BIO 101 Organism Physiology Paper

Fish Organs and Adaptation



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The Organs of a fish

Figure 1 the parts of a fish adapted from Module 6: Vertebrates – Part 2. (2014, November 23). Retrieved July 15, 2015, from, J., & Fay,

The fish has many parts, which are internal as well as external, and are discussed below

Eyes – The eyes are an external part of the fish that help the fish see through the water. Although eyes of all vertebrates are similar, Fish eyes are quite different from those of terrestrial animals, mainly because their shape are structured in a way to accommodate the watery environment which they live in, and adaptive in a way to see through the aquatic medium (Atema & Fay, 1988). Fish eyes don’t have eyelids, and can always be seen with their eyes wide open.

The Nostrils

Primarily, nostrils in fish are used for smelling food, and are as a rule supposed to be very sensitive so that they can smell food from far. They don’t open in the back of the mouth as those of mammals, and aren’t therefore used for breathing (Randall, 2014). The reason why fish can amazingly smell in water is because fish use the olfactory system, which connects the olfactory epithelium to their brain. They have the ability to detect dissolved substances/chemicals from considerable distances, even in low concentrations, in terms of smell and taste. Fish have either one or two pairs of nostrils, which allow water to flow through into the nasal cavity, where the olfactory epithelium contains sensory cells which detect the molecules that are dissolved in water.


The mouth of a fish is mainly for eating. Typically, fish have undergone evolution to have different mouth types, depending on what they feed on and how they feed, which gave rise to the four different types of fish mouths namely superior mouth, inferior, prostusile and sucker mouth. The mouth adaptation is in how they attack their prey, their feeding position and what they feed on. The mouth also takes in the water used to provide oxygen.


Gills are internal organs in the body of a fish that help the fish in gases exchange. They allow fish to absorb the oxygen found in the water so as to use it for energy, mainly. They work in the same way as human lungs and other mammals, only that the gills are able to absorb smaller oxygen concentrations that are available, while at the same time allowing the fish maintain a necessary level of Sodium Chloride in their bloodstream. All these happens in the filaments, which are contained in the gills, and aide in the transfer of water and ions and the exchange of oxygen and carbon dioxide. The gills, which are very sensitive are covered by the operculum, which also releases the water.

Spinal cord

It is the bone that runs through the spine, and connects the rest of the body to the brain. It is also important to the relaying of sensory information and other instructions to the brain from the different parts of the body.

Lateral line

This is one of the primary sense organs of a fish. This is because it is used to detect vibrations underwater, and determining their source and direction. This sensory ability is mostly achieved through the hair cells, which are basically epithelial cells that are tailor made to respond to any displacement that is caused by motion, as explained by Coombs (2013).


Fish fins are arguably the features that are distinctive to fish, and they are external organs that are composed of spines that protrude from the body, with skin covering them thereby forming a web-shape. They are used by fish mainly for moving, stopping, turning and having an upright position. According to Encyclopedia Britannica, fish generally have 7 fins, where 3 are single fins (caudal, dorsal and anal) and two fins which are in pairs (pelvic and pectoral) and each fin has its own specified functions when it comes to movement in the water.


Scales in fish form part of the fish’s integumentary system. They are arranged in an overlapping way like rooftop shingles, ensuring that the skin not exposed. They are generally meant to protect the fish. They do not protrude out of a fish, since the epithelial layer covers them. There are basically 2 types of scales. Although they are both round, one type has its edges serrated while the other type is completely smooth.

Fish Adaptation

Overtime, fish have evolved a lot in their physiology so as to adapt to their surrounding through natural selection. As complicated as they are, they have a very long history of evolution that spans over 150 Million years. These years have seen them developing features they did nit originally have, while other features just developed so that they may be better placed to survive. The first evolutionary species of fish is reported not to have jaws (Graham, 1997) but are now better adapted to eat by formation of jaws. The swim bladder, in terms of the positioning and functioning is also evolutionarily homologous to the lungs. The fins are easier to flap and move through the water, as opposed to the hinds that were used by the ancient fish. Therefore, it can be concluded that the evolution of fish over the years is the reason fish are adaptive to their current environment.


90, S., & Britannica, I. (2013). Fish and Amphibians. Chicago: Encyclopaedia Britannica.

Coombs, S. (2013). The lateral line system. New York: Springer.

Graham, J. (1997). Air-breathing fishes evolution, diversity, and adaptation. San Diego:

Academic Press.

Module 6: Vertebrates – Part 2. (2014, November 23). Retrieved July 15, 2015, from, J., & Fay,

Randall, J. (2014). Trout sense: A fly fisher’s guide to what trout see, hear, and smell.

Mechanicsburg: Stackpole

R. (1988). Sensory Biology of Aquatic Animals. New York, NY: Springer New York


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