A shark’s skin is composed of 2 layers, the outer epidermis and the inner dermis. The epidermis consists of dead cells produced by the dermis which itself is comprised of connective tissue, muscle fibres, sensory nerve cells and blood capillaries. The dermis is also the anchorage site for the base of the dermal denticles which cover the shark’s skin.
Denticles are small tooth-like scales, which push up through the epidermis, the outer layer of the denticles is covered in enamel, and underneath this is dentine and then a pulp cavity in the center with a nerve and blood vessels. Denticles eventually fall out and are replaced by others throughout the shark’s lifetime.
The shape and orientation of the denticles direct water flow over the shark’s body which improves its swimming efficiency by reducing water turbulence.
A shark’s skeleton is composed of cartilage, this is a flexible structure that grows with the shark and is lighter than bone. It is composed of specialised cells called chondrocytes that produce an extracellular matrix of collagen fibres, proteoglycan and elastin fibres.
Bone is essentially cartilage that has been hardened by mineral calcification, during which calcium crystals provide extra hardness and rigidity. Calcification does occur in parts of the shark including its vertebrae, teeth, parts of the jaw, fin rods and denticles.
All sharks have 5-7 gill openings. Each gill arch contains a double fringe of feathery-looking gill filaments, made up of small leaf–like lamellae. These lamellae contain capillaries, as blood flows through these, oxygen diffuses from its relatively high concentration in the water to the lower concentration in the blood and carbon dioxide passes in the opposite direction.
This system is optimised by the counter-current principle; water flows over the gills from the front to back, blood flows within the gills from front to back, thus providing a concentration gradient between the oxygen in the water (high) and in the blood (low) increasing the transfer efficiency.
A constant supply of water to the gills is provided either by “ram ventilation”; as the shark swims forward water is drawn into the mouth and squeezed out through the gill slits or “buccal pumping” in bottom dwelling sharks which is often aided by the spiracles (located behind the eyes) which are vestigial first gill slits that provide oxygenated blood directly to the eye and brain. In some active pelagic sharks (e.g.: the shortfin Mako, Isurus oxyrinchus) the spiracles are reduced or missing and the ability to actively pump water across the gills has been lost, thus they are termed “obligate ram ventilators” meaning these sharks have to keep swimming in order to respire.
Sharks have 3 main muscle classes:
1. Cardiac muscle in the heart which works continuously.
2. Visceral muscles / smooth muscles are found in various internal parts such as the guts, arteries, excretory and reproductive organs. Contraction of these muscles allows passage of contents through these parts.
3. Skeletal muscles that move the skeleton are comprised of 2 types:
1) Red muscle present in thin layers underneath the shark’s skin, this works by breaking down fats in the shark’s body. It has a good blood supply and allows the shark to swim slowly for long periods without tiring.
2) White muscle works by using the energy from the breakdown of sugars, it has a poor blood supply and is used only for short fast bursts of swimming when chasing prey or avoiding danger.
Shark blood is composed of 55% plasma which contains: water (90%), dissolved proteins, glucose, minerals, hormones, gases, platelets and blood cells (erythrocytes – red blood cells, leukocytes – white blood cells and thrombocytes). Red blood cells contain haemoglobin; this binds to oxygen and gets transported around the shark’s body.
White blood cells are responsible for immune functions and include:
1. Lymphocytes, of which there are 2 types: B and T lymphocytes, they make up 40-60% of all circulating white blood cells.
2. Granulocytes, of which there are 3 types: heterophils, eosinophils and basophils, these vary in size, shape and number depending on the sharks present condition, however they generally constitute 20-50%, 2-3% and less than 1% respectively of the total white blood cells.
3. Monocytes and Macrophages constitute around 3% of all white blood cells.
Thrombocytes are thought to play a role in blood clotting and may also have an immune function they constitute up to 20% of shark blood cells.
Sharks possess 2 types of immunity:
1. Natural or Innate Immunity, this provides the first line of defence against invading pathogens (something that causes disease e.g.: a bacteria or virus). It is often referred to as non-specific as it does not depend on prior exposure or recognition of distinctive molecules.
2. Specific or Adaptive Immunity is when immunological memory is generated following an initial exposure to an infectious pathogen, thus if the pathogen is encountered again the immune system is able to recognise it and attack it much faster and more effectively.
Most sharks are poikilothermic meaning their body temperature is almost the same as the surrounding water temperature. However warm-blooded (homeothermic) sharks e.g.: shortfin mako (Isurus oxyrinchus) and the white shark (Carcharodon carcharias) minimize heat loss by passing their blood, which has been heated by muscle activity and biochemical reactions in the tissues, through rete mirabile (a vascular network of arteries and veins where blood flows in opposite directions allowing heat exchange) before it enters the gills and after it exits from them. There are 3 sets of retia in endothermic sharks, 1 in the swimming muscles, 1 in the anterior viscera (abdominal cavity) and 1 surrounding the brain.
The white shark (Carcharodon carcharias) for example, has been reported to raise its stomach temperature by as much as 14.3°C above ambient water temperatures. Higher temperatures allow sharks to operate faster and more efficiently.
Teeth and Jaws
Both the teeth and jaw shape are extremely varied amongst shark species, this is due to the varied diets and lifestyles, for instance bottom feeding sharks (such as the Heterodontiformes) feed primarily on hard shelled organisms and thus have a form of dentition known as molariform, which consists of hundreds of tiny flattened teeth.
The majority of the pelagic sharks have sharp, pointed teeth that are designed for piercing through fish, such as the shortfin mako (Isurus oxyrinchus), other sharks such as the Tiger shark (Galeocerdo cuvier) have more blade-like teeth designed for slicing and severing through large prey items.
The basking shark (Cetorhinus maximus) is a plankton feeder and possesses a line of thick brush-like strands that cover the gill openings preventing the plankton from escaping. Plankton feeders also have rows of tiny residual teeth that are thought to aid purchase during mating.
All sharks are able to replace their teeth continuously throughout their lifetime, they have pockets in the jaw sometimes covered by a layer of flesh where the new teeth grow and move forward in order to replace any missing or broken teeth.
The shark’s heart is situated just behind the base of its gills. It is comprised of 4 compartments:
1. Sinus Venosus is bulbous and thin walled this dictates the hearts contraction rate.
2. Auricle is also bulbous and thin walled this acts as a blood collecting chamber.
3. Ventricle is ovoid, thick walled and muscular it squeezes blood through the heart and onto the rest of the circulatory system.
4. Conus Arteriosus is conical with thin relatively inflexible walls with 3 rows of one-way valves, the exact function is not clear but it may uniform blood pressure leaving the heart and/or prevent blood washing back into the between contractions.
Heart rate varies between species with 19-48 beats per minute recorded for the spiny dogfish (Squalus acanthias), lesser and greater spotted catsharks (Scyliorhinus canicula, Scyliorhinus stellaris) and the leopard shark (Triakis semifasciata) and 28-78 bpm recorded for the shortfin mako (Isurus oxyrinchus).
The relatively short oesophagus extends into the U or J shaped stomach, which is located in the mid-line of the body, the first section of the stomach is called the cardiac limb and the second section the pyloric limb. The stomach walls are muscular allowing peristalsis (rhythmic squeezing) and have folds called “rugae” which allow the stomach to expand. Secretory cells in the stomach lining produce hydrochloric acid and mucus which enable the chemical breakdown of food. In addition the pancreas duct delivers protein-splitting enzymes into the stomach. Sharks have the ability to “evert” (push out) their stomachs in order to get rid of unwanted/indigestible items.
The shark liver also aids digestion by producing bile which gets stored in the gall bladder and delivered to the intestine. Bile emulsifies fats for absorption. However the liver is an important complex organ which performs a number of functions. The liver is composed of 2 large lobes and 1 central smaller lobe. The colour and size of the liver varies between species but generally comprises 15-35% of the total weight of the shark (humans 1.5-2.2%).
Some of the functions carried out by the liver include: storage and release of vitamins, manufactures a starch-like compound used as a fuel supply for white muscle, stabilises the blood sugar level, detoxifies poisons, builds enzymes, manufactures cholesterol and constitutes a major source of metabolic heat.
In sharks the liver is perfused with low density oils and hydrocarbons, one of the most important hydrocarbons is squalene which is less dense than seawater. Thus the liver provides the shark with lift/buoyancy (sharks do not posses swim bladders) therefore the shark invests little energy to prevent sinking and the vast majority of swimming effort goes towards propulsion.
The intestine of a shark is reasonably short, to compensate for this and to maximise nutrient absorption sharks have intestinal valves which increase the surface area of the intestine.
There are 3 basic types of valves:
1. Spiral valve, this is a helical-screw type shape found in cow sharks (Hexanchidae), spiny dogfishes (Squalidae) and catsharks (Scyliorhinidae).
2. Scroll valve, this resembles a loose roll of paper found in ground sharks (Carcharhinidae).
3. Ring valve, this appears as a series of tightly packed plates found in all mackerel sharks (Lamnoids).
Any remaining waste is excreted via the rectum and out of the cloaca.
But first a note on seawater…
Most seawater fish have fluids and tissues that have lower salt concentrations than the surrounding water. Thus water leaves their bodies by the process of Osmosis, which means they must drink large amounts of seawater to compensate. Sharks however have another answer; to prevent losing water they keep the salts in their body fluids and tissues at around 5% higher concentration than the surrounding seawater by maintaining high concentrations of waste products especially urea in the blood. They also have another substance TMAO (trimethylamine oxide) which counteracts the damaging effects of too much urea.
Ocean salinity changes with such factors as temperature, depth and proximity to shore so sharks must continuously balance their internal osmotic pressure.
Excess salts are removed from the bloodstream by the kidneys which extend the length of the body cavity on either side of the vertebrae. These salts are then excreted into the rectal gland located just inside the cloaca. Excess water is filtered by the kidneys and excreted out the cloaca as urine.
Shark species such as the bull shark (Carcharhinus leucas) which sometimes inhabit brackish/fresh-water have evolved the versatility to retain less TMAO and urea and to shrink their rectal gland so less salts are retained this prevents excess water entering the sharks body by osmosis.
Tissue sites responsible for immune cell production include the thymus, spleen, epigonal and leydig organs.
The thymus is a paired organ located dorsomedially (toward the back and near the midline) to both gill regions, it is organised in distinct small lobes and changes in size and location relative to growth and sexual maturation, making it difficult to identify.
The spleen is elongate and positioned along the outer margin of the cardiac and pyloric regions of the stomach, it is easily recognised by its rich dark red to purplish colour.
The epigonal along with the leydig organ are considered the bone marrow equivalents that are unique to elasmobranchs. The epigonal is located caudally from the posterior margin of the gonads (reproductive glands), its size and shape varies dramatically depending on the species.
Leydig organs can be seen as whitish masses beneath the epithelium on both dorsal and ventral sides of the oesophagus. Although the leydig is not always seen and it is speculated that its presence is linked with the size of the epigonal (the larger the epigonal the smaller/absence of the leydig organs).
The male reproductive system consists of the testes, the genital ducts, the urogenital papilla, the siphon sacs and the claspers.
The testes are paired, symmetrical structures situated at the anterior end of the coelom (fluid filled cavity) dorsal to the liver, suspended from the mid dorsal body wall. The testes secrete hormones and produce sperm which gets passed along a tube, the first part of which is called the vas efferentia and the second part is the vas deferens. Glands within this tube make a sticky, mucus-like substance that bind millions of tiny sperm into the bundles known as spermataphores, these are then passed into a storage bag called the seminal vesicle.
These are formed from the inner sides of the pelvic fins, in immature males they are small and flexible, upon reaching maturity the claspers calcify and harden. They contain muscle and erectile tissue and are used to channel sperm when inserted into the female, siphon sacs lie under the skin close to the cloaca and are believed to secrete lubricating fluid and seawater to reduce friction and aid copulation. A spur is attached to the clasper and is used to anchor into the female’s reproductive tract.
The female reproductive system consists of the ovaries and the oviducts, the ovaries can be paired or single structures located at the anterior end of the body cavity dorsal to the liver. In some species both ovaries are functional and in others just the right one is. The size and appearance of the ovaries change with age/sexual maturity. During breeding seasons the ovaries produce eggs which run the length of the body cavity on both sides of the vertebral column. A structure known as the “shell gland” present in the anterior part of the oviduct secretes an egg membrane/shell which encloses the egg as it passes. The posterior part of each oviduct is enlarged to form a uterus where the embryonic shark develops. The two uteri unite posteriorly to form the vagina which opens into the cloaca.
All sharks utilize internal fertilization, depending on how long females retain the fertilised eggs, sharks can be divided into 2 main groups 1) Oviparous (egg-laying) and 2) Viviparous (live-bearing). Viviparous species can be further divided into aplacental and placental forms.
Aplacental (or ovoviviparity), where the developing young do not form a placental connection to the mother include:
1. Yolk dependency, where the embryos depend solely on the yolk within the egg.
2. Oophagy where the embryo utilises its own egg yolk and then ingests other unfertilised eggs in the uterus. In the case of the sandtiger shark (Carcharias taurus) oophagy has gone one step further with intrauterine cannibalism, where the largest embryo kills/eats the other developing embryo.
Placental viviparity, where the embryos utilize their yolk for a few weeks then the yolk sac elongates and becomes highly vascularised and attaches to the mothers uterine wall, forming the yolk sac placenta. Nutrients are then passed to the embryo from the mother’s bloodstream.
Captive female sharks have been shown to be capable of “automatic parthenogenesis” during which an unfertilised egg merges with another cell from the mother (called the “sister polar body”) to form a foetus. DNA sequencing confirmed that all of the pup’s genes had come from the mother. This process is thought to occur when female sharks are faced with few/no male encounters in an attempt to rebuild a population. The downside to this is that the pup will have little genetic diversity which could ultimately result in a decline of the population.
The majority of shark species have a relatively streamlined shape designed to minimise both resistance and drag by a forward force (thrust). As a shark swims its body undulates from side to side creating “S-like” curves. These press on the surrounding water which itself pushes back with equal and opposite force. The sideways motions cancel each other out and the rearward force pushes the water back and the water forces the shark forward.
A shark’s main swimming muscles along the flanks are arranged in zigzag blocks called myomeres, each myomere is connected to the cartilaginous skeleton by a tendon called the myo-septa. The numbers of myomeres are equal to the number of vertebrae, but the zigzag shape extends the effect over several vertebrae giving more pull for less energy use. Each myomere attaches along the spinal column at 2 points, As they contract the 2 parts of the column are pulled towards each other, bending the column into a wave. Along the row of myomeres each contracts after the one in front of it and then relaxes as the one behind contracts. This sequence produces a wave that travels along the shark from head to tail. The amplitude of the wave gets larger as it passes along the shark’s body, which increases the push on the water. The final thrust is given by the tail which traces a spiral figure of eight through the water.
The caudal fin is two-lobed. The spinal column project into the upper lobe, in some species the spinal column has an upward “kink”, in others it is straight and the fin is thus bent down slightly. The shape/size of the fin lobes vary amongst species, with bottom dwelling sharks having a pronounced upper lobe and reduced lower lobe (heterocercal) with other pelagic sharks such as the white shark (Carcharodon carcharias) having almost equal upper and lower lobes (homocercal).
Dorsal fins cannot be folded or flattened and thus create turbulence, the sharks use these as pivot points to push themselves through the water. In some species the dorsal fins are aligned in such a way that the second dorsal uses the vortex of the first to propel against.
The pectoral and pelvic fins of most sharks are slightly tilted with the leading edge aimed upwards. This provides the shark with lift as it swims forward to counteract the sinking effects of the body and caudal fin shape. Sharks can adjust these fins slightly to control ascending and descending. Also in some bottom dwelling sharks these fins are used to “prop” their body up, or even to crawl as in the epaulette shark (Hemiscyllium ocellatum).
Note: The nostrils, eyes, ears etc. are discussed in the Shark Senses article.