Cusk eels (family Ophidiidae) are a grouping of fish that can be found in much of the world’s temperate and tropical oceans. Their name can be misleading since they are not a true eel. They share many features with eels, but one feature that distinguishes them is their fins.

Fins are the appendages that help fish swim, including moving forward, turning, stopping, and moving up and down. Cusk eels have a ventral, or pelvic, fin near to the underside of their throats. Most true eels do not have this.

Cusk eels spend much of their time near the seafloor where they have been observed to use their tails to burrow into the mud to hide from predators and sneak up on prey. Cusk eels include a diverse range of species, and some species live in shallow water areas and others in the deep ocean.

In 1970, a cusk eel was recorded in the hadalpelagic zone of the ocean at a depth of 8,370 meters (27,460 feet) in the Puerto Rico trench, the deepest trench in the Atlantic Ocean. Not many fish species are known to live in the hadalpelagic zone−the zone of Earth’s deep ocean trenches. This deep zone is named after the realm of Hades, the underworld in Greek mythology, and ranges from around 6,000 meters (20,000 feet) in depth to more than 10,000 meters (33,000 feet).

  1. How do fish use their fins?

    • Answer

      Fins are the appendages of fish that help with mobility. Fish use fins to move in different directions, to turn, and to stop.

  2. How do fish and other marine life that live in the deep oceans with little or no natural illumination (also called the aphotic zone) navigate and catch food?  

    • Answer

      Many fish and other marine organisms that live in the aphotic zone of the ocean rely on senses other than sight, such as sensitivities to changes in local pressure and smell, to navigate, catch their food, and avoid being caught. Other organisms have eyes that use bioluminescent light.

  3. How do deep ocean trenches form?

    • Answer

      Ocean trenches are formed by a geological process called subduction in which tectonic plates made up of the Earth’s crust and upper mantle (also called the lithosphere) converge and the older, denser plate is pushed beneath the lighter plate and is forced deep down into the mantle, causing the seafloor to bend, tilt, and form a steep V-shaped depression.

hadalpelagic zone

deepest zone of the open ocean, starting at around 6,000 meters (20,000 feet).


a long, deep depression in the ocean floor.


animal that hunts other animals for food.

Puerto Rico Trench

deepest place in the Atlantic Ocean, 8,400 meters (27,560 feet) deep.


surface layer of the bottom of the ocean.



ventral fin

one of a pair of fins on the lower surface of a fish's body, in a similar location to hind limbs of quadrupeds. Also called a pelvic fin.