Eel

Eels are long fish.

Most eels live in the salty sea. Some eels live in fresh river water. They hide in sand or rocks.

Eels start life as tiny, clear bits. They drift in the water to eat. They grow and change as they get older. Eels move by making waves with their bodies. They can even swim backward!

The European conger is a very heavy eel. It is a truly amazing fish.

Eels are long, thin fish. There are about 1,000 different species. Most eels live in the salty ocean. Some live in fresh river water.

Many eels are nocturnal. This means they are active at night. They often hide in sand, mud, or rocks.

Eels have a special way of swimming. They make waves move along their bodies. They can even swim backward by reversing these waves.

The European conger is the heaviest true eel. It can reach a length of 300 cm. It can weigh up to 36 kg. Eels start life as tiny, clear larvae. We call these leptocephali. They drift in the sea and eat marine snow. Marine snow is small bits of food in the water. They change into glass eels and then elvers. Most elvers move into fresh water. They may have to climb over dams or waterfalls.

Some eels, like the electric eel, are not true eels. They just have a similar shape.

Eels are amazing, long-shaped fish that belong to a group called Anguilliformes. There are about 1,000 different species in this group.

Most eels live in the salty ocean, but some live in fresh water. For example, the genus Anguilla spends most of its life in fresh water. Some eels are also predators that hunt other animals. You might hear the name "eel" used for other fish too. The electric eel is a famous example, but it is not a true eel. It is actually a knifefish related to catfish.

Eels have a very special way of moving through the water. They do not have pelvic fins like many other fish. Their dorsal and anal fins are fused together. This creates a single long ribbon that runs down their body.

To swim, they create waves that travel along their entire length. This is called propulsive waves. If they want to move backward, they just reverse the direction of the wave. This allows them to be very tricky swimmers. Most eels like to hide in sand, mud, or rocks. Many are nocturnal, which means they are most active at night.

We can learn about the history of eels from fossils. The earliest fossil eels were found in Lebanon from the Late Cretaceous period. These ancient eels were a bit different because they had pelvic fins. Modern eel fossils do not appear until the Eocene period.

Scientists have also found tiny ear bones called otoliths from older times. One type of mud-dwelling eel called Pythonichthys arkansasensis lived after a major extinction event. This shows that eels have been around and changing for a very long time.

Eels come in many different sizes and weights. The European conger is the heaviest true eel in the world. It can grow to 300 cm long and weigh 36 kg.

Other eels are much longer but much thinner. The slender giant moray can reach 300 cm in length. Eels also go through many stages in their life. They start as flat, clear larvae called leptocephali. These tiny creatures drift in the sea and eat marine snow. Later, they become glass eels and then elvers.

Many people around the world interact with eels. In Japan, people eat freshwater eels called unagi. In Spain, a treat called angulas uses tiny young eels called elvers.

Some eel species are now very rare. The European eel is considered critically endangered. This means there are very few of them left in the wild. Because of this, groups like Greenpeace have added them to red lists. It is important to learn about these fish to help protect them.

Eels are a diverse group of elongated, ray-finned fish belonging to the order Anguilliformes. This order is quite large, containing about 1,000 different species. These species are organized into 164 different genera and 20 families. While many people use the name "eel" for various long-shaped fish, most are not true eels. For instance, the electric eel is actually a South American knifefish related to carps and catfishes. Other examples include swamp eels and slime eels, which evolved their long shapes independently.

The physical design of an eel is specialized for its lifestyle. Most eels lack pelvic fins, and many species also lack pectoral fins. A key feature is that the dorsal fin and the anal fin are fused with the caudal fin. This creates a single, continuous ribbon of fin running along much of the body. Eels move through the water by generating propulsive waves that travel the length of their bodies. Interestingly, they can swim backward by simply reversing the direction of these waves.

Eels undergo a complex metamorphosis during their life cycle. They begin life as leptocephali, which are flat and transparent larvae. These larvae drift in surface waters and feed on marine snow, which are small particles floating in the ocean. As they grow, they metamorphose into glass eels. Eventually, they become elvers before reaching their final juvenile or adult habitats. Many species, such as those in the genus Anguilla, are catadromous. This means they live most of their lives in freshwater but must return to the sea to breed.

The history of eels can be traced back millions of years through the fossil record. The earliest known fossil eels were discovered in Lebanon from the Late Cretaceous period. These primitive eels were different from modern ones because they still possessed pelvic fins. While modern eel body fossils appear in the Eocene, scientists have found older evidence using otoliths. Otoliths are tiny ear bones that can be preserved in the fossil record. These bones suggest that many modern eel families were already diversifying before the Cretaceous-Paleogene extinction.

Eels vary significantly in their physical dimensions. The European conger is recognized as the heaviest true eel. It can reach a length of 300 cm and a weight of 36 kg. In contrast, other species prioritize length over mass. The slender giant moray can reach 300 cm in length but remains much thinner than the conger. Most eels are nocturnal predators that hide in the sand, mud, or rocks. Some species inhabit shallow ocean waters, while others live on continental shelves at depths of 2,000 meters.

Humans have a long history of interacting with eels through various cultures. In Japan, freshwater eels known as unagi are a popular part of the cuisine. In Spain, a delicacy called angulas uses sautéed elvers and can cost up to 1,000 euro per kg. In the United Kingdom, jellied eels were once a traditional East London food. However, many eel species face serious conservation challenges. The European eel is currently listed as critically endangered. Because of this, organizations like Greenpeace have added the European, Japanese, and American eels to their seafood red lists.

Understanding eels requires looking at their complex biological and ecological roles. Their movement through waves and their ability to migrate between fresh and salt water connect them to various marine and river systems. Even their biology carries unique traits, such as eel blood containing a protein that is toxic to mammals. Fortunately, this toxin is destroyed during the digestive process or by cooking. By studying their taxonomy and evolutionary history, scientists can better understand how these specialized fish have survived through major geological changes.