what is the main chactaristic of a shark?

Question

what are the main charactaristics of a shark

Answer 1

The main features distinguishing the sharks are the gills. The gills of other fish are covered by a gill plate and only have 1 visible slit, however sharks have not gill plate and have between 5 and 7 visible slits, nearly every species has 5 however. Gills also distinguish sharks from rays, the difference between the two is the ventral (or under side) positioning of the gills of the ray in contrast to the location of shark gills on the side of the their head.
Though not unique to sharks, sharks have a cartilaginous skeleton. This means it is made up of cartilage instead of bone. This makes it much more flexible and much lighter than the skeleton of a teleost (bony) fish. Other fish like sturgeon also have cartilaginous skeletons however.
Sharks scales are known as placoid scales or dermal denticles and are homologous to teeth. This makes the shark much more hydrodynamic than other fish are the denticles produce much less drag than a smooth surface. They also are extremely tough and give the shark good defense against many bites. They are also responsible for the rough feeling gotten when you stroke a shark from tail to head.
Not unique to sharks but most commonly associated with them is the ability to sense electrical fields and they even sense the one produced by your heart beating, this is done using an organ called the ampulli of lorenzini and is found spread across the face of the shark in tiny pores. Sharks also have a excellent sense of smell and can smell tiny amounts of blood or urine in the water. They also have excellent eyesight and like all fish have a lateral line for detecting pressure change and movement in the water around them.
Sharks have a wide array of mating methods. All sharks undergo internal fertilisation and sperm is delivered to the female from the male using modified anal fins called claspers which are unique to sharks. Birthing in sharks occurs in one of three ways. Oviparity - egg laying. Aplacental viviparity - the embryo has yolk but is inside the female. Placental viviparity - the embryo is attached by a placenta to the mother and nutrients are passed along the placenta.
To maintain osmo and ionoregulation sharks use large quantities of urea in their blood stream along with a substance called TMAO. Use of TMAO makes sharks obligate (i.e they have no choice) predators as TMAO is only produce by other fish and sharks have to consume other fish to intake it. Sharks have no swim bladder, but this is not something unique to sharks as many other fish such as tuna or mackerel or many deep sea fish do not have a swim bladder either as for a pelagic (open water) animal is of no use at all. Use gain lift and control their position in the water column by using active lift. This is pretty much the same way an aircraft works. By using its pelvic fins (the big ones at the front) to produce lift like the wings of a plane do for it. Sharks do reproduce their teeth on a constant basis and do eat just anything, most species are very selective about they eat and they have a wide array of techniques and tactics to obtain their preferred food (watch white sharks hunt different types of seal and the difference in approach is noticeable).
No species of shark is fresh water only all can enter the marine environment, a few species can move between both the marine and the fresh water such as the bull shark which is even found land lock in Lake Nicaragua in central America, however it is not a specifically fresh water species. Most shark species despite a lot of people saying they are prehistoric are in fact not prehistoric at all, of the sharks in the sea now, only maybe 5 or 6 species where around with the dinosaurs and some groups of shark have been on the earth for around the same amount of time as humans have.
Sharks are not stupid either, they have some highly complex social behaviour and are fast adaptable learners.
Sharks do also sleep, many can be found on reefs in "grottos". Some species can only rest half their brain at a time as then need to swim constantly to breathe, this is similar to the method used by dolphin.

Answer 2

Main charateristics of a shark...
Cartilage forms the skeleton.
Most have to keep moving to force water through gills or they "drown" .
They cannot swim backwards.
Usually have sand paper rough skin.
Have highly sensitive sences.. Ie smell and can sence vibrations in the water.
Taste very good when cooked as stakes on the grill

Answer 3

No calcified bones - only cartilage.
Flotation via a large oily liver not an air bladder.
Micropatterned skin reducing hydrodynamic drag.
Progressively replaced teeth throughout its life.
Must continuously swim to run water over its gills - a stationary shark drowns.

Answer 4

They have no swim balder, they will eat just about any thing, the skin has a coarse sand paper texture, and their teeth grow back when damaged. most live in salt water, but some types will enter fresh water for weeks at a time, and their is a fully fresh water shark in Central America.

Answer 5

The arteries in your wrist are pretty small, relatively. You would really need some help, but there is a chance that without it, you would stop bleeding and recover. Some people who try to cut their wrists do cut the arteries, but they coagulate before they can bleed out. If they get an artery in the upper arm or thigh, chances are small that anyone would live long because those arteries are huge.

Answer 6

Sharp teeth that fall out but grow back in
Rows of teeth
Fin
Back fin
Face of shark

Answer 7

I am not sure exactly what you are asking.

Are you asking what are the main features to tell them apart? If so I would say teeth/jaws.

Answer 8

They are well known for their cartilage skeletons, many rows of ever-growing and replacing teeth, and never sleeping.

Answer 9

A predator

Answer 10

there are two: mainly the skeleton is cartilage, and they have three rows of razor sharp teeth.