Animalia-Chordata--Fishes By Ryan Sullivan

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Edited By: Hilary Stepansky
Jesse CarmenSam Blatchford Yasheka Sharma
Denny Poliferno
Rachel Kornetsky
Josh CzikBecca Levenson
Omer Zaidi
Grace Rehnquist
Meru Nangia
Arielle Reiter
Brittany Marcus-Blank
Sarah Vlach
Kevin Nayer
Sarah Schwarzschild (SES)
Ethan Richman

· The diagnostic characteristics that define the group
Fish belong to the kingdom Animalia. Therefore, fish are multicellular eukaryotes, organisms with cells that have membrane-enclosed nuclei and membrane-enclosed organelles. Their cells lack cell walls. They contain nervous tissue and muscle tissue. They are heterotrophic, meaning they consume other organisms. As chordates, they have a spinal cord, a cord of nervous tissues that allows the organism to process and create stimuli in a central location. Fishes have jaws, hinged structures that allow the organism to grasp food, and two pairs of fins. Fish also have gills, respiratory structures that aid in the intake of oxygen.(1)

It is also said that fishes can be divided into two groups, the Agnatha, which are vertebrates without jaws and the Gnathostomata, which are vertebrates with jaws and other high order vertebrates. It is said that some fishes, such as the Hagfish and Lamreys are known Agnathans. Also, jawed fishes are divided into three of the following groups:
Placodermi, which are scaly-skinned fish, chondrichthyes, which are cartilaginous(consisting of cartilage skeleton)fish and osteichthyes, which are bony fish. (5. Sharma)
Chordates conatin a notochord, a rod shaped skeletal structure located behind the gut tube and closer to the abdomin than the nerve cord, which plays an important role in promoting and organizing the undeveloped nearby structures. The notochord dissaperas or becomes exxtremly modied in adults. The nerve cord of chordates develops in the back of the body as a hollow tube above the notochord. Togetehr the brain and the spinal cord make up the central nervous system which connects to the sensory system and the motor nerves. (12) (BMB)
Basic Anatomy (13, NK)

Acquiring and digesting food
Fish are heterotrophs, so they consume other organisms. Fish are predators that catch food with their jaws and teeth with help from the added speed of their paired fins. Fish are bulk-feeders because they eat large masses of food at once, as most animals do. After food is acquired from the environment, fish begin digesting their food. Digestion begins in the mouth of the fish when the biting of the jaws and teeth cleaves the food into smaller pieces with greater surface area. The macromolecule, large molecule, food is broken down in the stomach of the fish into monomers, small molecules, by chemical factors and by stomach movements. The small molecules are absorbed into the bloodstream of the fish for use in other processes in individual cells. This process of absorption takes place in the lower digestive tract. (1)

· Sensing the environment
As vertebrates, fish are able to draw from a large number of sensory stimuli and process them quickly and efficiently in a central location, the brain. Fish have single-lens eyes, eyes that have a single lens that allows them to take in visual stimuli of the environment. They are able to sense changes in light and surrounding areas. This ability allows fish to both catch prey and evade predators by means of perceiving the other organism's presence visually. Fish have inner ears that are located near the brain. Within their ears, fish have sensory hairs that are able to conduct the vibrations of the sound waves under water. Because fishes' ears are not open to their outside environment, sound waves must first be conducted through the skeleton. Fishes also have a lateral line system, a system of nervous tissue that senses water currents, waves caused by moving objects, and sounds conducted in water. Fish have a series of neural cells that form webs on the outside layer of the body, the epithelial tissue, that allow the fish to sense pressure on the outside of its body.(1)
Some fish, such as swordfish and sharks, have special adaptations which allow their brains and eyes to remain at a temperature slightly higher than that of their environment. This adaptation increases their reaction times and allows them to see better. (KN 4)

· Locomotion
Like other animals, fish must move in order to capture food. The active movement from one place to another is called locomotion. Fish must move through the dense water around them, thus, friction becomes a paramount issue in movement. A great example of this would be the rainbow trout. The rainbow trout swims upstream to do this efficiently it must be shaped to swim upstream. To counteract friction, most fish are very sleek and aquadynamic. The perfect design is a torpedo shape. This shape reduces the most amount of friction.(4sjb) Fish move by moving their fins, tails and body in an undulating motion up and down to create propulsion. These fins also help the fish maintain its stability in the water and prevent it from rolling upside down (8) (OZ). Locomotion in fish is caused by voluntary movements of muscle tissue.(1)

· Respiration
To take in underwater gasses necessary to life, fish use gills. Gills are portions of the body surface with large surface areas. Fish constantly take in water through their mouths as they swim. This water passes through the mouth and into the internal gills of the fish. The gills are shaped as a series of arches to maximize surface area. Each of the arches of the gills contains gill filaments, arm-like structures that extend from the arches. The gill filaments are made up of lamellae, flat disks that are lined up parallel to one another. As water passes through the gills, the capillaries, tiny blood vessels, in the lamellae, absorb oxygen from the water.(1) The lamellae are very fine, delicate structures that are specialized to each particular fish. Their exact structure and dimensions are unique to the fish's uses. The more active a fish is, the thinner and more compressed the lamellae are, so as to increase oxygen intake. A slow and sluggish fish, on the other hand, would have thick, distant lamellae.(10 Nangia)

Gas exchange in fish is enhanced by countercurrent exchange. Countercurrent exchange allows the gills to maximize oxygen intake. When the water passes over the gills, the blood, flowing in the opposite direction as the water, becomes saturated, oxygen-rich, with oxygen. However, as the blood moves in an opposite direction than that of the water, the water that is in contact with the blood is constantly at its most saturated level. Therefore, the gills are able to maintain a favorable gradient, a difference in concentration that allows blood to absorb oxygen through the entire length of the gills. Countercurrent exchange allows fish to remove more than 80% of the oxygen contained in the water. As oxygen is absorbed, carbon dioxide is released into the water by means of another gradient, as the concentration of carbon dioxide in the water is less than that of the blood. After the oxygen is absorbed into the blood, it is passed into the circulatory system of the fish.(1)

· Metabolic waste removal
After food has been digested and the nutrients have been removed from the mass of consumed food, the waste products are left behind. When the macromolecules are broken down by enzymes, chemical catalysts that make reactions faster, the nitrogen is broken down into ammonia, a very toxic molecule. Because fish have such an abundance of water both internally and externally, they are able to dilute the ammonia to nontoxic levels. In fish, ammonia is lost through the gills by means of a concentration gradient, as there is more ammonia in the organism than outside of it. Solid waste passes through the digestive system of the fish and is passed through the anus into the external environment.(1)

Fish reproduce sexually. Sexual reproduction consists of male gametes, sexual cells with half of the genetic material of the parent organism, meeting and fertilizing female gametes. The fertilized gametes together are the zygote, the single original cell of an organism that contains a full set of genetic material. This process takes place externally in the water surrounding the fish.(1)
There are various methods of reproduction. Sharks and a few bony fishes, for example, give birth to live young, while most other fish lay eggs. Those that do lay eggs either produce hard shelled egg for protection of the embryo or unprotected eggs that become fertilized after they are in the water. Those that lay hard shelled eggs only produce 7 or 8 at one time because the eggs have a good chance of surviving. Those that produce unprotected eggs produce a lot of eggs at one time because the eggs will float around in the water and get eaten by other carnivores. The large number of eggs ensures that a few will survive. (HS 3)
In most species of fish, the sperm and eggs develop in male a females, respectively. However, some fish are hermaphrodites (produce both sperm and eggs), and some can even change their sex over time. Fertilization can either be internal or external. Some species of fish are unisexual; there is no fusion (combining) of the egg and the sperm. Instead, the sperm is only necessary to trigger the egg to grow into a fish, and the sperm does not contribute to the genes of the resulting offspring. Because of this, offspring of unisexual species are always female. Thef emale offspring will mate with males of other similar species in order to produce more offspring (6) (DP).

· Circulation
Fish circulate nutrients and gasses necessary for life in a central cardiovascular system, a system of a heart and blood vessels that transport matter throughout the body of the fish. A fish heart has a single atrium, a chamber that receives blood returning to the heart, and a single ventricle, a chamber that pumps blood from the heart to the body of the fish. After blood is pumped from the heart, it travels to the gills. In the gills, oxygen is absorbed into the blood and carbon dioxide is released into the external environment through capillary walls, the thin walls of tiny blood vessels that border the external environment. The oxygen-rich blood travels from the capillaries into a more central vessel. Blood then moves to a series of capillaries in the body that allow oxygen to pass into the cells of the fish through capillary walls. At this point, carbon dioxide is absorbed back into the blood and is pumped back to the heart via another central vessel. When blood is flowing through the capillary beds of fish, blood pressure, the pressure created by the pumping of the heart that drives blood through the circulatory system, drops, limiting the ability of the circulatory system to deliver oxygen to the body tissues.(1)

image017.jpg (Jesse Carmen)
As seen in this picture, a fish has a heart that circulates blood throughout its body in one, single loop. The deoxygenated (blue blood, which lacks oxygen) leaves the heart and then enters the gills. At the gills diffusion occurs, converting the blue blood into red, oxygen-rich blood as oxygen is diffused into the blood. The blood then travels from the gills back to the heart where the red (oxygenated blood) is converted back into de-oxygenated (blue blood) and then the process repeats itself. (Jesse Carmen) · Self protection
Fish use a variety of self defense techniques to protect themselves from predators. Fish are very fast as a result of their fins and tail. Thus, they are able to propel themselves quickly through the water to evade predators. Many fish travel in what is known as a school, or group of similar fishes. (ER) This enables them to obtain greater security in their numbers, and the act of scrambling the school can help to evade predators. (ER) Some fish have spines or poison to deter predators. However, in many environments, fish are at the top of the food chain and require little defense.(1)
external image PufferFish.jpgThis is an image of an inflated puffer fish. When threatened, it quickly inflates and sticks out the spines covering its body to seem larger and more intimidating than it normally is. (JAC) all types of fish lack true eyelids, when fish sleep, they just move in a dreamlike state. In this vulnerable state, most species are just floating around in the water, though some species lie on the bottom. If they are attacked or disturbed, most fish are able to dart away (7 BL).

Osmotic balance
Osmoregulation is the regulation of water levels within the body of an organism. Because fish are surrounded by water, the levels of all dissolved substances, the total osmolarity, is the same as the seawater. But, fish regulate specific solutes. The water in the ocean is much saltier than the internal environments, so water is quickly lost through concentration gradients because the percentage of water to that of salt is higher in the fish than outside of the fish. Fish gain water through consuming food and drinking seawater. Some fish, however, regulate salt balance with kidneys, specialized digestive organs, and excretion, allowing the fish to maintain much of its internal water. These fish do not need to constantly intake water to maintain osmotic balance. By comparison, freshwater fish have much water and little salt. Freshwater fish must constantly excrete water to maintain osmotic balance. Also, salt must be replenished by food.(1)

As shown by the diagram fish have different and almost opposite mechanisms to maintain osmotic balance depending upon their environment (fresh water vs. saltwater). Because freshwater fish have a higher ion content than their surroundings their are naturally gaining water, while marine fish are less salty than their environment so they are constantly losing water. The urine of freshwater fish is dilute because it has a high water concentration since freshwater fish try to get rid of the excess water. The urine of marine fish is concentrated because they need to keep any water they can since they are constantly losing it. Another difference shown by the diagram is that freshwater fish must actively intake ions while marine fish must actively secrete ions. (my original analysis based on the diagram RK)

Temperature balance
Most fish are conformers, meaning that they stay very close to their surrounding temperature. Almost all metabolic heat is lost to the surrounding cold water when blood passes through the circulatory system. In some fish, heat is kept within the body by means of a divided circulatory system. Fish, such as tuna and mackerels are warm blooded fish, that have this divided circulatory system. In this system there is are paired ingoing and outoging blood vessels, with the warm blood going from the gills is transferred with the cool blood moving towards the gills, and the warmth is kept inside the fish (9, GR). Some blood vessels deliver blood to the surrounding tissue, but other vessels deliver blood to the deep muscle tissue, keeping the tissues a few degrees warmer than surrounsing tissue. Some other fish have heat-generating organs.(1) One such fish is the mackeral, which has a special heating organ under its brain. Some fish also migrate to maintain their body temperatures. The most successful fish at regulating heat are large fish and fish with tissues with a high oxidative capacity (oxygen content). (AR, 11).

Review Questions:
1. How do the lamellae aid in the intake of oxygen and gas exchange in fish? (SV)
2. Explain how circulation transports oxygen throughout the body of the fish and gets rid of carbon dioxide. (HZ)
3. How does the method of laying eggs cause a difference in the number of surviving fish? (VM)
4. Explain the fish nervous system and the main parts of it, along with their functions. (SES)
5. How do fish hear? Be sure to include why fish have hairs in their ears. (DJ)
6. Specifically, what structures do fish have in their gills that allow them to respirate efficiently? (SS)

(1) Campbell, Neil A., and Jane B. Reece. Biology. 6th ed. Boston: Benjamin-Cummings Company, 2002.
(3) See W. S. Hoar and D. J. Randall,
Fish Physiology (6 vol., 1969–71); J. E. Webb et al. ed., Guide to Living Fishes (1981) 3 Dec 2008. <>.
(4) (5) Fish. Biological Sciences. University of Paisley. 18 Nov. 2008. <>
"Reproduction." 2002. SeaWorld/Busch Gardens Animal Information Database. 7 Dec. 2008 <>.
(7) "fish." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 07 Dec. 2008 <>.
"External Fish Anatomy." Fish Anatomy. 1999. Florida Fish and Wildlife Conservation Commission. 7 Dec. 2008 <>.
(9) "Body Temperature." Fish. 7 Dec. 2008 <http:
(10) Ramel, Gorden. "Gills: Gaseous Respiration in Fish." Gorden's Fish Respiration Page. 29 Sept. 2008. Earth-Life Web Productions. 7 Dec. 2008 <>.
(11) "Endothermy in fishes: a phylogenetic analysis of constraints, predispositions, and selection pressures." Environmental Biology of Fishes. 07 Jan 2005. 7 Dec 2008. <>.
(12) Lundberg, John G. "Chordata." Tree of Life. 1995. 7 Dec. 2008 <>.