Chordata-Fishes by titoooooooooooooooo
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The diagnostic characteristics that define fish

Fish are multicellular organisms belonging to Kingdom Animalia. Fish are defined as animals with a skull, permanent gills, and limbs -most often displayed in fins- or non tetrapod craniates. (EK21) They consume other organisms for survival and are aquatic. As chordates, they have a spinal cord and a tail. The diagnostic characteristic of fish is their use of gills to breathe under water. In addition, they are cold blooded and control body temperature through external means. They also have fins for movement. Fish are the oldest vertebrates in the world, dating all the way back 500 million years. (EK)
Species of fish can be divided into three classes: Jawless fish (Agnathatha) such as lampreys, cartilaginous-skeleton fish (Chordrichthyes) such as sharks, and bony-skeleton fish (Osteichthyes), which comprise all of the remaining living fish species (such as trout or salmon) (KA).

Jawless fish:
  • backbone consists of a notochord (KA).

Cartilaginous fish:
  • the backbone consists of a notochord surrounded by rings of cartilage (KA).
  • have spiky scales that project through the skin; the skin is very rough like sandpaper (CH 4)
  • lack swim bladders, so if they stop swimming, they will sink (CH 4)

Bony fish:
  • backbone consists of united, bony vertebrae. The notochord is absent (2 KA).
  • have flattened discoid shaped scales and are covered with a skin and mucus; makes them very slippery (CH 4)
  • have swim bladders, an air-filled sac below the vertebral column that keeps the fish buoyant when it stops swimming (CH 4)

Acquiring and digesting food

Since fish have jaws, they can eat food that is not suspended. Their fins and tail allow them to move around in the search for food. When they bite things, they chew it, therefore increasing its surface area. The food then goes to the stomach where it is broken down further into its basic chemical components. Finally, the now small food particles are absorbed from the digestive tract into the bloodstream. The waste then exit from the anus. This is the same basic variation of the digestive design common to all vertebrates.
Lampreys, lacking jaws, act as predatory parasites. A lamprey will attach itself to and bore a hole through the flank of a fish with its circular mouth. It will live on the blood and tissues of its host. (4RM)

Digestive System (CS 9)

Sensing the environment

Most fishes have noses, which function only for smelling and not breathing. Their noses are open cavities with dead ends, like a cup. Interestingly, the entire shark body transmits the sound to the inner ear. Almost all fish have eyes to see as well. Some fishes, such as sharks, can detect the electrical fields caused by the muscle movements of other animals. Sharks along with other fishes also have a lateral line system, a microscopic organs sensitive to changes in water pressure. Lateral lines can also detect the movement of other fishes nearby. Other fishes have the sense of hearing as well. All these stimuli are perceived in the fish's brain, where it can understand the environment around it. Fish can also sense how deep they are, and the salt content of the water that they swim in. (JE)


Especially in the water, it is evolutionary beneficial to be able to move quickly. The tail has powerful muscles which propel the fish forward. In addition, the fish has fins which mainly function to control their movements. Continual swimming ensures that water will flow into the mouth and into the gills where gas is exchanged. Fish also secrete a slimy mucus to its outside. This reduces friction, lets the fish swim faster, and saves energy. Around 64 species of fish can "fly", gliding on top of the water, barely skimming the surface. (8 AN) These fish have pectoral fins which are much larger than fins of non-flying fish. Flying fish can geerally be found in tropical and subtorpical waters. (8 AN)
Swimming is one of the most efficient forms of locomotion known. Water is more dense than air, and therefore it resists the movement of any body through it much more strongly than air. However, water is also not compressible, which means it is easier to push against it to propel the body in a direction. Also, the density of water is close to the density of a body, so the fish do not feel the effects of gravity that land dwelling animals experience. Therefore, the muscles of fish do not get tired from a gravitational pull, making swimming so efficient. (AW 7).Since body tissue is denser than water, fish must compensate for the difference or they will sink. Many bony fishes have an internal organ called a swim bladder that adjusts their buoyancy through manipulation of gases.(19DO)


The structure and function of gills, showing the gill arches, gill filaments, lamellae, and capillaries. (SW 13)

Since the oxygen content in water is very low, fish have to be very efficient at extracting it.

Fish breathe through their gills. Gills are outfoldings of the body surface that are suspended in the water. To do this, they have numerous specially adaptive mechanisms. As water passes through the gills, it passes through several gill arches consisting of many gill filaments (see diagram above). Each gill filament consists of many flattened out plates called lamellae. The lamella have capillaries, which are basically really small blood vessels that are so thin that they can absorb oxygen. The blood and water flow concurrently. That is, in the lamella, the oxygen poor blood flows in an opposite direction to the oxygen rich water. As the water passes the capillaries in the lamella, the oxygen in the water goes to the blood. The concurrent arrangement maximizes respiration efficiency.

To keep water flowing into the gills, the fish have specially adaptations. For example, the sharks have to continually swim with their mouth open to keep water coming through their gills. Some fish are able to extract water in a stationary position by pumping water in with powerful muscles surrounding the gills.

Metabolic waste removal

When an animal breaks down compounds for digestion, nitrogenous waste in the form of ammonia results. Ammonia is a very toxic compound. Fortunately for the fish, it is also easily dissolvable in water. Fishes dilute the ammonia into water until it reaches non toxic levels. Afterwards, they secrete directly outside their bodies since the concentration gradient allows them to do so. They commonly secrete it directly through the gills.Kidneys also help remove waste from the fish and control the amount of ammonia in the body. (7-SC)

Like most vertebrates, solid waste is released through the anus in fish.


The circulation of oxygenated blood is essential for the survival of fish. Blood carries oxygen, nutrients, and waste. To provide the entire fish's body with blood, the heart pumps the blood throughout the body. The fish's heart has two chambers, the atrium and ventricle. The blood pumped from the ventricle first goes to the gills where it picks up oxygen and disposed of carbon dioxide through the thin capillary walls. The oxygenated blood gathers in one central artery and then goes to the rest of the body and eventually goes back to the atrium. From the atrium, it goes to the ventricle and the cycle continues. With this system, blood flows relatively slowly. The muscle contractions of the fish's swimming aids in the movement of blood throughout the body.

The ciculatory system of fish differ frrom those of  other chordates in that the heart has only one atrium and one ventricle. Oxygenated blood from the respiratory (gill) capillaries flows directly into the systemic capillaries without moving through the heart first (6 J. Stein).
The ciculatory system of fish differ frrom those of other chordates in that the heart has only one atrium and one ventricle. Oxygenated blood from the respiratory (gill) capillaries flows directly into the systemic capillaries without moving through the heart first (6 J. Stein).

Above is a video of a fish's blood circulatory system. (J. Sun 19)

Self protection

In the aquatic environment, speed is very important. Often the best defense is to swim away. Or swim with friends: schools of fish are not made just because certain fish may want company, but also because schools of fish give the illusion of a larger fish that predators might be less likely to attack. (15 AL) Other fish, such as sharks, are at the top of the food chain. Some fish, such as the electric eel, send powerful electrical shocks for self defense. Many fish have coloring called "countershading", where fish have lightly colored ventral sides and darkly colored dorsal sides. (15 AL) This coloring is used as a type of camoflage to hide from predators: when seen from above they blend in with dark ocean depths and when seen from below they blend in with the light filled waters above. (15 AL) Other brightly colored fish that live in coral reefs use their coloring to seem like peices of coral. (15 AL)

Osmotic balance

Osmotic balance in fish DPOD 14

For marine fishes, the ocean is a dehydrating environment since it is very salty and water tends to leave the fish's body. Water is constantly lost, especially through the gills. To make a balance, the fish constantly drinks water, and releases the excessive salt intake through active transport, meaning that they use energy to do so.

The shark exemplifies another strategy. The salt concentration outside the shark's body is much higher than the concentration inside it's body. To make up for the salt coming in, the salt is removed through the kidneys, an organ called the rectal gland, and feces. Also, sharks maintain high concentrations of urea and trimethylamine as well. These are two organic solutes that help stop the loss of water to the environment.

In freshwater, fishes have a higher salt concentration than the water they live in. (HL 17) Fishes are constantly gaining water through osmosis and losing salt since the concentration of salt is higher in the fish's body. They make up for this by drinking water and eating ions through their diet. In addition, they actively uptake salt through their gills. They also excrete large amounts of water that is very dilute with salt.

Temperature balance

Fish are generally ectothermic, so their body temperature is usually within a few degrees of the temperature of the surrounding water (EG 9). There are endothermic fishes as well, including tuna and mackeral sharks (EG 9). For them to avoid losing the temperature created by their muscle movement, they have countercurrent circulatory systems that keep the heat within the fish's body (EG 9). Being "warm-blooded" is advantageous because muscles function better when warm; less force is lost to muscle movement. This gives the fish's muscles more power, enabling it to swim faster, etc. (AR 14)
In terms of temperature, fishes are conformers and retain almost no heat from their muscle movements. This is because as water passes through the gills, a lot of heat is lost. In addition, the fish always in contact with water, which is a great absorber of heat. In some species, such as the shark, arteries direct blood deep into the body, where the muscles can be significantly warmer than the water since it is insulated. In some species, special organs heat the eyes and brain so they can function better.
Fun Facts
1) The largest known freshwater fish is Arapaima gigas (Catfish) in the Amazon River. It grows up to 15 feet and can weigh approximately 440lbs (5 MB).

Review Questions

1) What are the two chambers of the fish heart called? (8DC)
2) How do fishes remove metabolic wastes from their bodies? (11 VK)
3) Why would lungs not be able to sustain fish respiration (MB)?
4) What vital anatomical part came with the evolution of fishes? When did it occur, or in what subcategory of fish did it occur? (AS 19)

1. "Interesting Fish Facts." 24 Oct. 2009 <>.
2. "Fish Anatomy." Web. 25 Oct. 2009. < anatomy.html>.
3. "Information on Fish Body Temperature." LookD. Web. 25 Oct. 2009. <>.
4. Mackean, DG. "Biology - Characteristics of Fish - An Introduction." 8 Mar. 2007. 31 Oct. 2009.<>
5. "Fun Animal Facts." 31 Oct. 2009
7. Ramel, Gordon. "Locomotion in Fish." The Earth-Life Web. 2 Nov 2009 <>.
9. <>.
10. "Senses of Fish." Oracle ThinkQuest Library. Web. 04 Nov. 2009. <>.
12. Campbell, Neil A., and Jane B. Reece. __Biology__. Sixth Edition. San Fransisco: Pearson Education, Inc, 2002.