Imagine finding a dead shark on a beach. Yet sharks are unique. They have no bony skeleton. They are cartilaginous fish along with skates, rays and chimeras. Chimaeras are sometimes called ghost fish, ratfish or rabbit fish. Generally they live very deep on the ocean floor at depths greater than feet. These fish have bulky heads and a single gill opening.
Some taxonomists classify chimaeras into their own order. The largest shark is the whale shark, while the smallest is the dwarf lantern shark. Sharks are primarily saltwater fishes, but some, like the bull shark, can be found in freshwater.
There are four living orders of rays that compose the suborder Batoidea. There are over species in thirteen families and four orders. While they are closely related to sharks, they are distinguishable by their flattened bodies, enlarged pectoral fins, and gill slits on the lower surface of their bodies.
As they do not have bone marrow, red blood cells are produced in the spleen and special tissue around the gonads. Another unique organ is the epigonal organ which probably has a role in the immune system. The subclass Holocephali, which is a very specialized group, lacks both of these organs. Originally the pectoral and pelvic girdles, which do not contain any dermal elements, did not connect.
In later forms, each pair of fins became ventrally connected in the middle when scapulocoracoid and pubioischiadic bars evolved.
In rays, the pectoral fins have connected to the head and are very flexible. In most species, all dermal denticles are oriented in one direction, making the skin feel very smooth if rubbed in one direction and very rough if rubbed in the other. It is assumed that their oral teeth evolved from dermal denticles which migrated into the mouth. But it could be the other way around as the teleost bony fish Denticeps clupeoides has most of its head covered by dermal teeth as do Atherion elymus , another bony fish.
This is most probably a secondary evolved characteristic which means there is not necessarily a connection between the teeth and the original dermal scales. The old placoderms did not have teeth at all but had sharp bony plates in their mouth. Thus, it is unknown which of the dermal or oral teeth evolved first. It has even been suggested that the original bony plates of all the vertebrates are gone and that the present scales are just modified teeth, even if both teeth and the body armor have a common origin from long ago.
But for the moment there is no evidence of this. One of the main characteristics present in sharks is their heterocercal tail, which aids the shark in locomotion. Class Chondrichthyes - Cartilaginous Fish Imagine finding a dead shark on a beach. Holocephali — Chimaeras Chimaeras are sometimes called ghost fish, ratfish or rabbit fish. Skates and Rays: Batoidea suborder There are four living orders of rays that compose the suborder Batoidea.
General Characteristics of Chondrichthyes Their digestive systems have spiral valves and, with the exception of Holocephali, a cloaca. A spiracle is found behind each eye on most species.
The Largest Fish is a Shark! The world record for the largest fish is a whale shark measuring 40 feet Blue Shark Prionace glauca.
Coral Catshark Atelomycterus marmoratus. Caribbean Reef Shark Carcharhinus perezi. Salmon Shark Lamna ditropis. Sharks swim by moving their heads and tails from side to side in the water. They use fins to stabilize themselves when they swim, to steer themselves in the water or to propel themselves through the water.
Sharks have fins on their back, sides, and stomachs. The fins on their back are called dorsal fins. Some species have one dorsal fin and others have two dorsal fins. In some species, the dorsal fin has a stiff spine, and in other species, there is no spine. There are even some species that have a poisonous spine on their dorsal fins. Dorsal fins help keep sharks from rolling over in the water!
The fins on the side of a shark are the pectoral fins. Pectoral fins are used for steering and to give the shark lift in the water. For example, the skates members of Rajidae and angel sharks Squatinidae are benthic species. The Lamnidae white sharks are pelagic , but in the upper depths only Fowler et al. For example, the mantarays Myliobatiformes make annual migrations, while the great white shark Carcharodon carcharias has been known to travel between South Africa and Australia Fowler et al.
The elasmobranchs occcupy the widest range of habitats - even the icy waters of the Arctic and Antarctic Ocean. But their physiology cannot withstand the most extreme conditions such as high salinity or the oxygen deficient depths that some teleosts have become specialised to endure.
Therefore most sharks do not inhabit very deep water, although some species have been sighted at depths up to m Helfman et al. The Holocephali are found only in temperate waters up to m, and prefer the cooler depths below 80m Fowler et al, ; Helfman et al. The majority of these are in the lowest risk category Vulnerable , but 42 species are Endangered , and 25 are placed within the highest risk category Critically Endangered. Seven of the remaining ten species are Data Deficient , and so may themselves also be under threat.
Chondrichthyes are an ancient and successful clade , having survived million years of changing environments. Yet human presence and intervention has put them at risk. The life history of Chondrichthyes marks them as vulnerable to extinction. As a K-selection species, they give birth to only a few young after a long gestation, which grow slowly and reach sexual maturity late Fowler et al.
Although their rate of survival is usually high, if populations become rapidly depleted, recovery can be difficult and lengthy. The primary threat to sharks, rays, and chimaeras is fishing Fowler et al. Commercial demand for shark meat and fins is high and fisheries are unmanaged, pushing population levels to dangerously low numbers.
Many species are also often killed as by-catch from bottom trawlers. Loss of habitat is also a significant threat, and those species which inhabit regions closest to human activity are at most risk from habitat degradation and pollution. Estuaries, whose shallow waters provide invaluable nurseries for chondrichthyans are being destroyed or polluted.
0コメント