Eel catfish

Quick Take

• This fish hunts on land, though its technique is nothing like what you'd expect from a creature with no legs. See how it hunts on land →
• Scientists believe this fish holds a clue about how life first crawled out of the ocean 400 million years ago. Explore the evolutionary clues →
• Its body has a hidden feature that lets it survive outside water, though it is not what most fish have. Discover its air-breathing organs →
• Keep one as a pet and there's one mistake that will guarantee it escapes the tank. Get the aquarium care tips →

The eel catfish are air-breathing fish with long, snake-like bodies. Local people catch and eat them, and institutions and private collectors around the world keep them in aquariums. What’s most intriguing about these fish is that they breathe air, and they reach out of the water to catch beetles on land! Scientists think that they may be an evolutionary link between fish and lizards.

Eel Catfish Facts

• This fish is related to catfish. It is called an “eel” catfish because of its similar appearance to eels.
• Beetles are its favorite food. It reaches out of the water to snatch them from land.
• It breathes air. Scientists think it may be an evolutionary link between fish and land-dwelling animals.
• Eel catfish are not dangerous. People eat them and keep them in aquariums.
• Although not endangered, they are vulnerable to parasites and polluted water.

Classification and Scientific Name

Eel catfish refers to catfish whose tails are elongated in an eel-like fashion, classified in the family Plotosidae (“eeltail catfishes”), which contains roughly 35 to 41 species across 10 genera. They are native to the Indian Ocean and western Pacific from Japan to Australia and Fiji.

However, “eel catfish” is also used as a casual English common name for individual species within that family — most often Plotosus canius and Plotosus lineatus.

Appearance

The eel catfish are species of ray-finned fish. They are black or dark brown. Because they live in dimly lit, muddy conditions, they have small, hidden eyes.

Although they are not scientifically classified as eels, their snakelike bodies resemble those of eels. They have long, slender bodies with widely spaced spines. They have no pectoral fins, but their dorsal fins run around their entire body, forming a single fin that includes the caudal and anal fins.

Their jaw muscles are hypertrophic — enlarged at the cellular level — an adaptation that gives them an especially powerful bite.

The eel catfish has special tree-like organs growing from its second and fourth gill arches, which allow it to breathe oxygen directly from the air for short periods of time.

They can grow up to 12.9 inches long, which is about the same as the height of a bowling pin. Females are broader in the body than males. They are one of the smallest kinds of catfish regularly hunted by anglers.

Behavior

Eel catfish are nocturnal and enjoy dim light conditions. They like to hide in submerged branches, burrow into sand and gravel, and make nests in tangled exposed tree roots on river banks and swampy areas. They congregate with their own kind. In an aquarium, it is best to keep three or four of them together.

One of its most interesting behaviors is that they have two different ways of eating. In the water, they suck water into their mouths to capture pieces of vegetation, worms, or small fish. However, they can also hunt on land. They watch from the water until a beetle walks by, then propel themselves out of the water. Using their specially adapted, flexible spines, they stabilize their bodies in the water while reaching out, bending their heads down, and using their jaws to snatch up the beetle.

It’s interesting to compare this method with another fish that feeds on land — the mudskipper. Unlike the eel catfish, the mudskipper actually uses its pectoral fins to pull its whole body onto land and drag itself short distances.

Scientists think these types of behaviors represent an evolutionary transitional step between fish and lizards. The eel catfish appears similar to fossils from the Devonian Period, 400 million years ago, when scientists believe sea creatures first evolved into terrestrial creatures.

Habitat

Plotosidae habitat and distribution span a wide range of aquatic environments, and the family splits fairly cleanly into marine/brackish and freshwater lineages.

Geographic range

Eeltail catfish are native to the Indian Ocean and western Pacific, ranging from Japan to Australia and Fiji. The family is especially diverse around the Indo-West Pacific, with strong representation in Australia and New Guinea.

If you decide to keep one as a pet, provide plenty of driftwood for hiding places, and keep it in a covered tank with a weight on top to prevent it from climbing out. The ideal temperature range for this type of fish is 71.6 to 80.6°F. The ideal pH range for their water is 6.5 to 7.8.

The best tank mates would be more of the same species. But they can be combined with other West African species, such as medium-sized African barbs, tetras, and cichlids that keep to the upper layers of the water, while the eel catfish burrow in the sand.

Diet

They are omnivores. Their favorite foods are Coleoptera (beetles), worms, and shrimp. They are not picky eaters. In captivity, it will eat any kind of pelleted or frozen fish food, such as bloodworms and shrimp, or live earthworms.

Predators and Threats

In their natural environment, eel catfish are hunted by larger fish and jungle carnivores, but humans are one of their main predators.

The full size of their population is unknown, but they are widely distributed and are not considered endangered. They are listed as of “least concern” by the IUCN Red List of Threatened Species.

They are vulnerable to environmental degradation, including water pollution from human activities and bacterial or parasitic infections.

Reproduction and Life Cycle

Eel catfish make nests in tangles of exposed tree roots near river banks. Females are larger than males. They reproduce sexually, but not much is known about their breeding habits. Researchers have found egg-filled individuals, but these fish have not been successfully bred in captivity.

Eel catfish are slow-growing. Their average lifespan ranges from 5 to 18 years. As they get older, they tend to become more territorial and solitary. They may even eat smaller specimens of their own species.

Population

Their exact population size is unknown, but they are not considered endangered.