Chordata - Fishes

by Alysse S.

A clown fish.

(photo 1)
Table of Contents:

  • Diagnostic characteristics

  • Acqusition and digeston of food
  • Sensing surroundings
  • Locomotion

  • Respiration
  • Metablolic waste removal
  • Circulation
  • Self protection
  • Osmotic balance
  • Temperature balance

What are the diagnostic characteristics of fish?
Fish belong to the kingdom Animalia. All Animalia are eukaryotic organisms (organisms that have nuclei but do not have cell walls). Further classified, fish are chordates (hence,he term chordata). This means that they possess a spinal

cord. This cord aids in carrying action potentials throughout the organism’s body to facilitate movement and response to stimuli. Fish breathe through gills, and have a counter-current gas exchange system of respiration. Fish live in fresh water, salt water, estuaries (where fresh and salt water meet), and even on land (fish like the mudskipper can survive on land for certain periods of time). Early fish did not have jaws, but as they developed, they acquired them.

external image fish-anatomy.jpg (AG)

Digestive tracts of fish (AZ)

How do fish acquire and digest food?

In general, fish eat a large amount in a short period of time (although some fish feed on very large organisms, and then will not feed for a very long time, such as some species of sharks). Many animals practice this method of consumption. For example, overfed goldfish have been known to eat until they explode! Fish are heterotrophs, meaning that they eat other animals as well as plants. They do this by either puckering their mouths to “inhale” small organisms in the water, or by catching food in their mouths and clamping their jaws tight (an example of a fish of this type is the piranha). The first step of digestion occurs in the fish’s mouth. Fish with jaws masticate (chew) food to enlarge the consumed material’s surface area. Enzymes in the fish’s mouth that begin to break down things like simple sugars and carbohydrates as soon as they enter its mouth. As in humans, when the food reaches the fish’s stomach, it is churned to be broken down into even smaller components that are useful to the fish. Most types of fish have pyloric ceca whose role is to digest the food upon reaching the stomach

(Walker). These smaller components are absorbed by the blood and spread throughout the body.


At approximately 40 feet long, the largest fish in the ocean, the whale shark, is a prime example of a filter feeder. As it swims with its mouth op

en, it takes in water filled with a number of tiny organisms floating around in the water. The water is filtered out through its gills and the organisms are digested. [KS]

How do fish sense their environment?
One of the defining characteristics of chordates is their ability to respond to a wide range of sensory triggers, or stimuli. The spinal cord allows impulses to travel from the fish’s brain to its nerves and vice versa. Fish are also able to "see." See is a relative term in this sense, though, because fish do not see in the same manner that humans do. Fish can sense changes in light and pressure with their eyes, and this allows them to “feel out” their surroundings. This ability is beneficial for fish because they have a heightened sense of awareness when a predator is near. Fish also have “ears.” Near their brains, fish have microscopic hairs that are able to sense vibrations of the sound waves at varying depths of water.

[Most fish possess highly developed sense organs. Many fish also have specialized cells known as chemoreceptors that are responsible for extraordinary senses of taste and smell. Although they have ears in their heads, many fish may not hear sounds very well. However, most fishes have sensitive receptors that form the lateral line system. The lateral line system allows for many fish to detect gentle currents and vibrations, as well as to sense the motion of other nearby fish and prey.Some fish, such as catfish and sharks, have organs that detect low levels electric current.Other fish, like the electric eel, can produce their own electricity.RS]

The lateral line system of a fish (JM)
The lateral line system of a fish (JM)

How do fish move? (locomotion)
Have you ever tried to run while in the water? It is very difficult. The water is dense and does not facilitate that kind of movement.
Fish undulation.
Fish undulation.
Have you ever swam? It is a lot easier; your body glides through the water in even strokes, and you are more hydrodynamic. You may even wear "flippers" to swim better. Fish have these flippers built in as an anatomical features. Some fish move by beating their tails in a side-to-side movement. Some swim by beating theirs up-and-down. This is beneficial for fish because it helps them to propel themselves th

rough the water. Fish are able to swim at great depth, great speeds, and for great distances. The combination of the powerful fins that fish have and their smooth bodies help them to move. Some fish can even swim against the current.
Most fish have an organ called a swim bladder located in the body cavity. The swim bladder is filled with gas and acts as a hydrostatic organ, enabling the fish to stay at a constant depth so that it does not float upward or sink. (SF)
[Fish orient themselves using landmarks and may use mental maps of geometric relationships based on multiple landmarks or symbols. By studying fish in mazes, it has been determined that fish routinely use spacial memory and visual discrimination.RS]

(photo 3)

How do fish breathe? (respiration)

Obviously, the majority of fish live in water. As mentioned previously ( in the "diagnostic characteristcs of fish" section) fish breathe through gills, slit like structures on the anterior (front) lateral (side) region of the f

ish. Water enters the fish through its mouth and gills. Through diffusion, gases in the water enter the blood stream of the fish, and through counter-current exchange the oxygen spreads throughout the body of the fish while carbon dioxide diffuses out. Gills are very efficient in fish. They have a large surface area for gaseous exchange, meaning that more oxygen can enter the bloodstream over a given period of time. A single gill consists of a curved fill arch bearing a double row of gill filaments. Each filament has many tiny folds in its surface, increasing the amount of surface area along a given length (LS).


How do fish metabolize and remove waste? (metabolic waste removal)
In any organism, once the water and nutrients essential to the organism's survival have been extracted, only waste remains. Once waste has traveled through the digestive system, it is passed through a bodily orifice and out t to the environment, where is floats down to the bottom of the ocean as detritus. Fish release ammonia and nitrogen into the environment as well.

How do fishes' bodies circulate blood? (circulation)
Fish have a double chambered heart consisting of a ventricle, and an atrium. Blood is pushed from the ventricle to the body, and blood is sent from the body back to the atrium. The heart is where the blood is oxygenated, and the oxygen is taken by the body to sustain life. As oxygen is taken from the blood, carbon dioxide diffuses into it. This counter-current exchange allows for carbon dioxide to be eventually removed from the organism.
external image CircFish-2.jpeg

Above is a diagram of the circulatory system of a fish including the idea of the two chambered heart.(CP)

How do fish protect themselves? (self protection)
Fish protect themselves in different ways. Some fish are poisonous, some are colored to warn predators, some can produce electricity, and some are pointy/spiky. These defense mechanisms can protect fish from predators. The Pufferfish inflates its self to triple its size in order to deter predators from approaching.(JAS) Also, many fish travel in groups, or schools. A question to consider is: is this altruistic (kind in nature) behavior so that the fish can protect each other, or is it so that one fish may escape while another is in danger, should a predator attack?

This is an example of fish travelling as a school. Because there are so many of them, they may be able to either resemble a larger fish and scare away predators, or protect one another from other animals.

(Photo 4)

A Japanese fighting fish.
A Japanese fighting fish.

All that water! How do fish maintain osmotic balance?
Osmotic balance means the balance of water in the organism, in this case fish. Because fish live in aquatic environments, their osmolarity is the same as that of the water they live in; however, to avoid "drying out" due to salts, or becoming waterlogged due to lack of them, fish regulate through channels the solutes in their systems. Some fish have kidney-like structures to help with this.

How do fish maintain homeostasis? (temperature balance)
Fish generally maintain a body temperature that of the temperature of the water that they reside in. A fish in the Caribbean has a higher body temperature than a fish that lives in Lake Massapoag. The water constantly traveling over and through the body of the fish affects its internal temperature. Though fish are cold blooded, signifying that the internal temperature fluctuates with the external temperature, fish do produce some metabolic heat. Most of this heat is lost to water as it passes through the gills, but some fish have adapted to maintain this heat. These fish, such as large tuna or sharks, have developed a counter current system in which their blood vessels are paired so that cool blood is in contact with warmer blood which allows heat to travel from the warm blood to the cooler blood. This allows the body is able to stay warmer in general. (JAF)

How do fish reproduce?
[Fish can reproduce sexually. The first step to this type of reproduction in fish is the process of the female laying her eggs. After the eggs have are available to a male, a male fish will fertilize the eggs by spraying them with a liquid called milt, which contains sperm. The eggs that now contain developing fish also contain a yolk which will feed the fish until, and maybe even a little bit after the egg hatches. (DG)]
(photo 2)Review Questions:
1. What does it mean that fish can "see"? How does this relate to their ability to sense their environment? (Ali Kirsch)
2. How does the counter-current exchange system work? (KTD)
3. How does the anatomical structure of fish allow it to live in an aquatic environment? Describe in terms of digestion, locomotion, and respiration. [Ben Yudysky]
4. How do gills differ from lungs from a air breathing animal?(msr)
5. What adaptations allow a fish to float in water without sinking or rising (depending on their density)? (DRM)
6. Describe the process of metabolism and waste removal in fish. (APS)
7. Why would using the self protection method of swimming with a school of fish be beneficial for an individual fish and the school as a whole?(LD)
8. What is the lateral line system and what sense(s) does it replace in fish? Some say that certain fish have a "sixth sense" what is this extra sense? (JL)

Sources used:
"Biology of Fish." Web. 22 Oct. 2009. <>.(Walker)

Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin-Cummings Company, 2002.
photo 1
photo 2
photo 3 (Photo 44 - Sara Waugh) (JAF) (Leo Schwartz) (Sarah Fleming) (JJF) l & (KNS) (AG) (Jake Schwartz) (AZ) (Jackson Murphy) (DG)