Shark Evolution & Classification
True sharks in the class chondrichthyes evolved approximately 420 million years ago, in the Silurian period. They had cartilaginous skeletons, their skin was covered in tooth-like denticles and their jaws were lined with rows of teeth. We know this due to various types of fossil evidence such as teeth, denticles, fin spines and rare skeletons. The oldest undisputed evidence that sharks existed during this time, comes from Silurian deposits found in Siberia of skin denticles. These have been identified as belonging to “Elegestolepidida” which is recognised as the oldest cartilaginous taxon to develop dermal denticles. The oldest shark fossil found dates back to the Early Devonian period almost 409 million years ago. Fossils like this are rare because cartilage usually decays quickly leaving little time for fossilisation to occur.
During the early Carboniferous period (c. 360 mya) shark forms diversified greatly, with species evolving and adapting to their environment and their food source. This was followed by a decrease in diversity with many forms becoming extinct by the beginning of the Permian period (c. 300 mya). However by the end of the Permian (c. 250 mya), there is evidence to suggest an abundance of ray finned fishes (Actinopterygians) inhabited the oceans, these Actinopterygians were an excellent source of food for the early shark forms and in response sharks began to diversify once more.
The fossil record for the early Triassic period (c.250 mya) shows a species known as Palaeospinax, this shared a similar morphology (dorsal fin spines, a sectioned vertebral column, position of jaw) to shark species found today such as the dogfish family (Squalidae), most notably the Spiny dogfish or Spurdog (Squalus acanthias) which has retained the dorsal fin spines.
Present day shark species have retained their diversity and variation, this can be seen by taking a look at their physical appearance; from flattened bottom dwelling sharks, to large filter feeding sharks, slow moving deep water sharks, camouflaged sharks, eel like sharks and of course the more familiar sleek torpedo shaped sharks.
Different body shapes and features are among the characteristics used to classify the nine separate orders of sharks which are as follows:-
- Squatiniformes (Angelsharks)
- Pristiophoriformes (Sawsharks)
- Echinorhiniformes (Bramble sharks)
- Squaliformes (Dogfish)
- Hexanchiformes (Frilled and Cow sharks)
- Carcharhiniformes (Ground Sharks)
- Lamniformes (Mackerel sharks)
- Orectolobiformes (Carpet sharks)
- Heterodontiformes (Bullhead sharks)
Smell, Sight, Taste, Hearing, Touch and Electroreception are the six sensory systems sharks are equipped with allowing them to successfully exploit the environment they live in by locating prey, avoiding danger and finding a mate.
Different species of shark utilize these senses relative to the environment they live in, or the conditions they may encounter. For example bottom dwelling sharks such as the Angel shark which have their nostrils partially obstructed by the sea bed, would use its sense of smell less when locating prey than an open ocean dwelling shark such as the Oceanic Whitetip.
Smell (Olfaction): Sharks posses a pair of nares, just under the edge of their snout. They are completely separate from the mouth and throat and do not aid in respiration, instead they are used purely for olfaction. Each nare is divided into two channels by a nasal flap, the water enters one channel gets passed over an area called the olfactory lamellae which contain neuro-sensory cells, these then send chemosensory information via the olfactory bulb to the large olfactory lobe in the sharks forebrain. After passing through the olfactory sac the water is then channelled out through the other nare. If the shark detects a smell which it wants to investigate (odour from prey or pheromones from a potential mate), it will swim in the direction of the scent moving its head back and forth (similar to its natural swimming motion), this motion will allow it to detect the direction of the smell by following the most concentrated signal.
Sight: In the majority of shark species the eyes are well developed, complex structures containing rod (highly sensitive to light intensity) and in some species cone cells. They can control the amount of light entering the eye by dilating or contracting their pupils. Focusing is controlled by the rectus muscles, these pull the lens closer to or further away from the retina, when used in conjunction with the oblique muscles movement of the entire eye is achieved.
Most sharks possess excellent vision in dim light conditions; this is due to the retina containing millions of rod cells together with a structure called the tapetum lucidum, this is a layer found behind the retina which reflects light back onto the retina amplifying the image. Sharks possess an upper and lower eyelid but these usually do not meet and therefore do not provide a full cover for the eye. Some sharks such as the tiger shark (Galeocerdo cuvier) have a “third eyelid” known as the nictitating membrane, this rolls up from the base of the eye to completely cover the eyeball, the use of this nicitating membrane is demonstrated regularly in shark documentaries, and most notably when the shark is attacking its prey.
Species like the great white shark (Carcharodon carcharias) which do not have the nicitating membrane often employ a different strategy to safeguard the eye; they roll the eye into the back of the socket exposing a hardened pad at the rear of the eyeball. The existence of such strategies designed to protect the eye, highlight the importance of sight as a sensory function to the shark.
Taste (Gustation): As a shark bites into an object (prey or otherwise), chemicals are released and attach themselves to gustatory sensory cells present in the sharks mouth and throat, these gustatory cells then send messages to structures (the thalamus and the hypothalamus) located in the sharks forebrain. The shark will then either accept or reject the object it has bitten. Taste often comes up when human-shark interactions are discussed. The entire consumption of a human by a shark is very rare, most instances are comprised of a single bite which (depending on the location of the bite) are usually non fatal. The question is why do sharks bite and release humans? There are a couple of possibilities:
i) the chemicals detected do not trigger an acceptance signal – perhaps a much higher fat/blubber content is required, or the interpreted signals are too different from their usual prey source;
ii) the shark is employing an energy saving tactic of inflicting a potentially fatal wound, allowing the prey to tire and then intending to finish the meal.
This latter possibility is difficult to verify due to the circumstances of most sharks attacks, for example in the majority of instances the shark has been reported to swim away following the first bite, but why is this? The victim is rarely alone in these situations with fellow ocean goers on hand to help the victim get out of the water; hence it is not always clear if the shark would hang around in the absence of commotion caused by other people. Shark bites can occur for various reasons including; mistaken identity – surfers looking like seals (although sharks have reasonably well adapted eyesight, studies on the white shark Carcharodon carcharias, have shown that when towing a seal cut-out behind a boat some sharks will still attack), poor water visibility, curiosity, shark dependent (some sharks are more curious/energetic/excitable than others), ill/weak sharks (if a shark is sick or hasn’t eaten for a long time perhaps any prey is better than none). It is important to remember however that by going in the water you are becoming a part of the sharks’ environment and it is important to respect that, the shark is a top predator; you wouldn’t for example enter a lions territory without some caution!
Hearing (balance and pressure detection): The shark ear is located in the frontal skull (chrondocranium) and is completely internal with only a tiny opening on the sharks’ head – not the spiracle which is involved in respiration. The ear detects sound with frequencies ranging from <20 to about 800 Hertz, most sharks show an attraction to infrasound (<20Hz) this is most likely due to the low frequency sounds emitted by struggling prey. The shark ear is also used for balance/orientation (by utilising the fluid filled semi-circular canals, with the movement of the fluid activating sensory hair cells) and pressure detection (by direct activation of the hair cells within the canals allowing sensory signals to be relayed to the brain via the auditory nerve).
Touch: A shark can feel a certain amount of direct contact due to free nerve endings embedded in the skin, mouth, jaws and even teeth. They can also sense things internally due to the presence of proprioceptors (microscopic sensory cells) found throughout the muscles, joints, digestive system and blood vessels. Sharks have a heightened sense of indirect touch via water displacement around the sharks’ body, this is accomplished by the movement of sensory hair cells which are present in the neuromasts that make up the lateral line system. This system is comprised of a series of canals or channels usually visible to the naked eye as a series of pores or lines, these run from the head all the way to the upper lobe of the tail.
This ability to detect water displacement aids the shark in prey detection and increases its awareness of any moving object in the water column. As a fish swims near a shark it will displace water and send waves outwards, these waves then directly move the cupula (a jelly like substance covering the cells) which bend and activate the sensory hair cells inside that send a signal to the medulla oblongata in the brain. The shark will also obtain an orientation on the prey item/object based on the strength of the signal received by the lateral line canals. The lateral line is also used to detect odour through a process referred to as “eddy chemo-taxis” meaning the tracking of odour and turbulence simultaneously.
Electroreception: The Ampullae of Lorenzini are specialised pores that are thought to have evolved from the lateral line system. They consist of a small chamber (the ampulla) and a sub-dermal canal which projects outward to the surface of the skin, the ampulla contains hundreds of sensory hair cells. The wall of the canal contains a double layer of connective tissue fibres and epithelial cells, which are tightly joined together to form a high electrical resistance between the inner and outer wall of the canal. The canal and ampulla themselves are filled with a high potassium low resistance gel that forms an electrical core conductor with a resistance equaling that of seawater.
Fish carry an electrical charge different to that of seawater and so a weak voltage is created (by the movement of positive and negative particles moving back and forth shifting electrons in an attempt to become stable). Because the salt in the water contains both sodium and chlorine ions which can move freely in the water the electricity itself is transported, and this is what the ampullae of lorenzini is able to detect.
Sharks inhabit every ocean on the planet; this is a testament to both the adaptability and the variety found among shark species as the oceans themselves provide very diverse conditions. Constant variation in water temperature, concentrations of dissolved oxygen and salts, light levels and movement of water masses are all encountered periodically by sharks.
The classification of shark species in terms of habitat in relation to landmass can be considered subjective. This is largely as a result of human occupancy or research having been conducted reasonably close to shore and hence any shark encounters within these areas have been recorded. Just because a shark hasn’t been seen in a particular place does not definitely mean it isn’t there. A classic example of this is the white shark (Carcharodon carcharias) considered (the females in particular) to be a coastal water species. However in November 2003 a female white shark was tagged off the coast of South Africa, she then made a transoceanic migration to the North East coast of Australia and then migrated all the way back to the site of tagging, resulting in a round trip of 20,000km in just under 9 months!
Why do our Oceans need sharks?
Sharks As Controllers: Commercial fishing practices, finning, trophy hunting and destruction of habitat, are all factors which result in the decline of sharks. It is reasonable to assume that the removal of sharks from the ecosystem, will affect the population numbers of their prey species as well as other species within the food web. Examples of such effects have been documented in other ecosystem interactions; for instance the Sea Otter was almost hunted to extinction off the West Coast of North America, this led to a population explosion of sea urchins (the otters main food source), which in turn led to a dramatic decrease in Kelp (the sea urchins food source). The Kelp provided the Pacific Herring with breeding grounds, with this area vastly depleted the Herring disappeared and without them the salmon, tuna, sea lions, sharks, dolphin and whales also left the area.
Sharks as Indicators: The loss or lack of sharks from certain areas are indicators of the balance of the ecosystems, provided the sharks have not been fished out of certain areas it is reasonable that the disappearance of sharks from an area is as a result of or in addition to the destruction of a suitable habitat. The fact that shark species are so diverse and inhabit every ocean on the planet makes them key players essential to the ocean environment. It is important to remember how vital the functioning of the ocean ecosystem is to earth itself. With phytoplankton being the greatest consumer of carbon dioxide which it uses for the process of photosynthesis, oxygen is the by-product of this process and it is estimated that 70 % of the oxygen in our atmosphere is produced by phytoplankton!