How about some fish jokes to start?
What is the difference between a piano and fish? You can't tuna fish!
What did the boy octopus say to the girl octopus? I want to hold your hand hand hand hand hand hand hand hand.

And Now... For the exciting stuff!

Fish first began to appear during the late Silurian period and early Devonian period.
The two main categories are Chondrichthyes and the Osteichthyes.Main Types of Chondrichthyes: sharks and raysMain Types of Osteichthyes: ray-finned fishes, lung fishes, lobe-finned fishesPlacoderm: extinct fish that also had a jaw but were found in fossilsSuperclass Agatha: Jaw-less fish, the most primitive group. The present day descendants are lampreys.(ORS 11)
There are about 750 different species in the Chondrichthyes classDuring the Devonian period, placoderms and acanthodians (jawed fish) became were the new forms of life.

The Main Diagnostic Characteristics

The structure of fish differs in two categories: the chondrichthyes and the osteichthyes. When fish first developed, the main characteristic that separated them from previous forms of life was the structure of their jaw. This allowed, and still allows, them to catch prey and eat it efficiently. Fish were able to become predators unlike earlier species, who had to eat whatever was available such as floating food particles. When the jaws developed through modification the skeletal rods were no longer in place to support the gill slits used for feeding. The slits remained and became used for respiration.
Countercurrent exchange, discussed further down, is the mechanism by which fish breathe.

Chondrichthyes- Endoskeletons made of cartilage; this is a derived characteristic due to modification (the process in which cartilage becomes hard bone was prevented causing the separation between chondrichthyes and osteichthyes) The tail fin especially helps them to swim quickly. The dorsal fin on top helps the fish to remain stable.

Located on the head of a shark, receptors can detect muscle contractions of close animals by contractions that produce electrical fields.

Osteichthyes- Endoskeletons made of bone. The operculum is a structure that covers several sets of gills that are used for respiration. The swim bladder is an air sac that controls the "lightness" of a fish.

Internal Anatomy (MT)
external image circulatory_system_red_fish.jpg
Acquiring and Digesting Food
  • With the development of the jaw, fish were able to become predators and eat other living organisms. They catch their prey and eat bits and pieces of the flesh. In the shark intestine is the spiral valve, which increases area and makes the path for food travel slightly longer.
  • Most fishes are predacious, feeding on small invertebrates or other fishes and have small conical teeth on the jaws. Most predacious fishes swallow their prey whole and the teeth are used for grasping and holding prey. Once it has reached the throat, food enters a short and greatly distensible esophagus. The stomach varies greatly in fish, depending on the diet. In most fish, it is a simple straight or curved tube with a muscular wall and a glandular lining. Food is largely digested here and leaves the stomach in liquid form. Between the stomach and the intestine, enzymatic liquids enter the digestive tube from the liver and pancreas. The liver is large and clearly defined and the junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca occur in some fishes at this junction and have a digestive or absorptibe function. The intestine absorbs nutrients into the blood stream. (LW) (3)

Sensing the Environment
  • The lateral line system is a line of organs that respond to changes in water pressure which allow them to, in a sense, hear. Both chondrichthyes and osteichthyes have a lateral line system, making it a primitive feature of fish. A nose is used strictly for the sense of smell and not for respiration. Fish also have eyes although they cannot distinguish colors very well.
  • (YA)The lateral line system which runs along the sides of the body onto the head, is divided into three branches, two to the snout and one to the lower jaw. It is constantly at work, allowing the fishes to be aware of its surrounding. For example, as the fish approaches an object near by, the pressure waves around its body are distorted, and these changes are quickly detected by the lateral line system, enabling the fish to swerve or to take other suitable action. The big question is how does it all work?
  • The basic sensory unit of the lateral line system is the neuromast, which is a bundle of sensory and supporting cells whose projecting hairs are encased in a specialized cap known as gelatinous. The nueromasts continuously send out trains of nerve impulses. When pressure waves surrounding the fishes cause the gelatinous caps of the neuromasts to move, the enclosed hairs move, and the frequency of the nerve impulses changes accordingly. Depending on the intensity of the pressure wave, neuromasts may occur singly, in small groups called pit organs, or in rows of canals. (YA)(1)


  • The main moving mechanisms of the fish are the fins. The dorsal fin on top of the fish help to stabalize it while swimming. The caudal fin (tail fin) helps to push them forward. The pectoral and pelvic fins, both located on the bottom of the fish, help to lift the water. In bony fish, glands in the skin produce the slime that helps to reduce water resistance while swimming. Rays use their enlarged pectoral fins to propel themselves across the sea.
  • The slime a fish produces reduces the friction it experiences by at least 65%. Also, because fish are flexible, the turbulence it creates is greatly reduced, and they experience much less drag than a "rigid model of a fish." (SM)(5)
  • There are different types of muscles present in fish. Smooth muscles are located in the digestive tract, air bladder, eyes, reproductive and excretory duct, and other organs. Fish also have striated muscles. These muscles are shaped in irregular vertical bands and different fish have different patterns of striated muscles. Striated muscles are the most common muscle in fish and function by creating undulations that allow the fish to thrust forward and allow them to swim. These muscles are also present on the rays and fin spines which also help the fish move. Fish have muscle segments called myomeres. Myomeres separate the superior and inferior portions of the fish along the midbody. These also help the fish move. (9)(MF)


  • Fish breathe by a process called countercurrent exchange. Because water is so low in the concentration of oxygen, gills must be very efficient when collecting oxygen.
  • Gills are specialized tissues that are structured as foldings, and each gill is covered by cells and contains many blood capillaries for respiration. (AC)(20)
  • In a gill, blood flows in the opposite direction of the water. As blood is going through a capillary, it becomes more concentrated with oxygen. At the same time, water, which has an even higher concentration of oxygen, is passing over and a diffusion gradient of oxygen is created between water and blood.
  • Sharks take in the water through their mouth which travels over the gills to exchange oxygen.
  • Gills are beneficial in fish respiration because they are very efficient in several ways. One, they have a large surface area, which means that more oxygen can enter the bloodstream over a period of time. Two, gills shorten the distance the oxygen has to travel to reach the bloodstream because the blood in the filaments of the gills is very close to the body. Three, gills create a concurrent circulation, which keeps the blood oxygenated. (RG) (21)
How Gas Exchange Works in Gills (CSR, 8)

Metabolic Waste Removal
  • Fish excrete waste through the anus. Fish excrete nitrogenous waste in the form of ammonia. Ammonia is highly soluble in water but can only be tolerated at low concentrations, thus requiring a large amount of water to excrete. Fish excrete ammonia through the gills in the form of ammonium ions.(KL)(2)

  • The Fish excretory system has four main parts: the kidneys, tubes that connect to a storage site, the bladder, and an exit tube (urethra).

  • Below is a picture of energy flow in a fish. [MS] 22

  • A fish heart consists of an atrium, which recieves blood, and a ventricle, which sends blood. The blood from the ventricle travels to the gills to recieve oxygen through countercurrent exchange and travels through the body to deliver oxygen-rich blood. This process is called systemic circulation. Blood then returns to the atrium through a vein.
  • The fish circulatory system is much like that of humans. The fish heart is two-chambered and consists of an atrium, a ventrical, a sinus venosus (a sac-like structure with thin walls), and a tube(bulbus arteriosus). It is located between the posterior gill arches. Like mammalian blood, fish blood consists of plasma and red blood cells which contain oxygen-carrying hemoglobin. (ZXU)(2)

external image fish_gills_2.jpg
Circulation overview in Fishes (GR)(10)
Self Protection
  • The fins are made to swim fast from predators. Fish also use their teeth to defend themselves and fight off attackers. Some types of rays have venemous barbs that function as a defense mechanism against predators.
  • Some fish protect themselves through camouflage. Camouflage allows fish to blend in with their environment in at attempt to hide from potential predators. For example, the peacock flounder can change its color and pattern to match the sea floor. This adaptation has allowed flounder to protect themselves from predators and fishermen. (SI) (4)
  • A common and important behavior seen in fish is "schooling" or the grouping of fish into swimming in a likewise manner. In many different ways, there is safety in numbers. Schools are important for large groups of fish because they protect the majority of fish, and there is a number of reasons for that. First, it is much harder for predators to identify, let alone remove individuals from a group than it is to hunt down a lone fish. Second, for an individual, the larger the group, the statistically less likely that death will occur. Third, a larger group of fish is more imposing- it can discourage some attacks by the school behaving like a larger organism and fooling others into believing it is. Schools are also useful because they improve awareness as a whole for the group- more eyes means superior foraging and predator detection. In addition, schools are great for reproduction- a large local population means a wider range of mates, as well as timed mating en masse that causes a "spawning onslaught" that overwhelms egg/spawn predators. Lastly, schools are useful for migration- according to hydrodynamics, fish swimming together aid each other by forming a "slipstream"- a small current that makes it easier for fish to swim, thus saving energy. (TM)(12,13)
  • Mucus is the first physical and chemical barrier that prevents disease organisms from entering the fish. It contains enzymes and antibodies that can kill invading disease organisms. In addition to mucus, scales and skin function to protect the fish against infection and water loss. Thirdly, inflammation is the fish’s natural immune response to a foreign protein, virus, toxin, etc. However, external stresses, like temperature (both extremely warm and cold), may inhibit the inflammatory response. (MT) (6)
  • Scales which can vary in size, shape and substance, serve in protection. Scales protect the fish from its environment by keeping the more vulnerable skin out of the open. By having scales it allows for some protection while still being able to move freely. Also an interesting fact is that you can tell the age of a fish from looking at its scales. As the fish grows the scales grow, and in the scales "rings" are produced as in a tree. (RL) (16,17)
Osmotic Balance
  • The high concentration of salt in salt water makes it hard for fish to maintain a balance. They take in lots of water through drinking and eating. The accompanying salt, however, must be disposed of. Many different species of fish use different mechanisms to keep an osmotic balance. In sharks, the kidneys remove most of the salt and the rest that is unnecessary is removed by the rectal grand or excreted as waste. In fresh water the fish have the problem of losing salt and gaining too much water, the opposite of salt water creatures. Fish excrete very diluted urine and maintain a leveled salt concentration by active uptake.

  • Fish prevent excessive salts diffusing into their bodies by passing out salt through their gills via active transport. (CC) (18)

external image T1623E04.gif
Osmoregulation of a fish. (NI) 19
Temperature Balance

  • Fish are described as cold-blooded, meaning that their body temperature varies with the external temperature. Fish do, however, produce metabolic heat (that is, heat derived from the oxidation, or "burning," of food and from other processes), but much of this heat is lost to the outside at the gills.(ZS) When blood passes through the gills, most body heat that is generated is lost to the surrounding waters. Most fish tend to be very close in temperature to the water surrounding them. Bigger, specialized endothermic fish will retain heat in their swimming muscles. This happens through the arteries that bring the blood to the tissues under the skin. The blood travels through branches to the muscles. Here the vessels are arranged in a countercurrent blood exchange, which means that the arteries and veins are parallel so the warm blood can give off heat to the cold blood. This helps to keep the internal muscles warm. One of the advantages of warm-bodiedness is an increase in muscle power. Muscles contract more rapidly when warm without loss of force. If with a 10 C degree (18 F degree) rise in body temperature a muscle can contract three times as fast, then three times the power is available from that muscle. More muscle power means more speed in pursuing prey, escaping enemies, and shortening the time required for long-distance migration.(ZS)

Review Questions:

1. Describe how gas exchange works in the gills? (SR)
2. How is the development of jaws beneficial to fish? (RG)
3. Which muscles do fish use for movement? How do each of these muscles contribute to movement? (AK)
4.Fish use a lateral line system to sense their environment. The neuromast is the sensory unit that drives the lateral line system. Describe what the neuromast detects and how it detects this stimulus. (SD)
5. How have swim bladders affected the ability of fish to survive in aquatic environments? How else are swim bladders beneficial? (CW)


Campbell, N.C., Reece, J.R. (2002). Biology. (Sixth Edition). San Francisco: Benjamin Cummings.(2)(KL)

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