Sierra College Department of Biological Sciences

Discover the Biological Sciences

The introductory information for this site has been contributed as a joint assignment by students in Bio. Sci. 10.

Zoology - Ichthyology

By: Katelyn Lindsay


Ichthyology is the scientific branch of biology that studies fish. Fish can be classified into three different categories, namely Osteichthyes, Chondrichthys, and Agnatha. Over 25,000 distinct fish species have been discovered so far, and it has been estimated that approximately 250 new species are officially reported every year. Ichthyology has a long history, and includes a multitude of research fields, of which the main ones will be covered in this essay.

To start, we should specify what exactly constitutes as a fish. According to the dictionary, a fish is defined as "any of various cold-blooded, aquatic vertebrates, having gills, commonly fins, and typically an elongated body covered with scales. "

Ichthyology stretches all the way back to the pre-historic era, and has been researched throughout the ages. In fact, the first fish scientists were hunters who watched and studied the fish to learn how to catch them, and when they were the most plentiful to catch them. Even Aristotle talked about ichthyology! He observed the differences between fish and other aquatic animals, and wrote out descriptions of them. The science of ichthyology was finally properly formulated in Europe in the 18th century.

There is much that has been studied and learned in ichthyology. The main topics explored are the classes of fish, reproduction, swimming, breathing, senses, and schooling. We will go into more detail on those topics now.

As stated in the introduction, fish are sorted into three main classes: Agnatha, Chrondrichthyes and Osteichthyes. The Agnatha, a very primitive-like species of fish, are classified under one category because they do not have any bones, or even a jaw. Meanwhile, the Chrondrichthyes class is distinguished by its characteristic skeleton of cartilage. Finally, the Osteichthyes make up the largest class of the fish, and its skeletons have been calcified which results in a harder skeleton compared to the other classes. Every fish can be divided into one of these classifications.

There are three main ways for fish to reproduce, and some fish are even hermaphrodites, which we will focus more on later. The most common way of reproduction is when the female fish lays unfertilized eggs, after which the male fish comes to fertilize them. This is called oviparity. Secondly, ovoviparity is a type of internal development. However, here the parent does not directly nourish the youngling, and when born, the young fish still have some developing to do. Most sharks and rays reproduce using ovoviparity. The last form of reproduction is viviparity. This is also an internal way of reproduction but, unlike ovoviparity, the young are nourished by the mother while still in the womb, much like human and other mammal births. Since the young are given nourishment while growing inside the mother, they are fully developed when born.

As already mentioned, some fish are hermaphrodites and there are two types of hermaphroditic fish. The first type is called a simultaneous hermaphrodite, which is just as it sounds; the fish has both male and female parts. This is particularly useful when it is difficult to find a mate, as they are able to pair with any other fish they come in contact with, making it a little easier to carry on their line. The second type of hermaphroditic fish is the sequential hermaphrodite. These fish can change sex only once in its lifetime; going from male to female or the other way around.

Swimming might seem easy; a fish just swishes its tail, and off they go. In reality, however, it is a little more complex than that. A fish’s skeleton, muscles, and fins must all work together in order to propel forward. The skeleton acts as a lever and allows for the correct movement of the fish. Muscles provide the power for swimming; a fish's muscles are ordered going in all directions, allowing the fish to turn any which way it pleases. Lastly, the fins provide the thrust and also are able to change the direction the fish is swimming. The shape of the fins, as well as that of the body, affects the way a fish swims. We can discover two types of swimmers, burst swimmers and cruisers. Cruiser fish are fish that are continuously moving, they have a high blood capacity and are rich in myoglobin, an oxygen holder; allowing the fish to be in continuous movement for long periods of time. A burst swimmer are fish who only swim for short periods of times and usually stick close and take shelter within the reefs.

Gills, as we all recognize, are the organs a fish uses to breath, but we do not generally know exactly how they work. Well, this is a simple process! A fish's heart only has 2 chambers (a human’s heart has 4) and the blood pumps in only one way. When the blood passes by the gills, it picks up oxygen from the water and carries it back to the heart.

Like humans, fish also have senses. These senses come from various sources including, but not limited to, light [for vision], chemicals in the water [for smelling and tasting], and vibrations [for hearing].

Now, let’s focus on the schools of fish phenomenon. They are more than they're made out to be in kids’ movies. Research has shown that swimming in large groups is advantageous for fish in many ways. The main advantage is protection from predators, since swimming in a large group makes the fish appear to be a larger animal, which in turn might lead to scaring off the predator. If this doesn't work, then there is also truth in the idea of safety in numbers. A fish swimming alone will surely be surprised, caught and eaten by a predator; but swimming with 50 other fish will lower each fish’s chance of being seized to 1 in 50. Also, with 50 other fish there are 100 more eyes to look out for predators, and a better chance of spotting the predator before it spots them.

Reproduction is another reason why fish swim in groups. Some fish get together only when it is time to mate, and then they create a frenzied mating school of fish. The females release all their eggs and the males fertilize the eggs in mass quantities. The reason this is done is because there are some fish out there that eat other fish eggs. Reproducing in mass quantities (also in a large group) give the eggs more of a chance to develop safely. A predator may still be able to eat some eggs, but some will undoubtedly make it through to hatch and grow.

A final reason why fish swim in large groups is for traveling. The fish in front have the most difficult time in the group, because they have to do most of the work. They must cut through the water, making it easier for the other fish to swim through the water as well. However, this makes it easier for fish to travel long distances without getting tired. Moreover, with so many fish, their directional abilities get better.

Imagine being able to tie your body into a knot. This is what the Agnatha class of fish is able to do, and it is one of the things I personally found most interesting. This fascinating class of fish is the most primitive one, yet unfortunately it is considered to be close to extinction. In fact, there are only two species in this class that are currently still in existence, namely the hagfish and the lamprey. What makes these two fish species so intriguing and why are they assigned to this primitive class? Well, first of all, there is the lack of bones in their bodies, making the fish extremely flexible and yes, they can tie themselves into knots, and do! Something else that distinguishes them from other fish is their lack of scales. Instead of the scales that most fish have, the Agnatha have a smooth slimy skin covering their body. Another interesting fact is that these fish are jaw-less; they have a sucker instead of a jaw. This sucker sticks to prey, and tears right through the skin, allowing the Agnatha to suck out food. Sounds like something from an old 70's horror movie, right? Here's an additional fact that would make it fit right in with those movies: the hagfish have no eyes! Imagine the way that must look. The lamprey fish, on the other hand, have very attuned eyes. Where can you find these fish? Well, the Agnatha's can live in both fresh water and salt water, and some can even live in both during their lifetimes.

In conclusion, ichthyology covers a wide area of biology, and there is a great deal to learn. This lecture discussed only some of the primary concepts that are researched in this scientific branch, namely the classification of fish, their reproduction, as well as their swimming, breathing, senses, and schooling. If you want to learn more, I suggest you take a look at the following web sites: - A database of information of fish - A brief history of Ichthyology - The Florida Museum web site\ - A web site with some great information on Ichthyology

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