A shark is the ultimate predator of the ocean and they have developed many adaptations to help them hunt, reproduce and survive in their marine environment. Sharks belongs to a class of fish known as the Chondrichthyes or cartilaginous fish, which also includes rays, skates and chimera.
By developing well-refined senses that work in conjunction with dangerous speed, strength and camouflage, sharks have become the most feared predator in the sea. In addition, they have evolved interesting methods to deal with problems faced with life at sea such as regulating buoyancy and reproduction.
A Shark’s Skeleton
The skeleton of a shark is made entirely from cartilage rather than bone. This is the feature that defines the group of fish that sharks belong to called Chondrichthyes. It is thought that the cartilaginous skeleton is the ancestral form for all vertebrate skeletons and that bones evolved more recently. Cartilage is lighter and less dense than bone which makes sharks naturally more buoyant than other fish.
A shark’s eyesight is also incredibly strong. An increase in the number of rods in their eyes gives sharks the ability to see in waters with poor visibility, such as murky water and moonlight. Some sharks, for example the blue shark, have a nictitating membrane that slides over the eyes to prevent damage from prey and UV light.
Sharks have a total of six senses, five of which are involved in locating food. Their hearing is amazingly good regardless of the fact their ears are hidden under their skin. They are able to detect sounds from over one mile away and the heartbeat of fish close-by; by detecting an increase in heart rate, it is thought that they are able to detect fear.
Smell and taste
Smell is one of the most important senses a shark has in terms of hunting and selecting food. Experiments have shown that 10% of a shark’s brain is devoted to smell and they are able to detect one part blood in one billion parts sea water. They are able to detect healthy fish from a long distance and react strongly to injured or distressed fish from great distances. Related to smell is the sense of taste, and taste is important for making the final decision of accepting or rejecting food.
A line of sensitive skin that runs down the side of a shark’s body, known as the lateral line, is used to feel any movement of water caused by surrounding fish. The lateral line provides a shark with information on how big and how close a fish is, and whether or not it is injured. The lateral line runs along the length of their body and is made up of tiny pores, filled with a jelly-like fluid and hairs.
Ampullae of Lorenzini
Arguably, the most impressive of the shark’s senses is the ampullae of Lorenzini. Tiny sensory pores on a shark’s nose that can detect very small electrical fields. Each pore contains a jelly-filled canal with sensory cells at the end. The hammerhead shark hunts on prey completely hidden from view but is able to find them by picking up on their electrical field through the ampullae of Lorenzini. This means that sharks can continue to hunt in very dirty water and moonless nights. It is also thought that sharks are able to pick up electrical fields from the Earth’s surface to aid in navigating.
Sharks use three different methods of reproduction. Each method begins with internal fertilization of a female shark. In many species, males will bite onto a female to ensure he is close enough to fertilize her using one of his claspers, a specialized pelvic fins used for fertilization. The clasper is bent at a 90° angle and inserted into the female’s cloaca. The clasper is locked in the cloaca by a mix of barbs, hooks and spines.
Oviparous is the first of three methods for reproduction used by sharks. Oviparous sharks lay very large egg cases and wedge them into safe areas in rocks or seaweeds. Nutrition for the embryo is provided by the yolk of the egg and molecules taken from the flow of water into the egg. It takes around 6 – 10 months for an embryo to fully develop and hatch.
The seven gill shark, along with others, uses a method for reproduction known as ovoviviparous. The young are developed within the mother but all the nutrition still comes from an egg yolk. Inorganic molecules are transferred from the maternal circulation to the developing young. The young may hatch inside the oviducts and spend half their fetal life surviving by cannibalism of their brothers and sisters.
The most developed sharks are viviparous sharks, such as the hammerheads and bull sharks. Young are developed inside the mother and the nutritional supply is continuous and not limited to a yolk. Some viviparous sharks develop extensions of the oviduct wall which deliver nutrition to the young. Others continue to ovulate and young feed on the eggs. Most commonly the yolk sac connects to the uterine wall and nutrition is obtained from the bloodstream via the yolk sac.
A common problem faced by fish is the challenge of maintaining neutral buoyancy in water with changes in depth. There are a number of methods fish use to combat this issue and sharks manage to overcome the problem through adaptations in their liver.
A shark’s liver is mostly made from a substance called ‘squaline’ which can make up as much as 25% of a sharks total body weight. Because squaline is positively buoyant in water, adjustments to the squaline content and size of a shark’s liver balance out the weight of dense body parts that may otherwise cause a shark to sink. In addition to their adaptive livers, cartilage is about half as dense as bone, so does not sink as bones do, however denticles (unique scales), teeth, and calcified cartilage add density to sharks.
The metabolic rate of sharks is mostly determined due the fact that they are cold-blooded animals and derive heat from the environment. Cooler body temperatures means they have very low metabolic rates.
To lower their metabolic rate further, they have very low energy expenditure because of efficient swimming. This means the frequency that sharks need to feed is very low in comparison to warm-blooded animals such as humans.
The camouflage method most sharks use is counter-shading. A good example of counter shading is seen in great white sharks, where the top of their body is a darker color than the bottom. Having a lighter underside and a darker topside means that when they are above fish they are more likely to blend in with the lighter surface waters and when they are below they can blend in better with the dark, deeper water. Bottom dwelling sharks such as the carpet shark and wobbegong use cryptic camouflage to blend into their environment and catch prey unaware.
The impressive senses of sharks in addition to their stealthy camouflage, strength and speed show just how sharp sharks are as a predator. Sharks evolved from an ancient line of fish and maintain the cartilaginous skeleton that was the predecessor to bones. Their ancestors have been the dominant predators in the oceans for millions years due to the adaptations they have evolved to improve their ability to catch evasive prey. They truly are spectacular animals.
Last edited: 17 December 2015