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The Taxonomy of the Fish

There are approximately 26,800 aquatic vertebrates referred to as fish. Forty percent of all the fishes evaluated in 2007 (3,100 fishes assessed) are classified threatened (IUCN 2007).

The classification of fishes is a subject of considerable debate. For a long time, the living fishes have divided into three different classes. These included the jawless hagfish and lampreys (Class Agnatha), the cartilaginous sharks and rays (Class Chondrichthyes), and the boney fish (Class Osteichthyes), but Class Agnatha was then split into two classes, Class Myxini (hagfish) and Class Cephalaspidomorphi (lampreys).

Vertebrate systematists do not consider hagfishes or lampreys to be fish because they belong to a much more primitive lineage than any other group that is more commonly defined fish (Chondrichthyes and Osteichthyes). In fact, the taxonomic term fish refers only to a specific clade of vertebrates, the actinopterygians, which are the ray-finned fish. However, we will still describe hagfish and lamprey below.

Existing vertebrates comprise two clades: the Hyperoartia (lampreys) and the Gnathostomata (jawed vertebrates).

Hagfish (Class Myxini, also known as Hyperotreti)

Hagfish, also referred to as blind eels, are found only in marine environments and feed mainly on dead fish. Despite being nearly blind, hagfish easily find food with their keenly developed sense of smell and touch. They have no jaws but with their rasping tongues, they burrow into carcasses and feed from within.

Hagfishes have a skull of cartilage but lack jaws. They also lack vertebrae but have a notochord, which they retain in adulthood as a strong, flexible rod of cartilage. Hagfish give off large amounts of slime possibly to repulse other scavengers or to deter a potential predator. They are an unusual “fish” (disputed if fish) as they are hermaphrodites. They produce only sperm or eggs at any one season, but they can produce sperm one year and eggs the next.

  • No jaws

  • No complete eyes

  • No true stomach, and skin respiration in addition to gills

  • Marine scavengers

  • Capable of producing large amounts of mucus or slime

  • Spawn repeatedly throughout their lives and produce few large eggs each time

  • Bind, toothless ammocoete larvae typically burrow in the bed of silty streams

  • Many adults are parasitic on other vertebrates

  • Eel-like body without scales and without paired fins

  • No dorsal fin, only a confluent caudal fin

No species of hagfish are listed endangered under the IUCN Red List.

For more information on hagfish click here

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An early fish
© Endangered Species International

Hagfish (Eptatretus cirrhatus) feeds mainly on dead fish
© 2006 Paddy Ryan

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