What are the characteristics of fish?
Fish are vertebrate animals (animals with a backbone) that have gills and live in the water (although some fish are amphibious and can come onto land for periods of time). Fish come in many different shapes and sizes. And not all of them look like fish!
To the right is a photo of a spiny seahorse. Is it a fish? It certainly lives in water. It has a backbone. And yes, it has gills. Seahorses are fish! Like most fish, seahorses have a swim bladder which helps them move up or down in the water. Seahorses are not very good swimmers though. For that reason, they often stay in one place for days, holding onto something by wrapping the end of their tail around it. While many seahorses are camouflaged in their environment, others can change their color to further blend in.
Would you like to hear about some other fish, and learn more about the common characteristics of fish?
Do you say "Fish" or "Fishes" to mean more than one fish? What's correct?
(You'll find the answer farther down this page, but first, see if you can answer it!)
Characteristics of fish
- Gills - Water goes into the fish's mouth and then over their gills (which are made up of rows of gill filaments) before leaving the fish out of the gill slits. There are many blood vessels in the gills. Oxygen from the water moves into these blood vessels. Once the oxygen is in the blood, it is carried throughout the fish's body, supplying the fish's cells with oxygen.
Fish gills have countercurrent flow, which means that water passes over the gills in one direction, while blood moves through the capillaries in the opposite direction. Countercurrent flow increases the efficiency of respiration.
- Single loop blood circulation (except for lungfishes) - This means that the blood flows through the heart to the gills, to rest of the body's tissues, and back to the heart in a single loop. Fish have two pumps, but four parts to their heart: the sinus venosus ( a blood collection chamber), the atrium, the ventricle (a pump), and the conus arteriosus (another pump).
- Backbone - which is sometimes made of cartilage and sometimes made of bone. Fish brains are encased in a skull.
- Fins - help fish swim in a similar way as a paddle helps us on a boat. Fins provide a large surface the fish can use to push against the water.
- Reproduction - Most types of female fish lay eggs that are fertilized by the male, by a cloud of sperm cells, once the female releases the eggs from her body. This is not true for all types of fish though/ The eggs of sharks, for example, are fertilized while they are still in the mother's body, and begin their development within her body, being born when they are mature enough to live on their own.
Fish? Or Fishes?
Did you know that if you are talking about several of the same kind of fish it's correct to say "fish," but if you are talking about several different species, it's correct to say, "fishes?" That's why biology textbooks refer to the "Fishes of the Amazon River."
Jawless fish have no jaws, no scales, and no bones. Their mouths contain structures for sucking, scraping, or stabbing their food. Their skeletons are made of cartilage.
- Hagfish are one type of jawless fish.
Hagfish look like large worms and are scavengers that eat dead fish.
Hagfish have a couple of interesting defense mechanisms. They create slime and can tie themselves into a knot. When another fish tries to bite a hagfish, the hagfish gives off an enormous amount of slime, causing it's predator to release it to avoid choking on the slime! Being able to tie themselves into a knot also helps hagfish get away from predators!
Short Video of Hagfish Producing Slime and Tying Itself In A Knot
- Lamprey are another type of jawless fish.
Some species of lamprey are parasitic, using their mouths as suckers to attach to other living fish. Once attached, they suck the blood of the larger fish. You can see a lamprey attached to a bigger fish in one of the drawings below.
Although there are 38 species of Lamprey, only 18 of these are parasitic.
See if you can identify the parts of this Lamprey (below). You can look at the labeled image above as a guide.
Cartilaginous fish - Sharks, Rays, and Skates
Cartilaginous fish have jaws, rough scales, and skeletons made out of cartilage and calcium carbonate. Their skeletons are light and strong. Sharks, rays, and skates are all cartilaginous fish..
Sharks have an excellent sense of smell because the part of their brain associated with smell is twice as large as other parts of their brain. This allows sharks to detect a tiny drop of blood in 25 gallons of water!
Most sharks rely on swimming or water currents to keep water passing over their gills. They must even take care to position themselves in a current while they are sleeping / resting.
Sharks have cone-shaped scales which are very similar in structure to their teeth. They have 6 to 10 rows of teeth, with the sharp, mature teeth in the front rows and immature teeth growing behind them. The teeth in the back move forward to replace the front teeth, as needed, operating almost like a conveyor belt.. A shark may lose 30,000 or more teeth over the course of his lifetime!
Rays and Skates
Rays and Skates are graceful flatfish that almost seem to fly like birds through the water. When they are not swimming, they often bury partially under the sand on the ocean floor.
Rays and Skates have tails. The tails of some types of rays, such as those of a stingray, have barbed stings and venom glands on them. Stingrays use their tails only in self defense.
Watch this beautiful Manta Ray as she swims over a reef.
National Geographic Documentary on Stingrays
Bony Fish - Bony fish are what you probably think of when you think of a fish!
Flounder, bass, and trout are a few of the many kinds of bony fishy.
Bony Fish Have:
- Skeletons made of bones.
- Lateral line system - a sensory system along the sides of a fish's body. It helps the fish perceive its rate of movement through the water as well as its position. A fish's lateral line system can also help it detect a motionless object. Similarly to the way our inner ear works, the cilia on a fish's sensory cells detects vibrations and passes that information on to the brain.
- Operculum - These can open and close over the gills, producing a cover. These gill covers allow the fish to move water over its gills even when the fish is not moving.
- Swim Bladder - This is a gas sack which keeps a fish from sinking. As the swim bladder fills with gas, the fish rises higher in the water. Likewise, as the swim bladder empties, the fish sinks.
The anatomy of Hector's lanternfish (above)
A: dorsal fin
B: fat fin
C: caudal fin
D: anal fin
E: pelvic fins
F: pectoral fins
G: operculum (gill cover)
H: lateral line
I: caudal peduncle
There are two groups of Bony Fish
Ray-Finned Fish and Lobe-Finned Fish are the two groups of bony fish. Shown here is a Ray-Finned Fish. Here are the parts of a Ray-Finned Fish.
A: lateral line
C: swim bladder
D: Weberian apparatus
E: inner ear
H: internal sex organs (ovaries or testes)
J: gall bladder
O: dorsal fin
P: fin rays
Q: tail (caudal fin).
R: anal fin
T: ventral fins
LisaDH on September 10, 2014:
The diversity in fish is amazing. It seems strange to think that seahorses, rays and sharks are all fish. But then again... horses, moles and bats are all mammals, and they're pretty diverse, too.
Tony Payne from Southampton, UK on September 08, 2014:
Wow that was really interesting information. I have seen a lot of fish in my time, mostly when snorkeling and diving in a lot of locations, but I never thought so much about the different types.