Most sharks are cold-blooded poikilothermic animals. This means that their internal body temperature matches the temperature of their surroundings.
The term cold-blooded is a little confusing, as sharks living in cold water will have a cold body temperature, but sharks living in warm water will have a warm body temperature.
Some sharks however are warm-blooded. That is, they have an internal body temperature higher than that of the surrounding water. Fast swimming sharks such as the Mako and Great White sharks have specialized blood vessels called rete mirabile which act as a countercurrent heat exchanger, allowing them to retain heat generated by the action of red muscle.
Being able to retain heat in this way allows these sharks to survive in many different environmental conditions.
Chemical processes occur faster at higher temperatures. This includes metabolism, meaning that these sharks are able to be faster and more active, and therefore better predators, than other sharks and animals.
An entire new article could be dedicated to shark senses. Not only do they have the same five basic senses that we humans do (smell, sight, sound, touch, and taste) they also have the ability to sense electrical currents and pressure changes as well.
These senses are incredibly sharp. A shark’s brain is as complex as a mammals, which is a rare thing for non-mammalian animals. This huge complex brain allows them to be such successful hunters.
Up to 75% of a shark’s brain is dedicated to its sense of smell. As such they have incredibly keen noses, and can detect a teaspoon of blood in a swimming pool! This equates to one part of blood to one million of water.
Shark’s ears are found behind each eye. They are seen as two small holes, they don’t have ears like ours! Three cartilage tubes are lined with hair cells and filled with fluid pick up, and are very sensitive to low frequency noise, especially those made by a wounded animal.
A shark’s sense of taste isn’t as sensitive as their other senses. This could be because it doesn’t directly help them find food. However sharks are known to test bite prey, which could well be the reason why shark attacks have a high survival rate. They bite a victim once, only to realize that they don’t taste nice and spit it back out.
A shark’s sense of touch is very sensitive as they have lots of nerve endings under their nose. Bottom dwelling sharks often have barbells which help them to search the substrate for prey such as bivalves and crustaceans. Barbells are covered in nerve endings which makes them incredibly sensitive.
Sight isn’t a main sense in sharks as light doesn’t travel well in water, even in crystal clear water. If the water is murky or silty then sight doesn’t come to the fore much at all. However as their eyes are situated either side of their head, they are able to see almost 360°. Their eyesight gets much more acute within 15 meters of an object. Similar to cats they have a shiny layer on their eyes called a tapetum lucidum, which enables them to see better in low light conditions.
Now onto the two coolest of sharks senses, electroreception and pressure changes.
Special organs found in the head and snout called ampullae of Lorenzini allow for electroreception. These are seen as small pores in the snout. Ampullae of Lorenzini are incredibly sensitive and enable sharks to not only detect the muscle movements of hidden prey, but also the Earth’s geomagnetic field. This allows sharks to be incredibly successful hunters and navigate huge distances accurately.
The lateral line organ is found in many aquatic vertebrates and enables sharks to detect movement, vibration, and pressure changes in the surrounding water. This enables them to follow prey by the vortices they leave in the water. It also enables them to create a pressure map of their surroundings, and lets them know what it is around them.