Over the last few million years, sharks have evolved highly developed and diverse characteristics that make them impressive and formidable predators:

Gills Sink or Swim Muscles Temperature Control

Kitefin skeleton © Wikimedia Commons, Ryan Somma.  

SKELETON – A shark’s skeleton is made of cartilage, which is very strong and durable, yet much more flexible and lighter than bone. Being lighter helps the shark to stay afloat and expend less energy through movement. The flexibility of cartilage also allows the shark to make tight turns quickly - a key adaptation that makes them one of the most advanced and efficient animals in the ocean.

FUN FACT - Scientists can tell the age of a shark by counting the rings on a shark’s backbone!

Shark skin illustration © Marc Dando.

SKIN – Shark skin is covered with millions of tiny teeth called dermal denticles, which point backwards, reducing surface drag and helping the shark to swim faster. As a shark grows their denticles are shed and replaced with slightly larger ones.

Denticles vary in shape according to the type of shark and also where they are on the body. Some denticles are so big that the shark uses them as defensive spines or shields!

FUN FACT - the hydrodynamic efficiency of shark skin has been copied by swimming costume manufacturers, such costumes are said to reduce swimmers times.

Tiger teeth © Wikimedia Commons, Stefan Kühn.  

TEETH - Made of enamel, shark teeth are strong and appear in huge numbers in the fossil record.

Sharks fend for themselves immediately after birth and so are born fully equipped with their own teeth. They have multiple rows that are formed in the mouth - as the front rows wear down or fall out, a new row will move forward from the back of the jaw to replace them, ensuring the shark always has a full set of razor sharp teeth.

FUN FACT – sharks replace their teeth approximately every two weeks. Some species can lose 30,000 teeth in their lifetime!

We can tell what a shark eats by the shape of its teeth – flat crushing teeth are used for shellfish, pointed teeth for gripping fish and sharp serrated teeth for larger prey, such as seals. A shark’s teeth may change with age as the diet of a pup may differ to that of an adult.

Shark fin © James Lea.  

FINS – The fins of a shark provide balance and stability in the water:

Dorsal Fins - This triangular-shaped fin on the back of a shark is required for balance. Usually a shark will have an additional smaller dorsal fin further back towards their tail.

Pectoral Fins - Sharks have two pectoral fins, one on either side of the body, which they use to steer and lift themselves in the water.

Pelvic Fins - Located behind the pectoral fins are two smaller pelvic fins. In male sharks, claspers will be associated with these, which are used in shark reproduction.

Anal fins – Some sharks have anal fins, which are believed to aid balance.

Caudal Fin – Sharks use their tail to propel themselves forward. The size and shape of their tail can vary greatly - faster sharks (such as the Shortfin Mako) tend to have shorter crescent shaped tails, whereas slower moving sharks (such as the Bluntnose Sixgill Shark) have longer thinner tails.

A high demand for shark fins worldwide has contributed to the decline of many shark species.

Shark gills illustration © Marc Dando.  

GILLS – As water passes over the gills, oxygen is absorbed into tiny blood vessels and transported around the body.

Many sharks must swim continuously in order to extract oxygen from the water. Through forward movement water is driven through the mouth and out over their gills in a process known as ‘ram-ventilation’. If these species become trapped for any reason they’ll be unable to breathe. Although, some less active sharks, such as Nurse Sharks and Zebra Sharks, can survive by sucking water into their mouth and squeezing it over their gills.

Sandtiger Shark © Dray van Beeck.  

SINK OR SWIM - Sharks rely on a number of adaptations to prevent them from sinking - a light skeleton reduces their body weight, the continual movement of water under their fins creates lift, and a large liver stores low density oils.

In some sharks the liver can comprise 25% of their total body weight, whereas in mammals this is only 5%. Unfortunately there is huge demand for shark liver oil (squalene), which is used in a number of cosmetic products, so this key adaptation also makes sharks vulnerable to human exploitation.

Some sharks, such as the Whitetip Reef Shark, are able to rest on the seabed - to resume swimming they’ll propel themselves forward with their tail.

FUN FACT - the Sandtiger Shark is able to hover motionlessly by gulping water at the surface, trapping air in their stomach.

Oceanic Whitetip Shark © Andrew Bellamy.  

MUSCLES – Sharks have two kinds of muscle – red and white. A thin layer of red muscle lies just under the skin. This has a good blood supply and helps the shark to swim slowly for long periods of time. Red muscle works by breaking down the fat in the shark’s body. White muscle is found under the red muscle. This has a poor blood supply and works by using the energy from the breakdown of glycogen (sugars). Sharks use their white muscle to make short fast sprints when catching prey or moving away from danger.

Long bundles of muscle fibres run from the top of a shark’s head to the tip of its tail. When these are contracted a series of undulations are produced along the body, allowing the shark to accelerate through the water using its tail. More pronounced muscle contractions produce faster acceleration, however the more a shark’s body bends, the less efficient its use of energy. To conserve energy a shark will build acceleration with a series of muscle contractions and then stiffen its body to cruise through the water.

White Shark © Sean Sequeira.  

TEMPERATURE CONTROL - The body temperature of a fish tends to be the same as the surrounding water. However mackerel sharks, which include the White Shark, Porbeagle Shark, Salmon Shark, Shortfin Mako and Longfin Mako, are warm blooded.

Retaining warmth makes them more efficient predators. Heat is mainly lost through a shark’s gills which provide a large surface area for heat exchange causing the blood to be exposed to cooler seawater temperatures. To counteract this and maintain an increased body temperature, mackerel sharks have a network of tiny capillaries which act as a heat exchange system, known as a rete mirabile. Blood vessels carrying warm deoxygenated blood to the gills pass alongside cold oxygenated blood that is going to the body (in the opposite direction), and as a result pass over some of their heat. This system helps the warmth from the body return to the muscles rather than being lost to the sea.

FUN FACT – the body temperature of mackerel sharks can be 10°C higher than the surrounding water.


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