Fishes have been dominating the world's suitable aquatic environments for millions of years, developing into an estimated 28,000 species today. Multiple species have been forced to adapt to their environments, other species, and the impact of humans. For example, many sharks have the ability to add salt to waters where the salinity levels are low. And the skin of many fish has softened over time.

A fish is defined to be an aquatic vertebrate, with gills, fins, and scales. Fish came from an unknown common ancestor, but certain ostracoderms gave rise to multiple fish species, generating into the dense populations we know today. It has even been discovered that the ancestor of tetrapods emerged from one group of fish, the Sarcopterygians.

*NOTE - The term "fishes" refers to more than one species, while "fish" refers to individual(s) within the same species.

Myxinomorphi


The superclass known as Myxinomorphi began to rule the seas about 1.5 million years ago during the Ordovician Period of the Paleozoic Era. The Myxinomorphi are a type of Agnatha, or a jawless fish with no apparent appendages. This superclass is comprised of the Myxini, or Hagfish, which is made up of about 70 different marine species. The best known Hagfish species known to North America are the Atlantic and Pacific Hagfish. Hagfish have the...

Myxinomorphi (continued...)

unique ability to secrete slime, and a developed vertebrae is not present. Although Hagfish are jawless, they are not parasitic like their distant cousin, the Lamprey. They are mainly scavengers, that have been known to (in the past) pester commercial fishermen by eating the contents of captured fishes, leaving the useless parts behind. Surprisingly, Hagfish are almost completely blind. They have an incredible sense of smell and touch that quickly attracts them to other foods, such as mollusks, and other crustaceans.

Petromyzontomorphi

The Petromyzontomorphi appeared during the Ordovician Period of the Paleozoic Era, along with their fellow Agnathans, the Myxinomorphi. This superclass is composed of Petromyzontida, or Lampreys, which are made up of 38 different species. Lampreys live in both fresh and salt water , and have been known to live in the Great Lakes. Of the 20 species of Lamprey that are found in North America, half are parasitic, and the other half never feeds after metamorphosis. Spawning usually takes place in freshwater streams during the spring and winter, and Lampreys usually die soon after.

Gnathostomata

The Gnathostomata superclass emerged from a group of Agnathans during the Silurian and Devonian periods of the Paleozoic Era. Gnathostomes are easily recognized compared to Agnathans, since their characteristics are what usually come to mind when people are asked to describe a fish. Gnathostomes include all fish that have jaws and paired appendages. The Gnathostomata classification includes other major classes such as the Chondrichthyes (sharks, rays) and the Actinopterygii (gars, sturgeons). Gnathostomes can be found wherever there is water, as long as the conditions are suitable.

Gnathostomata (continued...)

A major of Gnathostomata is the Chondrichthyes, which contains all cartilaginous fish, such as sharks and rays. Chondrichthyans first appeared during the Devonian period of the Paleozoic Era, and have built their numbers up to an estimated 970 species today. The class is almost entirely marine. Bone is completely absent throughout the class, and the assemblage of Chondricthyans is less complex compared to other classes. However, certain adaptations, such as sensitive sense-organs and powerful jaws have helped this class flex a keen role as a dominant predator. This class also contains the subclasses of Elasmobranchii (sharks) and Holocephali (chimeras).

Gnathostomata (countinued...)

The final major class of Gnathstomata is the Actinopterygii. This class emerged during the Devonian period of the Paleozoic Era. This class is the largest, with an estimated 27,000 species today. The Actinopterygii can be found in all aquatic environments, and have a uniquely complex physical assemblage. This class contains three subclasses, the Cladista, the Chondrostei, and the Neopterygii. All of which have herbivorous and carnivorous species.

BIBLIOGRAPHY

• Hickman Jr., Clevland P.. "Animal Diversity: Fifth Edition". McGraw-Hill Companies Inc., 2009

Vocabulary

Tetrapod ~ land creature
Ostracoderm ~ ancient fish with armored skin
Marine ~ saltwater (ocean)
Parasitic ~ latches onto prey and sucks blood
Appendage ~ fin, tail, etc.
Assemblage ~ body structure

FISH