Are sharks perfectly adapted predators?
This essay will discuss how sharks are thought to be one of the most perfectly adapted predators in the ocean food chain due to their physiology and interactions with their environment. However we must not disregard the fact that there are many species of sharks which may possess different features meaning that only certain species may be considered ‘perfectly adapted’. It is thought that sharks have been around for around 400 million years, in which they quickly evolved to the environment they live in. The sharks we see today are believed to be the result of evolution that took place 100 million years ago, making them largely unchanged over this time period. The most recently evolved sharks are the sphyrnids (hammerhead sharks) and the carcharhinids (requiem sharks). Although the reason behind why these sharks have survived could be due to early adaptation to their ecological role, we must consider those species of sharks that are now extinct and understand why certain species adapted quicker than others.

Sharks main defining feature is their cartilaginous skeleton, placing them in the class Chondrichthyes, which is thought to make up only 1% of living fishes. By having cartilage as opposed to a bony skeleton they are lighter and more flexible in the water, as well as the cartilage strengthening the body frame of the shark. This class of fish are commonly known for their paired fins, paired nostrils and jaws and can then be further divided into two subclasses; Holocephali and Elasmobrachii. Sharks fall under Elasmobrachii and are classified by their flattened bodies, an upper jaw not fused to the cranium, placoid scales and 5-7 pairs of gill slits. All sharks are carnivorous, and almost all are predators occupying a large range of ecological niches.

The physical appearance of sharks varies substantially across the 360 species there are believed to be, however many possess common adaptations. Sharks do not have a swim bladder to regulate their buoyancy which can be seen as an adaptation as this means they do not have to wait for gas pressures to equalise when making fast changes in depth. This allows them to make sudden bursts of up to 30kmph when needed to catch prey. This is a necessary adaption as it allows sharks to hunt from the surface of the water to 100’s of metres deep within a few seconds and without this characteristic may be seriously affected in terms of finding prey. Instead sharks control their movement throughout the water column by having a light skeleton and an oily liver as they are heavier than water and therefore tend to sink. By storing large quantities of oil in their liver they decrease their overall density, thus increasing their buoyancy and in turn reducing the amount of energy needed for swimming. A shark’s liver takes up around 90% of the space inside its body cavity. A great white shark weighing 3,312 kg (7,302 lb.) had a liver 456 kg (1,005 lb.) in weight.

Sharks have a diverse range of body shapes, but typically have a streamlined fusiform body meaning a rounded, torpedo shape at both ends. This contributes to their adaptations as it reduces drag and requires a minimum amount of energy to swim. They also have dermal denticles, also known as placoid scales, making up their skin which gives a rough texture. The denticles allow the shark to swim faster by reducing water resistance due to the fact they point backwards and also make them hydrodynamically quiet. This is advantageous as it allows the species to hunt for their prey with no noise being created from water turbulence, which in turn means their prey have little chance of escaping due to being unaware of the predator approaching. The skin of sharks has evolved within different species and also between males and females. For instance the females of some species tend to be considerably thicker than the males as males may hold the females during copulation which may inflict serious bites. Further adaptations that sharks possess are the 5-7 gill slits they use for breathing. Water flows through the mouth and is then forced over the gills where oxygen is absorbed. This allows them to stay completely submerged in the water their whole life as opposed to cetaceans such as dolphins which are required to make frequent trips to the surface of the water in order to breathe through their blowhole. However some species of sharks have failed to further evolve in the sense that if they are trapped or prevented from swimming forward they may suffocate due to poor ram ventilation. Ram ventilations are the idea that sharks swim forward most of the time in order to aid water flowing of their gills. Other shark species on the hand have been known to be able to actively pump water over their gills, known as buccal pumping, whilst resting on a sandy bottom. For example the nurse shark has this adaptation, which is necessary due to its feeding habits, such a bottom-dwelling fish. Nurse sharks are also a shallow water species that don’t migrate when the water becomes cooler, they simply become less active, and therefore this adaptation allows the species to continue to breathe.

The type of teeth sharks have depend on their feeding habits as different species require different uses for them. For example those that prey on fish have pointed teeth to allow them to catch their prey whereas sharks that eat seals or sea lions have razor sharp teeth for cutting out manageable chunks. Filter feeding sharks have also further evolved as they have a reduced amount of teeth due to the fact they are needed and instead utilize dermal denticles lining the gill plates. It is necessary that different species of sharks have different adaptations as they would no longer be able to feed on their current prey if they were to otherwise adapt. Sharks teeth are arranged in rows, and the species will usually have around 5 rows of teeth at a time which allows damaged or lost teeth to be replaced quickly. This means the shark will always be able to eat its prey because typically shark use their teeth to grab or tear apart their prey and not chew. Without this feature the shark would become unable to catch its prey.

A combination of physical and behavioural adaptations make sharks one of the most adapted species on earth and is key to why they are considered perfectly adapted. The reason many believe sharks to be perfectly adapted is due to the fact that these animals have remained unchanged for years and so if they have not furthering evolving then they must already be perfectly suited to their environment. However others may argue that the species isn’t perfectly adapted as there are still disadvantageous features sharks have as well as those few species that have already become extinct, which could be possibly due to poor adaptation. Some of the traits sharks have that have helped them become so successful have made them vulnerable to human impacts. For example because sharks are top predators they have very few natural enemies and therefore live for a long time and because of this adults tend only to reproduce at an old age. Sharks reproduce slowly and give birth to very pups. This means a disadvantage to be being so well adapted to their environment is that human impacts such as hunting can cause the species to dramatically reduce and collapse. By humans removing a large number of adults sharks quickly will results in too few pups being born to replace the losses as it may take a long time for the pups themselves to reach maturity and start reproducing. Overall it can be said that yes, sharks may be perfectly adapted to their environment and habitats, but this in itself can act as a reason to be considered why sharks are also not perfectly adapted.

Reference list
Bunting, A. E. (1979) The Sea World Book of Sharks. San Diego: Sea World Press
Evans, M. et al. The Great White Shark (2010) Series 2, Episode 13, Inside Natures Giants. London: Channel 4. Tuesday 08 June 2010 [television series].
Gilbert, P. W. (1970) Final report: studies on the anatomy, physiology, and behaviour of sharks. Washington, D.C.: Office of Naval Research 5: 17-22
Hamm, W. S. (1950) Liver oil properties of Philippine sharks and rays. Washington, D.C.: U.S. Dept. of the Interior, Fish and Wildlife Service
O'Daly, A. (2004) Encyclopedia of Life Sciences - Vol. 11. Tarrytown, NY: Marshall Cavendish 1487
Young, W. E. (1934) Shark! Shark! The thirty-year odyssey of a pioneer shark hunter. New York: Gotham House 19: 255-277