Chondrichthyes: Sharks, Skates, Rays a
Sharks:
Like mammals, birds, reptiles, amphibians, and other fishes, sharks (and skates and rays) are vertebrates belonging to the phylum Chordata and the subphylum Vertebrata.
The Class Chondrichthyes: , is comprised of two subclasses, the Elasmobranchii (sharks, skates and rays) and the Holocephali (the chimaeras, such as the ratfish and elephant fish). Chimaeras are big-eyed, stomachless, deep-sea creatures that possess an upper jaw which is fused to its cranium (unlike in sharks). The subclass Elasmobranchii consists of only one extant subcohort, the Neoselachii (modern rays and sharks - we will ignore the fossil elasmobranchs for the purpose of this discussion). The Neoselachii consists of four extant superorders and one extinct superorder (Palaeospinacomorphii). The four living superorders are: Squalomorphii, Squatinomorphii, Rajomorphii (or Batoidea - the rays), and Galeomorphii. Within the Squalomorphii, there are four orders, the Hexanchiformes (six- and seven-gilled sharks and frilled sharks), the Echinorhiniformes (the bramble sharks, genus Echinorhinus), the Squaliformes (dogfish sharks), and the Pristiophoriformes (sawsharks). The Squatinomorphii contains one order, the Squatiniformes (angel sharks). The Rajomorphii contains five orders: the Pristiformes (sawfish), the Rhinobatiformes (guitarfishes), the Torpediniformes (electric rays), the Rajiformes (skates), and the Myliobatiformes (stingrays). Although skates and rays are not called sharks, phylogenetically speaking they are sharks--just ones that have evolved a flattened body. The Galeomorphii contains four orders and most of the species that one thinks of when the word "shark" comes up. These orders are: the Heterodontiformes (bullhead sharks), the Orectolobiformes (carpet sharks), the Lamniformes (mackerel sharks) and the Carcharhiniformes (ground or requiem sharks).
In total, there are about 869+ extant species of elasmobranchs, with about 400+ of those being sharks and the rest skates and rays, and a primitive form called a ratfish. These numbers increase every year, as new elasmobranch species are trawled from the deep-sea or fished off coral reefs. Mostly marine, a few species have secondarily invaded freshwater habitats. Of these animals, it is the sharks that have captured the greatest amount of public attention. Indeed, a number of aspects of their anatomy and physiology have contributed to their widespread success as some of the ocean�s most formidable predators. Moveable upper and lower jaws studded with sharp, recurved teeth can open wide to firmly grasp prey, which can be located with a variety of sophisticated sense organs including those that detect chemical odors, water currents, bioelectric fields emitted by their prey and even the earth�s magnetic fields. Interestingly, the whale shark (which is the largest fish in the sea with a length that can reach 10 meters) feeds on some of the smallest prey (plankton) that is filtered from the water column with its gills! Although many people who enter the water fear sharks, attacks are relatively rare and almost always represent a case of mistaken identity in which a swimmer is mistaken for more normal prey (marine mammals or large fish). Indeed, it is usually humans that eat (or just kill) sharks, and many species are now threatened or endangered!
Sharks Lineage:
This is an ancient group of fish. Sharks have been around in one form or another for about 400 million years. Their current form has been relatively unchanged for at least the last 70 million years, possibly the last 150 million years. There are hundreds of species in the Chondrichthyes class. With so many species you can imagine the variety of shapes and sizes currently present. From the small "spined pygmy shark" to the gigantic whale shark. From the standard shape of the gray reef shark or the blue shark to the odd shaped goblin shark or hammerhead shark, the wide diversity of these creatures is remarkable.
Early shark fossils appeared in the Devonian period (385 million years ago). They then evolved into a huge range of shapes and sizes in the Carboniferous. In some fossil deposits they make up 60% of the species of fish present. Many of these sharks are similar to those still found in the oceans today. The most notable feature of these ancient sharks is that the males already had claspers on their pelvic fins, showing that they had internal fertilisation and the females gave birth to relatively few offspring. Sharks were affected by the major period of extinction 240 million years ago at the end of the Carboniferous period, and only a few groups survived. Those which did continued to evolve into still more sophisticated animals, which survived the reign of the dinosaurs, who became extinct about 65 million years ago.
Most sharks have a very distinctive cigar shape, with a triangular dorsal (back) fin, large pectoral fins just behind the head and a lobed tail fin. However, some sharks have become adapted to living on the seabed and have developed a flattened shape which makes them look more similar to their close relatives, the skates and rays. They have 5 to 7 gill slits on the sides of their heads (a characteristic of all sharks, skates and rays) and their mouth usually opens downwards. Shark skin is covered by tiny, but very sharp tooth-like structures called dermal denticles (because of its roughness it used to be used for smoothing wood before sandpaper was invented and was called shagreen) and their mobile jaws contain many rows of replacement teeth. Sharks have no air bladders inside their bodies to give them buoyancy.The main differences between sharks and rays is that the latter have pectoral fins attached to the sides of their heads in front of their gill slits, forming fiat wings, with the gill slits located underneath the head.
Sharks are found all over the world's oceans, from the poles to the tropics, and at varying depths. Some sharks live in very deep water, like the recently discovered megamouth shark that lives on plankton. Certain sharks probably never more very far during their lives, but others, particularly pelagic oceanic sharks, can travel great distances. Blue sharks tagged off North America and Europe have made trans-Atlantic migrations of over 6,000 kin. Fast-swimming sharks such as the shortfin mako have been recorded as travelling over 2,000 km in 37 days. Other sharks may live in shallow water and be found in bays, ports and harbours (like the lemon shark), or on coral reefs. Some, like the harmless basking shark and whale shark, disappear completely at certain times of the year and we don't know for sure where they live at these times. There are no completely freshwater sharks, although some rays are found in lakes and rivers. The bull shark is unusual because it often enters tropical river systems and freshwater lakes from the sea.
Sharks never
actually stop growing, so it is difficult to estimate how large they can get without
knowing how long they can live (probably from 25 to 100 years). The largest shark in the
world is the plankton-eating whale shark, which lives in tropical waters. This shark is
thought to reach 18m in length. The basking shark, found in colder waters, is the next
largest at up to 15m. Compare this with the largest great white shark caught; 'only' 7m
long (although some of its extinct ancestors are thought to have been the same size as
whale sharks).The smallest sharks known are deep-water animals. An adult dwarf dogshark,
found in the Caribbean, is only 20cm long. Some shallow water sharks are also quite small;
spadenose and catsharks are 50 to 75cm long.
Mention sharks, and most people immediately think of their teeth and jaws. Shark jaws are mobile (not firmly attached to their skulls like ours) so that they can be pushed out to take a large bite. Sharks can have up to 3,000 teeth in their mouth arranged in rows of 6 to 20. Only the front teeth are used for biting. New teeth continually grow and move forwards from the back rows to replace those that wear down and fall out in the front. A shark can get through up to 20,000 teeth in a lifetime. All sharks are flesh eaters, feeding mainly on fishes or shellfish (crustaceans and molluscs). Some are also scavengers. As everyone knows, shark teeth can be very sharp and serrated, like those of the great white shark, so that they can be used for cutting bites out of their prey to swallow whole. Other sharks have long curved teeth for catching and eating fish, like the grey nurse, or very blunt strong teeth to crunch up shellfish. Plankton-eating sharks have such tiny teeth that they cannot be used for feeding at all. Instead they use 'gill rakers' to filter their food, minute planktonic animals, out of the water.
Sharks can detect
their prey in a number of ways. They have acute senses of hearing (using an inner ear and
lateral line system to detect vibrations) and of smell (their nostrils are only used for
smelling, not for breathing). Lemon sharks can detect a concentration of one part tuna in
25 million parts of seawater. Sharks can also taste their food. Some sharks have acute
eyesight and can almost certainly see in colour. The lemon shark can see in the dark ten
times better than humans. Perhaps most interesting is the unique bioelectrical sensory
system that sharks can use to detect and home in on the weak electric fields produced by
all live animals, even if they are buried in sand. This system consists of several
lateral-line canals and receptor cells under their skin pores called ampullae of
Lorenzini. The electric currents produced by metal objects in water can sometimes fool
sharks - perhaps this is why they sometimes attack diver cages or boats, especially the
motors. It has been suggested that feeding frenzies may occur when all these sophisticated
senses become over-laden with stimuli from blood, large numbers of prey or injured,
struggling animals. Sharks may then attack anything that moves, even other sharks.
Reproduction in the
sharks and rays is very different to that of bony fishes, who release millions of tiny
eggs when they spawn for fertilisation in the sea. Sharks and rays produce fewer larger
eggs and fertilisation is internal. About 30% of living sharks are 'oviparous'. They lay
eggs covered by protective leathery cases into the sea bed; a mature female may produce up
to 100 of these a year and these are the "Mermaid's purses" sometimes found
washed up on beaches. Each baby fish develops inside the safety of the egg case, with its
food provided by the yolk. When the yolk is used up the baby shark escapes through a slit
at one end of the case, swimming away to hunt for its own food. The enormous whale shark
reproduces in this way, and is only 35cm long when it emerges from it's egg case.
About 70% of all sharks are viviparous, and give birth to live young, like the mammals. In some species the eggs hatch inside the mother shark and the pups are then born alive into the sea, miniature replicas of their parents. A few other species of embryo shark not only use their own egg yolk for nourishment, but also unfertilised eggs produced by the mother to feed her young. About 30% of viviparous sharks are placental. In these species the yolk sac develops into a placenta attached to the wall of the uterus (as in mammals), which nourishes the pups until birth.
The number of shark pups born to a single mother varies considerably between species. Some have only two large pups, while others, like the blue shark, may have 40 to 80 much smaller young. Females may be pregnant for between three or four months (in some small sharks) to two years (spiny dogfish), or perhaps longer. Most large sharks have a gestation period of nine to twelve months, and reproduce every other year, while smaller sharks often give birth annually.
Some sharks are born in nursery areas, like mangrove swamps or shallow bays, where there is plenty of food and fewer predators than in the open sea. (The destruction of up to 50% of the world's mangrove habitats means that many baby sharks have lost their nursery grounds.) Once born, baby sharks do not receive any more protection from their mothers, but are left to fend for themselves. Sharks, which do not use nursery areas for their babies, tend to give birth either to the largest young, to help them to escape being eaten by other sharks, or to very large numbers of pups. Because they have many natural enemies (particularly other sharks), very few shark pups survive to adult size. Once fully grown, their only serious threat is humans.
Because of their low reproductive rate, or fecundity, it can take a long time for an adult shark population to produce sufficient young to replace themselves. Their solitary and elusive nature also makes it largely impossible to find out the size of populations of some sharks, or what their breeding rate is.
In addition to sharks, the Class Chondrichthyes also contains a number of skates and rays. In these animals, the pectoral fins are greatly enlarged to form "wings" that undulate gracefully during swimming, and the tail fin is reduced or absent. Skates and rays differ from sharks in having few scales and being generally adapted for feeding on bottom-dwelling animals such as molluscs and crustaceans. Instead of sharp, recurved teeth, skates and rays have flat, plate-like teeth that are used for crushing prey. Their bodies are flattened dorsoventrally, enabling them to glide slowly over the bottom in search of prey. The eyes and spiracles (openings for taking in water) of skates and rays are located on top of the head, allowing them to take in water for gill ventilation while being partially buried in sand. Another difference between sharks and skates and rays is the fact that the former are normally viviparous (live bearers) whereas skates and rays are oviparous, releasing their eggs in rectangular cases sometimes called "mermaid�s purses". Although a few electric rays are capable of delivering a powerful electric shock and some sting rays have large, barbed stingers, in general, these docile animals pose no real threat to swimmers and divers.
Skates and Rays
Skates are a family of
flat-bodied rays found in warm and temperate seas. They are in the family Rajidae
of the order Rajiformes. Rays have broad, flattened bodies, with eyes located on
the upper surface of the body while the mouth and gills are located on the lower surface.
The upper surface of the body is dark colored, while the lower surface is light colored.
This is called countershading and is a technique many marine animals use for
protection as well as for predation. Countershading makes the animal almost invisible
because when another animal looks down at them, they are camouflaged with the darkness of
the sea bottom. When looked up at from underneath, the animal is camouflaged with the
light from the sun. Great White Sharks use this technique for predation. Skates and rays
swim using rajiform swimming. Rays and skates are similar species that are closely related
to sharks. Collectively, they are known as batoids. Like sharks, their skeletons are made
of cartilage instead of bones.
Skates or rays?
Skates and rays can be difficult to differentiate. Both are flat and (with a few notable exceptions, such as the eagle and manta rays) bottom-dwelling elasmobranchs sharing a similar diamond or rhomboid shape. Some of the most familiar forms occur together in coastal habitats. Adding to the confusion, the term 'ray' is also used in reference to the electric and torpedo rays (order Torpediniformes); fortunately, torpedinoids are unlikely to be confused with either skates or rays (especially if one handles a live specimen!), being easily identified by the following characteristics: a rounded pectoral disc; soft, flabby body; dorsal fins nearer the pelvic fins than the tail tip; and kidney-shaped electrogenic organs at the base of the pectoral fins. Distinguishing skates from rays is rather subtle, but once one has learned their respective field marks, it is relatively easy to tell them apart.
Skates (order Rajiformes):
each pelvic fin divided into two lobes tail relatively stocky, without a stinging spine
tail usually with two small dorsal fins near its tip and a tiny caudal fin (some forms lack dorsal and caudal fins)
many have enlarged, thorn-like scales ('bucklers') along the midline of back and tail; enlarged scales also occur along the side of the body (near the pectoral fin base) in some species
Rays (order Myliobatiformes):
pelvic fin with one lobe
tail relatively slender to whip-like, usually with a saw-edged stinging spine midway along its length
tail usually without a dorsal fin (in some forms, a single dorsal fin occurs near the tail base), caudal fin reduced and continuous or absent. No bucklers along the midline of back or tail (although Hypolophus sephen, sometimes called the 'Pearl Ray', has three rounded scales in the midline of its back) or along the side of the bodymmmm
males lack malar or alar spines
Most rays are kite-shaped with streamlined tails that may have one or more poisonous barbs or spines. In contrast, skate tails are fleshier and heavier, and have small fins. In general, skates commonly found in North American waters have elongated noses.
Size is another way to differentiate rays from their skate cousins. Rays can grow to tremendously large sizes. For example, the manta ray (Manta birostris) can reach a width of 22 feet, weighing several tons. The five species of skates commonly found in North Carolina waters range in size from 16 inches to 5 feet in length.
Rays and skates use different mechanisms to defend themselves, too. The stinging spine on the ray’s tail is an effective weapon against predators. The edges of the spine are serrated so that once driven into a victim it is very difficult to remove without creating more damage. A thin skin covers the entire structure, and when ruptured, venom is released into the victim. Since skates do not have stinging spines, they rely on large thorns on their backs and tails to deter predators.
Skates and rays differ from each other in the way they give birth. All skates lay eggs, which are leathery cases very similar to shark egg capsules. These cases sometimes wash up on beaches and are called mermaid’s purses. Rays, on the other hand, give birth to live young.
There are seven main families of rays in the Order Myliobatiformes, including stingrays, butterfly rays, devil rays, eagle rays, river rays, round stingrays and six-gill rays. Some of the individual species common off the North Carolina coast are the smooth butterfly ray (Gymnura micura), the spotted eagle ray (Aetobatus narinari), the cownose ray (Rhinoptera bonasus) and the southern stingray (Dasyatis americana).
There is only one family of skates, the family Rajidae. Skates common to North Carolina include the clearnose skate (Raja eglanteria), the winter skate (Raja ocellata), the little skate (Raja erinacea), and the barndoor skate (Raja laevis).
Stingrays are surprising animals. Most have a flat body with wide pectoral fins that resemble wings. There are two smaller pelvic fins and a long, thin, whiplike tail that may or may not have a small dorsal fin or two on top. Rays have skeletons of cartilage rather than bone. (People have some cartilage in their bodies, too - in their outer ears and the tip of their noses.) Cartilage is more flexible and lighter in weight than bone.
The skin covering of rays is usually soft and smooth, ranging in color from light to dark grays and brown. Some, like the spotted eagle rays, have a pattern of spots. The underside is usually lighter, a pattern called countershading that is found in many sea creatures and in some birds also.
Most rays have a hard spine on top of the tail. The spines have serrated edges with sharp points that point back toward the base. Spines can be lost and replaced, and sometimes rays have multiple spines. The spine is used only for defense. A sting might occur, for example, if someone accidentally steps on hidden ray and the animal tries to protect itself. (Experienced swimmers who wade in shallow water shuffle their feet along sandy bottoms to alert rays of their approach.)
Most rays are bottom-dwellers. As a ray rests on the bottom, its mouth and gill slits face the bottom. To prevent taking in muddy water, two holes (spiracles) behind the eyes lead to a space lined with muscles that pump clean water over the gills.
pelvic fin with one lobe
tail relatively slender to whip-like, usually with a saw-edged stinging spine midway along its length
tail usually without a dorsal fin (in some forms, a single dorsal fin occurs near the tail base), caudal fin reduced and continuous or absent. No bucklers along the midline of back or tail (although Hypolophus sephen, sometimes called the 'Pearl Ray', has three rounded scales in the midline of its back) or along the side of the bodymmmm
males lack malar or alar spines
Most rays are kite-shaped with streamlined tails that may have one or more poisonous barbs or spines. In contrast, skate tails are fleshier and heavier, and have small fins. In general, skates commonly found in North American waters have elongated noses.
Size is another way to differentiate rays from their skate cousins. Rays can grow to tremendously large sizes. For example, the manta ray (Manta birostris) can reach a width of 22 feet, weighing several tons. The five species of skates commonly found in North Carolina waters range in size from 16 inches to 5 feet in length.
Rays and skates use different mechanisms to defend themselves, too. The stinging spine on the ray’s tail is an effective weapon against predators. The edges of the spine are serrated so that once driven into a victim it is very difficult to remove without creating more damage. A thin skin covers the entire structure, and when ruptured, venom is released into the victim. Since skates do not have stinging spines, they rely on large thorns on their backs and tails to deter predators.
Skates and rays differ from each other in the way they give birth. All skates lay eggs, which are leathery cases very similar to shark egg capsules. These cases sometimes wash up on beaches and are called mermaid’s purses. Rays, on the other hand, give birth to live young.
There are seven main families of rays in the Order Myliobatiformes, including stingrays, butterfly rays, devil rays, eagle rays, river rays, round stingrays and six-gill rays. Some of the individual species common off the North Carolina coast are the smooth butterfly ray (Gymnura micura), the spotted eagle ray (Aetobatus narinari), the cownose ray (Rhinoptera bonasus) and the southern stingray (Dasyatis americana).
There is only one family of skates, the family Rajidae. Skates common to North Carolina include the clearnose skate (Raja eglanteria), the winter skate (Raja ocellata), the little skate (Raja erinacea), and the barndoor skate (Raja laevis).
Stingrays are surprising animals. Most have a flat body with wide pectoral fins that resemble wings. There are two smaller pelvic fins and a long, thin, whiplike tail that may or may not have a small dorsal fin or two on top. Rays have skeletons of cartilage rather than bone. (People have some cartilage in their bodies, too - in their outer ears and the tip of their noses.) Cartilage is more flexible and lighter in weight than bone.
The skin covering of rays is usually soft and smooth, ranging in color from light to dark grays and brown. Some, like the spotted eagle rays, have a pattern of spots. The underside is usually lighter, a pattern called countershading that is found in many sea creatures and in some birds also.
Most rays have a hard spine on top of the tail. The spines have serrated edges with sharp points that point back toward the base. Spines can be lost and replaced, and sometimes rays have multiple spines. The spine is used only for defense. A sting might occur, for example, if someone accidentally steps on hidden ray and the animal tries to protect itself. (Experienced swimmers who wade in shallow water shuffle their feet along sandy bottoms to alert rays of their approach.)
Chondrichthyes: Sharks, Skates, Rays a
Sharks:
Like mammals, birds, reptiles, amphibians, and other fishes, sharks (and skates and rays) are vertebrates belonging to the phylum Chordata and the subphylum Vertebrata.
The Class Chondrichthyes: , is comprised of two subclasses, the Elasmobranchii (sharks, skates and rays) and the Holocephali (the chimaeras, such as the ratfish and elephant fish). Chimaeras are big-eyed, stomachless, deep-sea creatures that possess an upper jaw which is fused to its cranium (unlike in sharks). The subclass Elasmobranchii consists of only one extant subcohort, the Neoselachii (modern rays and sharks - we will ignore the fossil elasmobranchs for the purpose of this discussion). The Neoselachii consists of four extant superorders and one extinct superorder (Palaeospinacomorphii). The four living superorders are: Squalomorphii, Squatinomorphii, Rajomorphii (or Batoidea - the rays), and Galeomorphii. Within the Squalomorphii, there are four orders, the Hexanchiformes (six- and seven-gilled sharks and frilled sharks), the Echinorhiniformes (the bramble sharks, genus Echinorhinus), the Squaliformes (dogfish sharks), and the Pristiophoriformes (sawsharks). The Squatinomorphii contains one order, the Squatiniformes (angel sharks). The Rajomorphii contains five orders: the Pristiformes (sawfish), the Rhinobatiformes (guitarfishes), the Torpediniformes (electric rays), the Rajiformes (skates), and the Myliobatiformes (stingrays). Although skates and rays are not called sharks, phylogenetically speaking they are sharks--just ones that have evolved a flattened body. The Galeomorphii contains four orders and most of the species that one thinks of when the word "shark" comes up. These orders are: the Heterodontiformes (bullhead sharks), the Orectolobiformes (carpet sharks), the Lamniformes (mackerel sharks) and the Carcharhiniformes (ground or requiem sharks).
In total, there are about 869+ extant species of elasmobranchs, with about 400+ of those being sharks and the rest skates and rays, and a primitive form called a ratfish. These numbers increase every year, as new elasmobranch species are trawled from the deep-sea or fished off coral reefs. Mostly marine, a few species have secondarily invaded freshwater habitats. Of these animals, it is the sharks that have captured the greatest amount of public attention. Indeed, a number of aspects of their anatomy and physiology have contributed to their widespread success as some of the ocean�s most formidable predators. Moveable upper and lower jaws studded with sharp, recurved teeth can open wide to firmly grasp prey, which can be located with a variety of sophisticated sense organs including those that detect chemical odors, water currents, bioelectric fields emitted by their prey and even the earth�s magnetic fields. Interestingly, the whale shark (which is the largest fish in the sea with a length that can reach 10 meters) feeds on some of the smallest prey (plankton) that is filtered from the water column with its gills! Although many people who enter the water fear sharks, attacks are relatively rare and almost always represent a case of mistaken identity in which a swimmer is mistaken for more normal prey (marine mammals or large fish). Indeed, it is usually humans that eat (or just kill) sharks, and many species are now threatened or endangered!
Sharks Lineage:
This is an ancient group of fish. Sharks have been around in one form or another for about 400 million years. Their current form has been relatively unchanged for at least the last 70 million years, possibly the last 150 million years. There are hundreds of species in the Chondrichthyes class. With so many species you can imagine the variety of shapes and sizes currently present. From the small "spined pygmy shark" to the gigantic whale shark. From the standard shape of the gray reef shark or the blue shark to the odd shaped goblin shark or hammerhead shark, the wide diversity of these creatures is remarkable.
Early shark fossils appeared in the Devonian period (385 million years ago). They then evolved into a huge range of shapes and sizes in the Carboniferous. In some fossil deposits they make up 60% of the species of fish present. Many of these sharks are similar to those still found in the oceans today. The most notable feature of these ancient sharks is that the males already had claspers on their pelvic fins, showing that they had internal fertilisation and the females gave birth to relatively few offspring. Sharks were affected by the major period of extinction 240 million years ago at the end of the Carboniferous period, and only a few groups survived. Those which did continued to evolve into still more sophisticated animals, which survived the reign of the dinosaurs, who became extinct about 65 million years ago.
Most sharks have a very distinctive cigar shape, with a triangular dorsal (back) fin, large pectoral fins just behind the head and a lobed tail fin. However, some sharks have become adapted to living on the seabed and have developed a flattened shape which makes them look more similar to their close relatives, the skates and rays. They have 5 to 7 gill slits on the sides of their heads (a characteristic of all sharks, skates and rays) and their mouth usually opens downwards. Shark skin is covered by tiny, but very sharp tooth-like structures called dermal denticles (because of its roughness it used to be used for smoothing wood before sandpaper was invented and was called shagreen) and their mobile jaws contain many rows of replacement teeth. Sharks have no air bladders inside their bodies to give them buoyancy.The main differences between sharks and rays is that the latter have pectoral fins attached to the sides of their heads in front of their gill slits, forming fiat wings, with the gill slits located underneath the head.
Sharks are found all over the world's oceans, from the poles to the tropics, and at varying depths. Some sharks live in very deep water, like the recently discovered megamouth shark that lives on plankton. Certain sharks probably never more very far during their lives, but others, particularly pelagic oceanic sharks, can travel great distances. Blue sharks tagged off North America and Europe have made trans-Atlantic migrations of over 6,000 kin. Fast-swimming sharks such as the shortfin mako have been recorded as travelling over 2,000 km in 37 days. Other sharks may live in shallow water and be found in bays, ports and harbours (like the lemon shark), or on coral reefs. Some, like the harmless basking shark and whale shark, disappear completely at certain times of the year and we don't know for sure where they live at these times. There are no completely freshwater sharks, although some rays are found in lakes and rivers. The bull shark is unusual because it often enters tropical river systems and freshwater lakes from the sea.
Sharks never
actually stop growing, so it is difficult to estimate how large they can get without
knowing how long they can live (probably from 25 to 100 years). The largest shark in the
world is the plankton-eating whale shark, which lives in tropical waters. This shark is
thought to reach 18m in length. The basking shark, found in colder waters, is the next
largest at up to 15m. Compare this with the largest great white shark caught; 'only' 7m
long (although some of its extinct ancestors are thought to have been the same size as
whale sharks).The smallest sharks known are deep-water animals. An adult dwarf dogshark,
found in the Caribbean, is only 20cm long. Some shallow water sharks are also quite small;
spadenose and catsharks are 50 to 75cm long.
Mention sharks, and most people immediately think of their teeth and jaws. Shark jaws are mobile (not firmly attached to their skulls like ours) so that they can be pushed out to take a large bite. Sharks can have up to 3,000 teeth in their mouth arranged in rows of 6 to 20. Only the front teeth are used for biting. New teeth continually grow and move forwards from the back rows to replace those that wear down and fall out in the front. A shark can get through up to 20,000 teeth in a lifetime. All sharks are flesh eaters, feeding mainly on fishes or shellfish (crustaceans and molluscs). Some are also scavengers. As everyone knows, shark teeth can be very sharp and serrated, like those of the great white shark, so that they can be used for cutting bites out of their prey to swallow whole. Other sharks have long curved teeth for catching and eating fish, like the grey nurse, or very blunt strong teeth to crunch up shellfish. Plankton-eating sharks have such tiny teeth that they cannot be used for feeding at all. Instead they use 'gill rakers' to filter their food, minute planktonic animals, out of the water.
Sharks can detect
their prey in a number of ways. They have acute senses of hearing (using an inner ear and
lateral line system to detect vibrations) and of smell (their nostrils are only used for
smelling, not for breathing). Lemon sharks can detect a concentration of one part tuna in
25 million parts of seawater. Sharks can also taste their food. Some sharks have acute
eyesight and can almost certainly see in colour. The lemon shark can see in the dark ten
times better than humans. Perhaps most interesting is the unique bioelectrical sensory
system that sharks can use to detect and home in on the weak electric fields produced by
all live animals, even if they are buried in sand. This system consists of several
lateral-line canals and receptor cells under their skin pores called ampullae of
Lorenzini. The electric currents produced by metal objects in water can sometimes fool
sharks - perhaps this is why they sometimes attack diver cages or boats, especially the
motors. It has been suggested that feeding frenzies may occur when all these sophisticated
senses become over-laden with stimuli from blood, large numbers of prey or injured,
struggling animals. Sharks may then attack anything that moves, even other sharks.
Reproduction in the
sharks and rays is very different to that of bony fishes, who release millions of tiny
eggs when they spawn for fertilisation in the sea. Sharks and rays produce fewer larger
eggs and fertilisation is internal. About 30% of living sharks are 'oviparous'. They lay
eggs covered by protective leathery cases into the sea bed; a mature female may produce up
to 100 of these a year and these are the "Mermaid's purses" sometimes found
washed up on beaches. Each baby fish develops inside the safety of the egg case, with its
food provided by the yolk. When the yolk is used up the baby shark escapes through a slit
at one end of the case, swimming away to hunt for its own food. The enormous whale shark
reproduces in this way, and is only 35cm long when it emerges from it's egg case.
About 70% of all sharks are viviparous, and give birth to live young, like the mammals. In some species the eggs hatch inside the mother shark and the pups are then born alive into the sea, miniature replicas of their parents. A few other species of embryo shark not only use their own egg yolk for nourishment, but also unfertilised eggs produced by the mother to feed her young. About 30% of viviparous sharks are placental. In these species the yolk sac develops into a placenta attached to the wall of the uterus (as in mammals), which nourishes the pups until birth.
The number of shark pups born to a single mother varies considerably between species. Some have only two large pups, while others, like the blue shark, may have 40 to 80 much smaller young. Females may be pregnant for between three or four months (in some small sharks) to two years (spiny dogfish), or perhaps longer. Most large sharks have a gestation period of nine to twelve months, and reproduce every other year, while smaller sharks often give birth annually.
Some sharks are born in nursery areas, like mangrove swamps or shallow bays, where there is plenty of food and fewer predators than in the open sea. (The destruction of up to 50% of the world's mangrove habitats means that many baby sharks have lost their nursery grounds.) Once born, baby sharks do not receive any more protection from their mothers, but are left to fend for themselves. Sharks, which do not use nursery areas for their babies, tend to give birth either to the largest young, to help them to escape being eaten by other sharks, or to very large numbers of pups. Because they have many natural enemies (particularly other sharks), very few shark pups survive to adult size. Once fully grown, their only serious threat is humans.
Because of their low reproductive rate, or fecundity, it can take a long time for an adult shark population to produce sufficient young to replace themselves. Their solitary and elusive nature also makes it largely impossible to find out the size of populations of some sharks, or what their breeding rate is.
In addition to sharks, the Class Chondrichthyes also contains a number of skates and rays. In these animals, the pectoral fins are greatly enlarged to form "wings" that undulate gracefully during swimming, and the tail fin is reduced or absent. Skates and rays differ from sharks in having few scales and being generally adapted for feeding on bottom-dwelling animals such as molluscs and crustaceans. Instead of sharp, recurved teeth, skates and rays have flat, plate-like teeth that are used for crushing prey. Their bodies are flattened dorsoventrally, enabling them to glide slowly over the bottom in search of prey. The eyes and spiracles (openings for taking in water) of skates and rays are located on top of the head, allowing them to take in water for gill ventilation while being partially buried in sand. Another difference between sharks and skates and rays is the fact that the former are normally viviparous (live bearers) whereas skates and rays are oviparous, releasing their eggs in rectangular cases sometimes called "mermaid�s purses". Although a few electric rays are capable of delivering a powerful electric shock and some sting rays have large, barbed stingers, in general, these docile animals pose no real threat to swimmers and divers.
Skates and Rays
Skates are a family of
flat-bodied rays found in warm and temperate seas. They are in the family Rajidae
of the order Rajiformes. Rays have broad, flattened bodies, with eyes located on
the upper surface of the body while the mouth and gills are located on the lower surface.
The upper surface of the body is dark colored, while the lower surface is light colored.
This is called countershading and is a technique many marine animals use for
protection as well as for predation. Countershading makes the animal almost invisible
because when another animal looks down at them, they are camouflaged with the darkness of
the sea bottom. When looked up at from underneath, the animal is camouflaged with the
light from the sun. Great White Sharks use this technique for predation. Skates and rays
swim using rajiform swimming. Rays and skates are similar species that are closely related
to sharks. Collectively, they are known as batoids. Like sharks, their skeletons are made
of cartilage instead of bones.
Skates or rays?
Skates and rays can be difficult to differentiate. Both are flat and (with a few notable exceptions, such as the eagle and manta rays) bottom-dwelling elasmobranchs sharing a similar diamond or rhomboid shape. Some of the most familiar forms occur together in coastal habitats. Adding to the confusion, the term 'ray' is also used in reference to the electric and torpedo rays (order Torpediniformes); fortunately, torpedinoids are unlikely to be confused with either skates or rays (especially if one handles a live specimen!), being easily identified by the following characteristics: a rounded pectoral disc; soft, flabby body; dorsal fins nearer the pelvic fins than the tail tip; and kidney-shaped electrogenic organs at the base of the pectoral fins. Distinguishing skates from rays is rather subtle, but once one has learned their respective field marks, it is relatively easy to tell them apart.
Skates (order Rajiformes):
each pelvic fin divided into two lobes tail relatively stocky, without a stinging spine
tail usually with two small dorsal fins near its tip and a tiny caudal fin (some forms lack dorsal and caudal fins)
many have enlarged, thorn-like scales ('bucklers') along the midline of back and tail; enlarged scales also occur along the side of the body (near the pectoral fin base) in some species
Rays (order Myliobatiformes):
pelvic fin with one lobe
tail relatively slender to whip-like, usually with a saw-edged stinging spine midway along its length
tail usually without a dorsal fin (in some forms, a single dorsal fin occurs near the tail base), caudal fin reduced and continuous or absent. No bucklers along the midline of back or tail (although Hypolophus sephen, sometimes called the 'Pearl Ray', has three rounded scales in the midline of its back) or along the side of the bodymmmm
males lack malar or alar spines
Most rays are kite-shaped with streamlined tails that may have one or more poisonous barbs or spines. In contrast, skate tails are fleshier and heavier, and have small fins. In general, skates commonly found in North American waters have elongated noses.
Size is another way to differentiate rays from their skate cousins. Rays can grow to tremendously large sizes. For example, the manta ray (Manta birostris) can reach a width of 22 feet, weighing several tons. The five species of skates commonly found in North Carolina waters range in size from 16 inches to 5 feet in length.
Rays and skates use different mechanisms to defend themselves, too. The stinging spine on the ray’s tail is an effective weapon against predators. The edges of the spine are serrated so that once driven into a victim it is very difficult to remove without creating more damage. A thin skin covers the entire structure, and when ruptured, venom is released into the victim. Since skates do not have stinging spines, they rely on large thorns on their backs and tails to deter predators.
Skates and rays differ from each other in the way they give birth. All skates lay eggs, which are leathery cases very similar to shark egg capsules. These cases sometimes wash up on beaches and are called mermaid’s purses. Rays, on the other hand, give birth to live young.
There are seven main families of rays in the Order Myliobatiformes, including stingrays, butterfly rays, devil rays, eagle rays, river rays, round stingrays and six-gill rays. Some of the individual species common off the North Carolina coast are the smooth butterfly ray (Gymnura micura), the spotted eagle ray (Aetobatus narinari), the cownose ray (Rhinoptera bonasus) and the southern stingray (Dasyatis americana).
There is only one family of skates, the family Rajidae. Skates common to North Carolina include the clearnose skate (Raja eglanteria), the winter skate (Raja ocellata), the little skate (Raja erinacea), and the barndoor skate (Raja laevis).
Stingrays are surprising animals. Most have a flat body with wide pectoral fins that resemble wings. There are two smaller pelvic fins and a long, thin, whiplike tail that may or may not have a small dorsal fin or two on top. Rays have skeletons of cartilage rather than bone. (People have some cartilage in their bodies, too - in their outer ears and the tip of their noses.) Cartilage is more flexible and lighter in weight than bone.
The skin covering of rays is usually soft and smooth, ranging in color from light to dark grays and brown. Some, like the spotted eagle rays, have a pattern of spots. The underside is usually lighter, a pattern called countershading that is found in many sea creatures and in some birds also.
Most rays have a hard spine on top of the tail. The spines have serrated edges with sharp points that point back toward the base. Spines can be lost and replaced, and sometimes rays have multiple spines. The spine is used only for defense. A sting might occur, for example, if someone accidentally steps on hidden ray and the animal tries to protect itself. (Experienced swimmers who wade in shallow water shuffle their feet along sandy bottoms to alert rays of their approach.)
Most rays are bottom-dwellers. As a ray rests on the bottom, its mouth and gill slits face the bottom. To prevent taking in muddy water, two holes (spiracles) behind the eyes lead to a space lined with muscles that pump clean water over the gills.
pelvic fin with one lobe
tail relatively slender to whip-like, usually with a saw-edged stinging spine midway along its length
tail usually without a dorsal fin (in some forms, a single dorsal fin occurs near the tail base), caudal fin reduced and continuous or absent. No bucklers along the midline of back or tail (although Hypolophus sephen, sometimes called the 'Pearl Ray', has three rounded scales in the midline of its back) or along the side of the bodymmmm
males lack malar or alar spines
Most rays are kite-shaped with streamlined tails that may have one or more poisonous barbs or spines. In contrast, skate tails are fleshier and heavier, and have small fins. In general, skates commonly found in North American waters have elongated noses.
Size is another way to differentiate rays from their skate cousins. Rays can grow to tremendously large sizes. For example, the manta ray (Manta birostris) can reach a width of 22 feet, weighing several tons. The five species of skates commonly found in North Carolina waters range in size from 16 inches to 5 feet in length.
Rays and skates use different mechanisms to defend themselves, too. The stinging spine on the ray’s tail is an effective weapon against predators. The edges of the spine are serrated so that once driven into a victim it is very difficult to remove without creating more damage. A thin skin covers the entire structure, and when ruptured, venom is released into the victim. Since skates do not have stinging spines, they rely on large thorns on their backs and tails to deter predators.
Skates and rays differ from each other in the way they give birth. All skates lay eggs, which are leathery cases very similar to shark egg capsules. These cases sometimes wash up on beaches and are called mermaid’s purses. Rays, on the other hand, give birth to live young.
There are seven main families of rays in the Order Myliobatiformes, including stingrays, butterfly rays, devil rays, eagle rays, river rays, round stingrays and six-gill rays. Some of the individual species common off the North Carolina coast are the smooth butterfly ray (Gymnura micura), the spotted eagle ray (Aetobatus narinari), the cownose ray (Rhinoptera bonasus) and the southern stingray (Dasyatis americana).
There is only one family of skates, the family Rajidae. Skates common to North Carolina include the clearnose skate (Raja eglanteria), the winter skate (Raja ocellata), the little skate (Raja erinacea), and the barndoor skate (Raja laevis).
Stingrays are surprising animals. Most have a flat body with wide pectoral fins that resemble wings. There are two smaller pelvic fins and a long, thin, whiplike tail that may or may not have a small dorsal fin or two on top. Rays have skeletons of cartilage rather than bone. (People have some cartilage in their bodies, too - in their outer ears and the tip of their noses.) Cartilage is more flexible and lighter in weight than bone.
The skin covering of rays is usually soft and smooth, ranging in color from light to dark grays and brown. Some, like the spotted eagle rays, have a pattern of spots. The underside is usually lighter, a pattern called countershading that is found in many sea creatures and in some birds also.
Most rays have a hard spine on top of the tail. The spines have serrated edges with sharp points that point back toward the base. Spines can be lost and replaced, and sometimes rays have multiple spines. The spine is used only for defense. A sting might occur, for example, if someone accidentally steps on hidden ray and the animal tries to protect itself. (Experienced swimmers who wade in shallow water shuffle their feet along sandy bottoms to alert rays of their approach.)
Most rays are bottom-dwellers. As a ray rests on the bottom, its mouth and gill slits face the bottom. To prevent taking in muddy water, two holes (spiracles) behind the eyes lead to a space lined with muscles that pump clean water over the gills.
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