Take a closer look at what makes these animals top predators in the marine system!
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Sharks have been around far longer than most other animal groups. They first appeared some 400 million years ago… this is about 100 million years before the dinosaurs roamed the earth and 100 million years after the first fish evolved.Sharks evolved from the divergence of the earliest form of fish into many different groups, only two of which still survive today. The Chondrichthyans retain their early physical characteristics of having cartilaginous skeletal structures, however the Osteichthyans replaced cartilage with bone in their skeletal structures and this latter group remains the most abundant fish group today. Sharks belong to the Chondrichthyes; the divergence of these groups over 400 million years has led to some profound differences in biology.

Buoyancy

bcdThe Osteichthyans (bony fish), all evolved with swim bladders with which they regulate the amount of air enclosed, thus managing their buoyancy through height in the water column. Chondrichthyans do not possess swim bladders and as such are negatively buoyant, meaning that they would sink when not moving. To mitigate against sinking there are several behavioural and physiological traits that sharks have. Firstly the cartilage making up their skeletal structure is much lighter than bone. Cartilage is also much more flexible than bone, allowing for quicker movements and tighter turns. Most shark species also have unusually large livers, making up to over a quarter of the total body size. The liver contains an extremely high oil content and as oil is lighter than water, the liver aids in buoyancy. The main behavioural characteristic preventing sinking is swimming and by using their fins, sharks can essentially glide through the water and alter their position in the water column.

Teeth and Jaws

Sharks continually produce and shed teeth throughout their lifetime. The teeth are formed inside the mouth in rows which are constructed in grooves within the cartilaginous upper and lower jaws.

The lower jaw of a Bull shark – showing teeth emerging from the groove in the lower jaw.
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The cross section below illustrates the movement of the tooth rows from the grooves to the outer most point where they are eventually shed. Each tooth comprises an inner core of pulp encased in dentine and coated in enamel, as in mammalian teeth. However, in sharks each tooth is carried along a “conveyor-belt” course through weak attachments to the jaws by the dermis. Sharks can shed 30,000 teeth in a lifetime.

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The shape of a shark tooth is incredibly varied and unique to each species, related to the diet of each species. Sharp and pointed teeth are indicators of a diet mainly of fish and squid, while triangular and saw-edged teeth indicate a diet of larger animals such as turtles, seals or whales and dolphins. Saw-edged teeth indicate a diet from scavenging on large marine mammals.

The manner in which the teeth are used can also vary between species. Many sharks can produce a large amount of pressure with their jaws which can be used to crush hard-shelled invertebrates between more flattened teeth. Other species use little jaw pressure but shake their heads to create a saw-like motion with their teeth.

As the teeth rows are not attached or embedded within the skeletal structure, their teeth can be protruded away from the skull whilst feeding in some species. This means they are able to swallow larger chunks of prey without the hindrance of teeth. Another physiological trait that aids in feeding is the segmentation of the jaw structure. The upper and lower jaws are actually separated in the middle by a flexible cleft which allows the jaws to widen to a much greater scale (see image of Bull Shark jaw above).

Dermal Denticles

Sharks also possess a second set of teeth (also placoid) that cover the skin. These are dermal denticles meaning “skin teeth” often overlapping to create a chain mail-like armor. The shape of denticles is specific to individual species.

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The construction of denticles is similar to the oral teeth, but denticles are anchored in the dermis through the epidermis as seen in the cross section below.

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The overlapping of the denticles does not restrict the movement of animals and in fact can aid in resistance to build-up of bacteria and parasites on the skin of sharks. It also provides some measure of armored protection against attacks by larger predatory animals.

Even though there is great variation in size and shape of denticles through species, all have ridges which point to the tail end of the animal. This construction gives a hydrodynamic form to the shark’s skin which drastically reduces friction whilst swimming. In turn, it reduces the energetic cost of moving. This efficient design of shark skin has been used as a model in human engineering in two noteworthy cases, firstly a “shark-skin” swim suit. The artificially reproduced shark skin increases a swimmer’s performance. The effect was also applied to underwater craft such as personnel submarines.

The evolution of dermal denticles precedes the origin of sharks by around 20 million years. A group of fish known as the Thelodonti are the earliest form of animal that exhibited dermal denticles. Fossil records also show that some members of the group also had internal denticles which are considered by many to be the evolutionary heritage of teeth; these animals existed before the evolution of jaws!

Sharks have two unique sensory systems that give them a profound advantage whilst foraging and during vast migrations.

1. Lateral Line Sensory System

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The lateral line system exists in all shark species (and most bony fish species) and runs the length of their bodies, generally on either flank. It detects indicates movement and vibration to the shark through even minute changes in the surrounding water pressure.

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The receptors in the lateral line are called neuromasts which have hair cells encased within a gelatinous dome or cupola. The neuromasts in sharks are positioned mainly within canals connected to the surface of the skin by tubules. As water is effectively incompressible, the pressure change due to motion, vibration or depth is translated through the tubules and down the canals and registered by the effect on the neuromast hair cells.

Interestingly the hair cells of the neuromasts are similar to the construction of hair cells within the inner ear of vertebrates, suggesting a common origin. It has been long theorised that sharks are able to use their lateral line to aid in detection of underwater sound, as sound waves travel much like pressure waves.

2. Electro-sensory System

Electro-receptors in sharks are the ampullae of Lorenzini and allow sharks to detect electrical signals through these pores in their skin. They are most abundant on the head and face of sharks where they form intricate patterns in clusters. The pores also vary in width and depth.

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These electro-sensitive systems are found only in aquatic animals, due to the electrical conductivity properties of water rather than air. The electrical signals transmitted through water come in contact with the ampullae of Lorenzini pores in the skin of a shark. The signal is then transmitted through gel filled canals to the ampullae, comprising bundles of nerve fibres. The gel is a glycoprotein-based substance and has semi-conductor properties, allowing the transmission of electrical signals to the brain.

The ampullae also detect temperature gradients and electrical fields, present in all animals. Electrical impulses occur during muscle contraction and some shark species can detect their prey location by even 5/1,000,000,000th of a volt. A particularly novel use can be observed in hammerhead sharks, which are thought to be some of the most electrically sensitive species on the planet. They scan the sediment of the sea bed as if using a metal-detector, searching for buried invertebrates.

ampullae
As different parts of the face and head have individual clusters and patterns of pores, the information can be segmented to give the impression of strong or weak impulses, corresponding with changes in distance from the emitting object. Although little is known about the purpose of the patterns and sizes of the pores, they tend to be unique to species and individuals. Some forms of ampullae of Lorenzini are capable of detecting the earth’s magnetosphere, aiding a shark in determining its location in the sea. In the open ocean, this is certainly an advantage and it is thought that hammerhead sharks use this to navigate their vast migrations.