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Fish Classification

Explore the five classes of fish and their physiological characteristics.

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Primitive Fish Classification

Types of Primitive Fish

There are about 28,000 existing species of fish, and they are placed in five different classes. The classes are commonly referred to as hagfish, lampreys, cartilaginous fish, ray-finned fish, and lobe-finned fish (see the table in the previous lesson).


Hagfish are very primitive jawless fish. They retain their notochord throughout life rather than developing a backbone, and they lack scales and fins- Lacking paired fins means they are not strong swimmers and would struggle being predators. Hagfish are noted for secreting large amounts of thick, slimy mucus. The mucus makes them slippery, so they can slip out of the jaws of predators.

Slime...It's Not Just For Defense Anymore

A Pacific Hagfish (Eptatretus stoutii). Do you think that is the anterior or the posterior of the animal? Photo by Celeste Ramsay

Slime as a Fashion Statement

Are you ready for a hagfish suit? What's a hagfish? A hagfish is a type of jawless fish which has fascinated and grossed-out people for centuries. They may be best known for their habit of wriggling into dead things and eating them from the inside out, but a close second is the copious amounts of slime they can produce. A 6-inch hagfish can fill a gallon bucket with slime in minutes. Take a look at this clip to find out more about this amazing slime. Notice the white spots hovering around the hagfish, they are part of a mucus/slime veil which surrounds the fish. Can you see how far from the animal the veil extends before the hagfish is handled? Does it appear to be uniform around the hagfish?

Notice how cohesive the slime was even when removed from the hagfish, and how it can be pulled and manipulated. These properties come from protein threads which are exuded with the slime and will be important for making a hagfish suit. But let's go back to the hagfish for a little bit before returning to the suit.

Jawless fish are the earliest fish found in the fossil record at a date of 530 MYA. These fossils are part of the Chengjiang fauna and were found in shale deposits in Yunnan Province, China. Their discovery opens up all kinds of questions about the tempo of evolution and the Cambrian explosion, but we don't need to worry about that right now. The oldest hagfish fossil is only 320 million years old. It was believed in the 1800s that the hagfish's simple body plan had to do with it adapting to a parasitic lifestyle and losing characteristics it shared with other fish. This is a very plausible theory, however it is also untrue. We now know that hagfish do actively hunt (you'll see this in the below clip) and are not parasitic or solely scavengers. So what does this mean? Well, the oldest jawless fish are 530 million years old, but they are not hagfish. Scientists are still working out how they are related to hagfish. Moreover, despite what some of you may have heard, hagfish are not living fossils, they are not our ancestors, and they are not primitive though they do utilize a very old body form.

So what about the slime? Why do hagfish make this? Watch this clip carefully, remember the first clip and what the slime looked like, remember also its cohesiveness. Also, make special note of the hunting behavior of the hagfish. Why do you think this behavior was unknown for so long?


Did you notice the slime "ball" left in the shark's mouth? This is particularly bad news for the shark as the cohesiveness of the slime means it can completely cover the sharks gills if ingested. Shark's take in water through their mouths and pass it over their gills in order to obtain the oxygen they need. This system doesn't work so well if a slime membrane covers the gills or fills the mouth. Imagine what it would be like trying to breath with your face covered with plastic wrap or even if you tried to breathe through your mouth with a plastic bag in it. So this slime has definite defensive properties, but what about the predation later in the clip? The hagfish appears to secure its prey, wrap around it, exude slime and then carry the prey away from the slime cloud. Is this really what it is doing? Maybe, but it appears that the slime could also have functions beyond defense. As a matter of fact, recent research indicates that hagfish can absorb nutrients through their skin in a manner reminiscent of flatworms and some aquatic annelids. This certainly casts a different light on their habit of wriggling into corpses and suggests that their trademark slime may have yet another function ( Hagfish Just Got More Disgusting).

So you see hagfish are pretty sophisticated animals we are still learning much about, but what about a hagfish suit or a dress? Researchers are now investigating hagfish slime as a source of natural fibers which could be used for manufacturing clothes. Currently, many of our clothes are made from synthetic fibers. Apart from clothes, synthetic fibers like Kevlar® are used in bullet-proof vests and as structural reinforcements. But currently, our most widely used synthetic fibers are petroleum based, which means the carbon that makes up the backbone of these fibers comes from fossil fuels. Fossil fuels are a limited resource, so any possibility to replace petroleum-based products with products from a renewable resource like hagfish slime can have long-term positive benefits for humankind. Watch this clip to see fibers being pulled from hagfish slime.

So what’s going on in this clip? Well, researchers isolated the protein component of the slime (the component which gives it its elasticity), and that’s what they add to the container with the pipette. The proteins then spread out across the surface of the liquid. When the researchers then grab a piece of the protein film and pull it, the pulling action serves to align and arrange the protein molecules to form a fiber. This fiber is then further manipulated to alter the molecular arrangement of the proteins in the fibers and improve physical properties like strength. So are these textile-quality fibers? Can you buy a hagfish fiber suit? Not yet, but initial results are promising, and you may be able to in the near future. What we do know is that this marvelous substance which has helped hagfish survive as a species has applications far beyond the world of hagfish and provides another example of the unexpected things we can learn from studying the organisms with which we share this planet.

Extension Investigation

Use the below resources to answer the following questions

  1. Why is hagfish slime a more attractive natural source of fibers than spider silk?
  2. Hagfish hatch as small versions of the adult. How does this differ from development in other fish? Does this seem like an ancestral trait to you? Does this support the view that despite outward appearance hagfish are not primitive creatures? Why or why not?
  3. What is unique about the hagfish's gonads? Does this situation surprise you? Why or why not? Do you think they may provide some clues about how separate sexes evolved?


Like hagfish, lampreys also lack scales and are jawless, but they have fins and a partial backbone. The most striking feature of lampreys is a large round sucker, lined with teeth, that surrounds the mouth (see Figure below). Lampreys use their sucker to feed on the blood of other fish species.

Sucker Mouth of a Lamprey. The mouth of a lamprey is surrounded by a tooth-lined sucker.

Invading Species Awareness PSA - Sea Lamprey

Describes the problems associated with the Great Lakes sea lamprey invasion and how Canada and the United States have been able to control their numbers.

Cartilaginous Fish

Cartilaginous fish include sharks, rays, and ratfish (see Figure below). In addition to an endoskeleton composed of cartilage, these fish have a complete backbone. They also have a relatively large brain. They can solve problems and interact with other members of their species. They are generally predators with keen senses. Cartilaginous fish lack a swim bladder. Instead, they stay afloat by using a pair of muscular fins to push down against the water and create lift.

Cartilaginous Fish. All of these fish belong to the class of cartilaginous fish with jaws. (a) Oceanic whitetip shark (b) Ray (c) Ratfish

One of the most important traits of cartilaginous fish is their jaws. Jaws allow them to bite food and break it into smaller pieces. This is a big adaptive advantage because it greatly expands the range of food sources they can consume. Jaws also make cartilaginous fish excellent predators. It you’ve ever seen the film Jaws, then you know that jaws make sharks very fierce predators (see also Figure below).

Jaws of a Shark. Sharks have powerful jaws with multiple rows of sharp, saw-like teeth. Most other fish are no match for these powerful predators.

Introduction to Skates and Rays

Learn about the fascinating creatures skates and rays, which are fish that are closely related to sharks.
Open the resource in a new window.

Queen of the Rays

Reef manta rays (Manta alfredi) spend all their time hanging around their home reefs

You Mean Those Aren't The Same

Sometimes scientists can't make up their minds. They used to think there were lots of species of manta rays because of color differences. Then they decided there was only one species but lots of different color schemes based on mitochondrial DNA analysis. Everyone was happy with this except for some of the people who spent a lot of time in the field swimming with manta rays. They noticed behavioral differences between manta rays and a size difference to go with the behavior. Then they found the kicker--a vestigial stinger to go with the size and behavior differences. Yep, Andrea Marshall saw what everyone else had missed. There wasn't one species of manta ray. There were two... At least as far as science knows now.

Things to think about

  1. Do you think it matters if scientists look at specimens in the lab and don't observe animals in their native environment? Why or why not?
  2. What kinds of information do you think you could gather by having an animal in a lab that you could not gather in the field? Do you think this kind of information justifies holding animals in labs indefinitely? Why or why not? Do you think this kind of information justifies capturing them and then releasing them?
  3. Explain how the behavioral differences between these two species of manta rays could have led to the formation of the species? Explain your thinking as fully as possible.
  4. Why is the identification of the two species vital to the conservation of the species? What kinds of information do scientists need to have a good chance at being successful in conservation efforts?


Just like the largest whales and the largest sharks, the largest rays feed on some of the smallest creatures, plankton.

Interactive Anatomy: The Great White's Weapons

This interactive model of a shark demonstrates its anatomy by featuring its most significant body parts.
Open the resource in a new window.


  • There are about 28,000 existing species of fish, and they are placed in five classes: hagfish, lampreys, cartilaginous fish, ray-finned bony fish, and lobe-finned bony fish.


Use these resources to answer the questions that follow. These are really helpful: 

  1. Describe the body style of agnathans.
  2. Describe the body covering of cartilaginous fish.
  3. Compare rays to skates.
  1. Describe the main differences between the two groups of jawed fishes living today.


1. Assume that a new species of fish has been discovered deep in the ocean. It has a complete vertebral column made of cartilage. Which class should the new species be placed in? Name one other trait you would expect to find in the new species of fish. Explain your answers.

2. Fish with jaws may be very large. Infer how their jaws may be related to their large body size.

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