Is an eel a fish? They are aquatic animals who breathe with gills and swim through the water. Still, they look so much different than the other fish species we’re used to.
To understand whether an eel is a fish, we first need to know what makes a fish a fish. Then, we can decide if an eel fits into that category.
What Are Fish?
It’s hard to define what a fish is because the category is so vast. More than half of all vertebrates consist of fish.
There are about 54,000 known vertebrates in the world. A staggering 33,000 species consist of fish.
Many scientists argue over the definition of a fish. But, there are two widely accepted definitions.
According to the University of Hawaii, “Fish are aquatic vertebrate animals that have gills but lack limbs with digits, like fingers or toes. Recall that vertebrates are animals with internal backbones. Most fish are streamlined in their general body form.”
OR
Fish are “poikilothermic, aquatic chordate(s) with appendages (when present) developed as fins, whose chief respiratory organs are gills and whose body is usually covered with scales.”
To break this down, to be a fish, an animal must have at least some of these characteristics:
1. Poikilothermic — the animal’s body temperature adjusts along with the outside temperature. In the case of a fish, this means the temperature of the water.
2. Aquatic — the animal must live in freshwater or saltwater. There are some fish who can move onto land, but they must stay moist, and they must return to the water.
3. Chordate — a chordate is an animal with notochords. Notochords are stiff cartilage that protects and supports the nerve cord.
4. Gills — most fish have gills that they use for gas exchange. (Note that some fish breathe in other ways).
5. Scales — most fish have scales or some version of them. But, this IS NOT true for eels.
6. Fins — most fish have fins that are thin, flat, and moveable.
So, are eels a type of fish?
Eels are poikilothermic, aquatic, chordates, and have gills and fins. So, yes, eels are a type of fish.
What Are Eels?
All ‘true’ eel species have a few characteristics in common. These characteristics set them apart from other eel-like fish. Wolf eels and electric eels, for instance, are not eels.
A ‘true’ eel is a fish that:
- Has a continuous dorsal fin that connects with the caudal and anal fins
- Small, embedded scales or no scales
- They spend part of their lives in freshwater and part in saltwater
- They produce slime
- They have many sharp teeth
- Juvenile eels are flat and look like a ribbon
How Many Species Of Eels Are There?
There are 19 families consisting of over 800 types of eels.
Where Do Eels Live?
Where an eel lives depends on its species. There are both freshwater and saltwater species.
Eels live all across the world, including in the frigid temperatures of the polar oceans. Some species span the world’s oceans. Other species are more localized.
Most saltwater eels live in shallow waters. They can bury in the soft mud or sandy substrate of the ocean floor. There are also several species of saltwater eels that live in the deep ocean, but we don’t know much about them yet. There are even a few species that live in the open ocean.
Freshwater eels do not spend their entire lives in freshwater. They are catadromous, meaning they spawn in the ocean. So, for most of their adult lives, they live in freshwater. But, they are born and die in saltwater.
Eel Lifespan
The lifespan of eels also depends on the species.
On the low end, American eels only live for about 5-15 years. Some are able to live for up to 20 years, but that’s rare.
On the higher end, species like the European eel can live for over 80 years.
The interesting thing about eels is that they don’t reach sexual maturity until they’re ready to die. As soon as an eel reaches sexual maturity, they travel to the ocean to spawn. Then, they die almost immediately after spawning.
Eel Colors & Characteristics
We recognize eels by their long, slender, snake-like bodies.
They have a long dorsal fin that runs the length of their body, connecting with the anal and caudal fins. But, they lack pelvic fins, and most lack pectoral fins as well.
To swim, they use that long fin to move in a wave-like motion. The water glides across their fin, creating movement like that of a snake. They can even swim backward by changing the direction of the movement of their fins.
Unlike most fish, eels do not have scales. Instead, their bodies are smooth.
Eels come in all different colors. Those that live deeper in the ocean tend to come in dull colors of gray, brown, or black. Those that live closer to the surface and in reefs are more colorful.
Eel Temperament & Behavior
Most eels are nocturnal. They hide within crevices and rocks during the day before coming out at night to hunt.
Most species are solitary throughout the duration of their lives. Some species, like garden eels, live in colonies consisting of hundreds of individuals.
What Do Eels Eat?
A carnivorous, predatory species, eels hunt mostly for smaller fish, invertebrates, and crustaceans. They will sometimes eat insect larvae. As they are vicious predators, eels are even known to be cannibalistic.
How Big Can Eels Get?
Eels range in size from the tiny Snyder’s moray eel (Anarchias leucurus) that only reaches about 4.5 in (11.5 cm) in length.
Other eels can reach enormous sizes. The slender giant moray eel (Strophidon sathete) can grow up to 13 ft (3.9 m) in length. There is also the huge European conger eel (Conger conger) which can grow up to 9.8 ft ( 2.98 m) long and 350 lbs (158.8 kg).
How Are Eels Born?
Eels spawn in saltwater, so larvae are born in the ocean. As they grow, they move from saltwater to freshwater. Juveniles spend most of their time in estuaries and at the mouths of rivers. As adults, they live in purely freshwater, often in streams and rivers.
When it is time to spawn, adult eels travel from their freshwater home, back to the ocean. They spawn in the ocean in large groups and die shortly after.
Which Eels Are Not Really Eels?
There are fish that we refer to as ‘eels’ that are not really eels, although they are fish.
1. Pipefish are long, eel-like fish covered in armored plates. They live in shallow areas like estuaries and the mouths of rivers, as many freshwater eels do.
But, they are NOT an eel. Instead, they are relatives of seahorses and seadragons.
2. Cusk Eels are long-bodied fish with a continuous fin that runs the length of their bodies. Although they look like eels, they are relatives of tuna and seahorses.
3. Hagfish are long, slimy, eel-like fish. They are not eels because they do not have spines, and don’t even have a jaw. They are barely even a vertebrate
Still, they have a skull as well as teeth that attach directly to the mouth and throat muscles.
4. Electric eels are long fish that produce electricity and look like eels. But, they are a type of knifefish.
Conclusion
To be a fish, an animal must have a certain set of characteristics. Characteristics include being poikilothermic, aquatic, and a chordate. Most also have gills, fins, and scales. It’s not necessary for an animal to have ALL these characteristics, but it must have some of them.
An eel is a scaleless fish with a long, continuous fin down its back. Although it looks and moves like a snake, it is a fish.
FAQs
Eels used to be abundant across the world. But, because of human activity, many of their populations are dwindling. The pollution of waterways, overfishing, and habitat destruction all play a part in the dying eel populations.
Eric Feunteun, a French researcher, says, “When I was young, eels were in every river and estuary. My grandmother had a cafe… and sometimes customers who were down on their luck would bring a bucket of young eel to pay for their coffee.”
Less than 50 years later, the eel population in Europe is 10% of what it was in the 1960-70s.
The International Union for the Conservation of Nature (IUCN) lists European eels as ‘critically endangered’. American and Japanese eels fall just behind as ‘endangered’.
Although eels and sea snakes look similar, they’re two completely different animals.
Eels are fish that breathe with gills and have razor-sharp teeth. Sea snakes are reptiles that breathe with their lungs and have venomous fangs.
Eels have a single, continuous fin that they use to swim, while sea snakes have a paddle-shaped tail.
