The eel is a type of jawless fish. It has an elongated body like a snake, which has a long tubular body with sharp teeth on the upper and lower jaws. On sunny days, when the wind and waves are calm and the sea water transparency is high, they mostly stay in muddy burrows and reduce their feeding activities. When the wind and waves are high and the water is turbid, they come out to forage around, and are especially active during the period from sunset to early morning. There are more than 600 species of eels in the Indian and Pacific Oceans, and they generally have seasonal migrations.

Among the eels, the seven-gill eel is the most famous. They have no scales, and their elongated body is rounded, much like an eel. The seven-gill eel has only one nostril, located on the top of the head between the two eyes, and it has seven gill holes on each side of the body behind the eyes, which is why it is called "seven-gill eel". The eel attaches itself to other fish through a sucker mouth and feeds on the host's blood. Sometimes the eel abandons the host before it dies and finds a new victim, or sometimes it stays with the fish until it dies. In the waters of the Kamchatka Peninsula, the blind eel is the only fish in the world that breathes through its nose. The eyes of the blind eel are born with a layer of skin membrane, but it has receptors in its head, and the whole body is also full of supersensory cells, which can correctly determine the direction and distinguish objects, which is very useful for the blind eel's predation and avoidance of enemies. Unlike the seven-gill eel, which attacks live fish, the blind eel feeds on fish carcasses or weakened fish caught in nets. It will often enter through the gills or mouth of the food bait and eat the food whole. Because the blind eel has special glands on its body that produce a thick mucus, it sticks the surrounding seawater into a translucent mass when it encounters an enemy and rapidly changes its size. When the enemy is confused by this mucus, the blind eel has already taken the opportunity to escape. There is also a kind of cave eel that can not swim, it lives in the water but can not swim. In the waters of the Maldives Islands in the Indian Ocean, the cave eel lives in sandy dens. Its way of feeding is to poke out half of its body from the hole, open its mouth and swallow the plankton or small animals floating with the water.