FISHby Varuna Nangia


external image Oceans-Clownfish.jpg

The characteristics that define the group
Fish are vertebrates, meaning they contain a spinal cord. They belong to the Animala kingdom, a sub category of which is the deuterostome branch. The deuterostome branch consists of two phyla: chordates and echinoderms. Vertebrates are part of the chordates phylum. The four characteristics that distinguish the phylum chordate are a notochord, a dorsal, hollow nerve cord, pharyngeal slits, and a muscular, postanal tail.



Vertebrates are distinguished from chordates as they have more specialized characteristic mainly associated with a larger size and a more active lifestyle. Vertebrates also form neural crests, have a high degree of cephalization, a vertebral column, and a closed circulatory system.
There are two main groups of fish: Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish). The Chondrichthyes class is composed of sharks, rays, and their relatives; they are known for their relatively flexible endoskeletons that are made of cartilage instead of bone. The Osteichthyes class is composed of three main fish groups: the ray-finned fish, the lobe-finned fish, and the lungfish. Most of these bony fish are characterized by a hardened endoskeleton often covered with scales.
Fish are further distinguished by their characterizations of jaws, two pairs of fins, and gills which facilitate in their respiration.All fish live in water (whether in marine or freshwater) and some fish can even leave the water for short periods of time.

The term fish is very general and is generalized to include many species of aquatic animals including: shellfish, cuttlefish, starfish, crayfish, and jellyfish. The fish referred to in this page is known as the fin fish. (AR)
Diagram of Fish anatomy (CM)
Diagram of Fish anatomy (CM)

There is another separate group of fishes that are jawless. This group is divided up into hagfish and lampreys. These fish are cylindrical and do not have scales or controllable fins. (PS Source 5)

The earliest types of fishes were jawless fishes, such as the lamprey and the hagfish. In fact, the lamprey and the hagfish are the only two remaining extant jawless fishes. Both of these fish are wormlike and lack hinged jaws; also, as stated above, they do not have scales or controllable fins and are cylindrically shaped. The lamprey is considered a parasite and feeds on the blood and juices of live fish by attaching itself to the body of the fish with its teeth. The hagfish, on the other hand, is a scavenger, meaning that it feeds only by attaching itself to the bodies of dying or already dead fish. (MR; Sources 14 and 15)

Diagram of a bony fish. The swim bladder in the center helps bony fish control their buoyancy. (Matt B - Source 8)
Diagram of a bony fish. The swim bladder in the center helps bony fish control their buoyancy. (Matt B - Source 8)

Acquiring and Digesting food
Fish can be carnivores (eat other animals) , herbivores (eat mainly autotrophs such as plants ans algae) or omnivores (consume animals as well as plants or algae). While some fish are suspension feeders (they sift through food particles from the water), most fish acquire their food with the use of the jaws and paired fins. The jaws of fish allow them to grip food and tear it apart with ease. Also, a fish’s tail and fins enable easy maneuvering through the water. Sharks use their toothy mouth to grasp their food with incredible strength and power.The digestive systems of fish are similar but much simpler than other mammals. They have a complete digestives tract, digestive tubes extending between a mouth and an anus. They go through the four main stages of food processing ingestion, digestion, absorption, and elimination.

Lips are rare in fish with most species having a hard edge to their mouth. Tongues in fish are generally very simple. They are thick, horny, immovable pads in the lower jaw which can be sometimes have small teeth. A tongue is not necessary for the manipulation of food as much as it is for terrestrial animals due to the fact that food for fish can be buoyed up by water and moved through the mouth by controlling water flow and with help from the placement of teeth. (IL - source 10)

Sensing the environment
Fish are able to sense the environment around them through the inner ears located near their brains. These ears do not have an eardrum or open to the outside of the body. However, they do contain small sensory hairs that are stimulated by the movement of otoliths (fish ear bones). Any vibrations caused by sound waves in the water go to the inner ears, making the otoliths vibrate and as a result, stimulating the hair cells. Many fish also have a lateral line system along the sides of their body that allows them to monitor water currents and low frequency sounds conducted through water. This is a result of neuromasts (receptor units) in the lateral line that contain clusters of hair cells surrounded by a gealatinous cup called the cupula. When pressure from the moving water bends a cupula, the hair cells transmit the energy into action potentials sent to the brain. These potentials tell the fish where it is moving in the water or the direction and velocity of the water currents.
The spook fish has “four eyes” which in actuality, are four mirrors, making it the only fish to use mirrors instead of eyes. Two eyes point up at the environment and potential food and two point down. They are unique because they use mirrors to make the image. (LPE)

Although it is often said that fish cannot feel pain, a 2009 study proved otherwise. A group of fish was divided into two separate groups. One group was given morphine while the other group was given a placebo. Both groups were then exposed to painful heat. All of the fish reacted similarly to the heat with increased wriggling, suggesting that the morphine blocked only the pain and not the discomfort. After the heat was taken away, however, the fish that were given morphine returned to normal, but the fish given the placebo had increased wariness, fear, and anxiety. It was concluded from this experiment that fish do indeed feel pain and that even when fisherman throw fish back into the water after hooking them, the fish may in fact have lasting effects. (JS 13)
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The way fish are able to travel from place to place (locomotion) is by swimming. While gravity is less of a problem for swimming animals than for those on land, overcoming friction (resistance) is much tougher because water is denser than air. As a result, fish tend to have sleek, torpedo like shapes that help propel them easily and efficiently through the water. An adaption found in bony fish but not sharks are the swim bladder, an air sac that helps control the buoyancy of fish. This sac adjusts the density of the fish as gases transfer between the bladder and the blood preventing it from floating upwards or sinking. The fins of bony fish are also useful in steering and propelling them through the water; some fish can swim in short bursts up to 80 km/hr. The fins of sharks on the other hand, are more stiff and harder to maneuver with. Rays use their enlarged pectoral fins to propel the through water.

Mid-water swimmers are the most common type of fish. These consist of tunas and trouts, which are adapted for strong, rapid swimming. Fish that live in quieter waters like bays are usually not as strong or quick, but can almost 'sprint' with quick bursts of speed to escape a predator or catch evading prey. These fish often have flattened sides. Fish that inhabit the bottom are usually slow swimmers, and have many modifications of body shape, like flounders, rays and mudskippers. (JP- 4)

Fish move through the water with movements of their tail, here different kinds of fish locomotion are illustrated: A. A crucian carp's fin action for stabilizing and maneuvering. a. Anguilliform locomotion (eel); b. Carangiform locomotion (tuna); c. Ostraciform locomotion (boxfish). The blue area on these fish shows the portion of the body used in locomotion. (RW)

Fish are multi-cellular organisms, some cells are buried deep within the organism; these interior cells need to bring in nutrients (O2) and remove waste (CO2). Further, there is not a high amount of dissolved oxygen found in water. In order to survive, fish must therefore have a very efficient gas-exchange methods. Breathing through gills solves this problem because gills have a large surface area to volume made up of the many fine, threadlike filaments on the gills. Each filament consists of a thin layer of cells surrounding a network of capillaries (short narrow blood vessels). Further, the filaments are lined up parallel with the direction of the water. As water passes over the gill surface from front to back, oxygen and carbon dioxide are exchanged between the blood circulating in the opposite direction through the capillaries ( by diffusion). This process is known as Countercurrent exchange. As the CO2 diffuses across the organism’s gills, the oxygen diffuses into the blood down its concentration gradient and is then carried to the interior cells of the fish.

external image gills.gif

Metabolic Waste Removal
Some of the most important waste that living organism must dispose of are the nitrogenous breakdown products of proteins and nucleic acids. Fish live in water, and are therefore able to excrete these nitrogenous wastes in their natural form of ammonia, a small but extremely toxic molecule that can only be excreted in large volumes of dilute solutions. However, the way that marine and freshwater fish dispose of metabolic waste varies slightly. In marine fish, ammonia ions are easily moved across the epithelium (a layer of specialized epithelial cells that regulate solute movements) of the gills. The kidneys of fish also secrete nitrogenous waste, but only extremely small amounts. In freshwater fish, this process differs slightly to accommodate osmoregualtion and as a result the gill epithelium takes up Na + from the water in exchange for NH4 +, which helps to maintain a much higher Na + concentration in body fluids than in surrounding areas(explained in greater detail under osmosis of fish).

Fish have closed circulatory systems (blood travels only through veins and arteries) know to many as the cardiovascular system. As a result, fish have evolved a single circuit of blood flow with a double-chambered heart: an atrium (chambers that receive blood returning to the heart) and a ventricle (chambers that pump blood out of the heart). The blood carrying the nutrients and waste of the fish circle through veins (return blood to heart), arteries (carry blood away from heart to organs), and capillaries (narrow blood vessels with thin, porous walls). Diffusion of dissolved gases is exchanged across the capillaries.
In fish, as de-oxygenated blood is pumped away from the heart by the ventricle, it goes to the gills where oxygen is diffused in and carbon dioxide is diffused out by the capillaries. The now oxygenated blood travels through the body by converging into a vessel that carries oxygenated blood to the capillary beds in all other parts of the body. The now deoxygenated blood travels through veins back to the heart, to repeat the process.

The Circulatory System of Fish

The countercurrent exchange mechanism allows fishes to retain a higher concentration gradient of O2 for a longer amount of time, making gas exchange more effective.
The countercurrent exchange mechanism allows fishes to retain a higher concentration gradient of O2 for a longer amount of time, making gas exchange more effective.

Self protection
The two main features of fish that help in its protection and development are the jaws and the paired fins. Jaws help to grip and slice predators while fins enable fish to swim swiftly and maneuver. Some fish, rays in particular, have whip like tails that contain venomous barbs that function as a self defense method. Many fish such as stickleback fish tend to be very aggressive as well and have developed fixed action patterns or FAP (sequence of behavioral acts that are essentially unchangeable and carried to completion once initiated). These FAP’s are triggered by sign stimuli, usually a feature of another species. For example, when an intruder with a red belly (sign stimulus) invades the territory of a male three-spined stickleback fish, it will respond aggressively. These FAP’s are crucial in the protection of many fish.

Puffer fish can fill themselves up with water (and sometimes air), increading their size drasrically and therefore evading predators (LPE)
Other defense mechanisms include countershading, camouflage, and traveling in schools. Countershading is a coloration scheme where the fish have bright underbellies and dark overtops. When seen from above they blend in with the dark oceans but when seen from below, they blend in with the sky and light colored waters. This also relates to fish using camouflage as a hiding tactic by blending in with brightly colored coral. Predators can also use this tactic unfortunately to sneak up on unsuspecting prey. Traveling in schools is common among smaller fish because it gives the illusion that the school is one big fish; predators will never pick a fight with a big fish, they’ll only go after smaller ones. (AP)

Osmotic Balance
All fishes are osmoregulators, meaning they control their osmolarity (balance of water uptake and loss based on total solute concentration). To explain it in simpler terms, the fish adapts to the osmolarity of its surroundings. Fishes that live in the ocean (salt water) are hypoosmotic (one of two solutions has a lower concentration of solute) to their environments. They are constantly losing water by osmosis and taking in extra salt by diffusion to adapt to the high concentration of salt present in the ocean. The majority of marine fish balance this loss of water by drinking large quantities of seawater, and excreting only small amounts of water. The excess salt leaves the body by active transport through their gills. Further, fish have special cells called chloride cells which actively transport the chloride ion (Cl-) out. The sodium ion (Na+) is transported out passively through the gills. The kidneys of the fish excrete any excess calcium (Ca2+), magnesium (Mg2+) and sulfate ions (SO42-).

Fish that live in fresh water are hyperosmotic (one of two solutions has a higher concentration to their environments) to their surroundings. They are constantly taking in water and losing salts to counter osmotic loss. In contrast to the ocean fishes, freshwater fish excrete large amount of urine that is hypoosomotic to its body fluids in order to manage its water intake and salt loss. Freshwater fish also take in salt through food and uptake by the gills. For freshwater fish, the chloride ion (Cl-) is actively transported in and is followed by the passive transport of the sodium ion (Na+) by the chloride cells in the gills.

freshwater_fish_osmos.png marine_fish_osmos.png

Temperature Balance
Most fish (they are small) are ectotherms, organisms that have such low metabolic rates that the amount of heat they generate is too small to have much effect on body temperature. Almost all of the heat generated is lost to water when blood passes through the gills. As a result, the body temperatures of fish are determined by the temperature of the water that surrounds them. Usually, there body temperatures are between 1-2 ° C of the surrounding water temperature. However, there are some fish (big fish like tuna and sharks) that are endotherms, in which the organism’s high metabolic rate generates enough heat to keep the body warmer than the environment. These fish have circulatory adoptions that help them retain heat. For example, the Great White Shark has a countercurrent heat exchanger in its swimming muscles to reduce the loss of metabolic heat. Even though all fish lose heat as water passing through the gills cools blood, sharks have a small dorsal aorta that prevents the cold blood from moving to the core of the organism’s body. Instead, small arteries carrying cool blood inward from large arteries under the skin are paralleled by small veins carrying warm blood outward from the inner body. This process of countercurrent flow maintains the metabolic heat produced by the muscles. Also, it is believed that in order to allow more effective functioning, some fish have special heat-generating organs to warm the eyes or brain.

Temperature Regulation in fish through Concurrent flow

The Reproductive System

Most Fish are egg laying, in which eggs are released by the female and the male either releases sperm on the eggs or into the water around them. Some fish do bare young. Fish can be either ovoviviparous, the young develop in an egg inside the female and simple hatch inside the female, or viviparous, develop inside the female and are proved with nutrients through the female's tissues. In some of the egg laying species and in the young baring species different methods of internal delivery systems of the male's semen have developed. Often a fin slowly adapted as a phallus and can impregnate a female. This is the case with sharks, whose pelvic fins have been used as a sexual organ known as myxoptergia. In many fish species the individual, throughout its life, can transform from a male to a female and vise versa, guaranteeing a breeding population. (MS 12)

Review Questions:
1) How does the respiration system that fish have work? (MM)
2) What is the primary food of suspension feeders? (RK)
3) How is the fish circulation system similar and different than the human circulation system? (AR)
4) How do fish use bones in their ears called otoliths to sense the environment around them? (CP)
5) What is the purpose of the swim bladder found in all bony fish? (ZJ)
6) Why is countercurrent flow important in maintaining homeostasis? (SP)


1. Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin-Cummings Company, 2002. (Varuna Nangia)

2. (MB)

3."Defense Mechanisms of Ocean Fish: How Fish Avoid Predators in the Sea |" Megan Jungwi | Web. 29 Oct. 2011. (AP) (LPE) (LPE)


5. (TB)


7. Jr.Cleveland P Hickman, Larry S. Roberts, Allan L. Larson: Integrated Principles of Zoology, McGraw-Hill Publishing Co, 2001, ISBN 0-07-290961-7 (information found on (AR)

8. (AR) clownfish picture

9. (Matt B) Bony fish diagram

10. Ramel, Gordon. "Fish Anatomy 1: The Digestive Tract." Anatomy of the Digestive System of Fish. The Earth Life Web. Web. 12 Nov. 2011.

11. (RW)

"Information on Fish Reproduction." LookD - Information at Your Fingertips. Web. 13 Nov. 2011. <>.
13 (JS)

14. "Fish Evolution." Earth Sciences for Students. Detroit: Macmillan Reference USA, 2008. Gale Science In Context. Web. 15 Nov. 2011.
15. "Fish." UXL Complete Life Science Resource. Ed. Julie Carnagie and Leonard C. Bruno. Detroit: UXL, 2009. Gale Science In Context. Web. 15 Nov. 2011.