Cusk eels are a large and remarkably diverse group of fishes in the order Ophidiiformes, found in oceans worldwide from sun-drenched coastal shallows to the deepest trenches on Earth. Despite their name, they are not true eels at all. Their elongated, tapering bodies earned them the common name, but they belong to a completely separate branch of bony fishes. With roughly 500 described species spread across several families, cusk eels occupy an extraordinary range of habitats, and some hold records as the deepest-living bony fish ever documented.
What Exactly Is a Cusk Eel
The term “cusk eel” covers a broad group within the superfamily Ophidioidei, which includes the families Ophidiidae (the true cusk eels), Bythitidae (the viviparous brotulas), and their close relatives the Carapidae (pearlfishes). The relationship among these families has been debated for decades, and recent phylogenetic work has revealed that the traditional family Ophidiidae is actually paraphyletic, meaning it does not contain all descendants of a single common ancestor. The pearlfishes, once considered a distinct lineage, actually nest within one of the four main clades of the Ophidioidei rather than sitting outside them.1Molecular Phylogenetics and Evolution. Exploring the phylogeny and depth evolution of cusk eels and their relatives (Ophidiiformes: Ophidioidei) So the family tree is messier than textbooks suggest, and some species currently lumped together as “cusk eels” are less closely related to each other than they are to pearlfishes.
In body form, most cusk eels share a few hallmarks: a long, laterally compressed body that tapers to a pointed tail, with the dorsal and anal fins often fused into a continuous fin fringe. Many species lack pelvic fins entirely, while others have modified them into slender, whisker-like structures. Sizes vary wildly, from small species just a few centimeters long to commercial species like the kingklip that exceed a meter in length.
An Ancient Lineage That Invaded the Deep
Cusk eels are not recent arrivals in the deep ocean. Molecular clock analyses estimate that the broader order Ophidiiformes originated in shallow waters roughly 96 million years ago, during the Late Cretaceous. Their ancestors moved into the deep sea surprisingly early: the common ancestor of the Ophidioidei is estimated to have invaded deep waters around 90 million years ago, a timing that coincides with a massive global anoxic event known as OAE2.2Molecular Phylogenetics and Evolution. Exploring the phylogeny and depth evolution of cusk eels and their relatives (Ophidiiformes: Ophidioidei) During that event, large swathes of the shallow ocean became depleted of oxygen, which may have pushed ancestral cusk eels into deeper, more oxygenated waters. Once there, they diversified explosively, eventually colonizing every depth zone from continental shelves to abyssal plains and hadal trenches.
That 90-million-year head start in the deep sea helps explain why cusk eels are among the most species-rich fish groups in the abyss today. They have had an extraordinarily long time to adapt, speciate, and fill ecological niches that other fish lineages have barely touched. One species, Abyssobrotula galatheica, was famously trawled from more than 8,000 meters deep in the Puerto Rico Trench, making it one of the deepest bony fish ever collected.
Pelvic Fins That Work Like Whiskers
One of the more unusual features of certain cusk eels is what they have done with their pelvic fins. In many bony fishes, pelvic fins sit near the belly and help with steering or braking. In the cusk eel Ophidion rochei, however, the pelvic fins have been reduced to just four rod-like structures positioned beneath the lower jaw. As the fish swims, these slender rods sweep continuously along the seafloor, and detailed anatomical work shows they function almost identically to the barbels of catfish and other bottom-dwelling species.3PubMed. The barbel-like specialization of the pelvic fins in Ophidion rochei (Ophidiidae)
These modified fins are loaded with the same types of sensory cells found in true barbels: taste buds, solitary chemosensory cells, and goblet cells. They are also innervated in ways that parallel barbel wiring. The result is that O. rochei essentially tastes and probes its environment with what used to be locomotory fins. This makes sense for a species with nocturnal habits: in the dark, a pair of forward-mounted chemical sensors is far more useful than a pair of small swimming paddles. It is a striking example of how evolution can repurpose a structure for an entirely different function when the ecological pressure is strong enough.
A Remarkably Complex Sound System
Many fish make sounds, but cusk eels have one of the most elaborate vocal systems in the fish world. Their sound-producing apparatus involves modified vertebrae, specialized muscles, and a swimbladder that has been reshaped into something closer to a biological instrument than a simple buoyancy organ.
In the fawn cusk eel Lepophidium profundorum, the system works through antagonistic pairs of muscles, which is unusual among sound-producing fish. Most fish that vocalize rely on a single set of muscles that contract to deform the swimbladder, with the swimbladder’s own elasticity or internal gas pressure snapping it back into shape. The fawn cusk eel instead uses one set of muscles (the ventral and intermediate muscles) to pull a wing-like bony process and the swimbladder forward, pivoting a modified neural arch above the first vertebra backward. This stretches a thin window in the swimbladder wall and stores strain energy in a system of ribs, tendons, and ligaments attached to the front of the swimbladder. A second, shorter set of muscles then pulls the neural arch back the other way, releasing that stored energy and restoring the swimbladder to its resting position through a lever advantage.4PubMed. Functional morphology of the sonic apparatus in the fawn cusk-eel Lepophidium profundorum (Gill, 1863)
The cryptic cusk eel Parophidion vassali uses a similar but distinct version of this apparatus: three pairs of sonic muscles work in concert with a swimbladder that features mineralized caps at its front end, a fenestra (a thin-walled window), and a raised tube along its posterior edge.5PubMed Central. Sound production and mechanism in the cryptic cusk‐eel Parophidion vassali The mineralized structures likely help focus or amplify the sound produced when the swimbladder wall vibrates. Across the group, the pattern is consistent: cusk eels have invested heavily in vocal hardware, and different species have tweaked the same basic blueprint in species-specific ways. The sounds are thought to play roles in courtship, territorial disputes, and possibly predator deterrence, though confirming function in deep or nocturnal species is inherently difficult.
What Cusk Eels Eat and How Their Diet Shifts
Cusk eels are generally predators of small benthic and near-bottom organisms, but their diets shift depending on their size, the depth they inhabit, and where they live geographically. Detailed work on the pink cusk eel Genypterus blacodes in the southwestern Atlantic illustrates these patterns well. Stomach content analysis showed the species feeds primarily on fish and crustaceans, with smaller individuals eating proportionally more crustaceans and larger individuals shifting toward a fish-dominated diet.6Fisheries Research. Prey contribution to the diet of pink cusk-eel Genypterus blacodes (Forster, 1801) revealed by stomach content and stable isotopic analyses in the southwestern Atlantic
An interesting wrinkle emerged when researchers compared what was found in the stomach with what was actually being assimilated into the fish’s muscle tissue, measured through stable isotope ratios. In one subregion, stomachs were full of crustaceans, followed by cephalopods and then fish. But the isotope data told a different story: fish turned out to be the prey most efficiently absorbed into muscle tissue, with crustaceans and cephalopods contributing relatively little to long-term nutrition.7Fisheries Research. Prey contribution to the diet of pink cusk-eel Genypterus blacodes (Forster, 1801) revealed by stomach content and stable isotopic analyses in the southwestern Atlantic This is a good reminder that what ends up in an animal’s stomach on a given day does not always reflect what matters most to its nutrition over weeks and months. The pink cusk eel may eat a lot of shrimp, but the fish it catches are what really fuel its growth.
Larvae That Rise Before They Sink
Cusk eels include both egg-laying (oviparous) and live-bearing (viviparous) species, which is an unusual split within a single superfamily of deep-sea fish. You might expect that viviparous species, which give birth to relatively developed young, would skip the pelagic larval stage that egg-laying species go through. That turns out to be wrong. Otolith microchemistry from both viviparous blind cusk eels (Barathronus maculatus) and oviparous cusk eels showed that larvae of both reproductive types undergo a pelagic phase, drifting in shallower waters before eventually settling to the deep-sea floor.8Journal of Fish Biology. Vertical habitat shift of viviparous and oviparous deep-sea cusk eels revealed by otolith microstructure and stable-isotope composition
The vertical distance these larvae travel during settlement can be dramatic, ranging from about 200 meters to more than 1,000 meters depending on the individual. For a viviparous species like B. maculatus, this period of larval drift in shallower water may be critical for dispersal. Born on the sparsely populated deep-sea floor, the larvae rise into more productive midwater zones, feed and grow, and then descend to colonize new patches of abyssal habitat. This mechanism could explain how deep-sea cusk eels maintain surprisingly wide geographic distributions despite living in an environment where encounters between individuals are rare.
Swimming Backward in the Abyss
Baited camera landers deployed at depths between 4,500 and 6,300 meters in the Pacific Ocean have captured something rarely documented in bony fish: sustained backward swimming. Among the species filmed doing this were two cusk eels, Bassozetus sp. and Barathrites iris. These fish used reverse body undulations, essentially sending propulsive waves down their elongated bodies in the opposite direction from normal forward swimming, to back away from the camera or from other animals at the bait.9Journal of Fish Biology. Backward swimming in elongated-bodied abyssal demersal fishes: Synaphobranchidae, Macrouridae, and Ophidiidae
The cusk eels managed up to seven tail beats in reverse before pivoting and resuming forward swimming in a new direction, and they supplemented their reversed body waves with paddling from their pectoral fins, though this came with some lateral wobble of the head. The behavior appeared to serve as an escape or repositioning maneuver rather than a regular mode of locomotion. It is a neat trick for animals that live in a three-dimensional, pitch-dark environment where bumping into a predator or competitor may require a quick retreat without the luxury of turning around first. Their elongated, flexible bodies, the same shape that defines the group, make reverse undulation physically possible in a way that a stiff-bodied fish could not manage.
Commercial Fisheries and Conservation Concerns
Several larger cusk eel species support valuable commercial fisheries, particularly in the Southern Hemisphere. Kingklip (Genypterus capensis) is one of the most prized fishery resources in South Africa, where it is caught primarily as bycatch in trawl and longline fisheries targeting hake. Intense exploitation in the past drove substantial population declines, though current exploitation levels are considered sustainable under a management approach that sets a precautionary upper catch limit.10Fisheries Research. Genetic population sub-structuring of kingklip (Genypterus capensis – Ophidiidiae), a commercially exploited demersal fish off South Africa The word “sustainable” here comes with caveats: genetic analysis has revealed population sub-structuring within kingklip, meaning that fish from different regions may represent semi-independent stocks. Managing them as a single population could mask localized depletion even while overall numbers look healthy.
The South American pink cusk eel (G. blacodes) and the Chilean cusk eel (G. chilensis) face similar pressures. These species are long-lived and slow to mature, traits that make them vulnerable to overfishing because a depleted population takes many years to recover. Their deep-water habitat also means that bycatch mortality can be high: fish pulled up from hundreds of meters deep rarely survive the pressure change even if they are discarded. For consumers, kingklip and its relatives are marketed as premium white-fleshed fish, often sold in fillets under regional common names that obscure the fact that these are all cusk eels.
Cusk Eels at Methane Seeps
Deep-sea methane seeps, places where hydrocarbon-rich fluids bubble up through the seafloor, support unusual ecosystems fueled by chemosynthesis rather than sunlight. These communities are typically dominated by specialized invertebrates like tube worms, clams, and bacterial mats. Fish are generally visitors rather than residents at seeps, but the Chilean cusk eel (Genypterus chilensis) has been documented congregating at a deep-sea methane seep off the coast of Chile.11Ecology. Congregation of cusk-eels (Genypterus chilensis, Ophidiiformes) at a deep-sea methane seep off Chile
Why a cusk eel would hang around a methane seep is an open question. The dense invertebrate communities at seeps offer an obvious food source, and the elevated microbial productivity could support higher densities of small prey organisms in the surrounding sediment. For a benthic predator like G. chilensis, a seep might function as an oasis on an otherwise food-poor deep-sea floor. Whether the fish are permanent residents or occasional visitors taking advantage of a rich feeding patch remains to be established. Either way, the observation adds to a growing list of vertebrates that interact with chemosynthetic ecosystems in ways scientists did not expect a few decades ago.
Why Cusk Eels Are Understudied
For such a large and ecologically important group, cusk eels remain poorly known compared to many other fish families. The reasons are mostly practical. Most species live at depths that are expensive and logistically difficult to sample. Trawling the deep sea is slow, costly, and often destructive to the specimens themselves. Many cusk eels are soft-bodied and fragile, arriving at the surface in poor condition after being dragged through hundreds or thousands of meters of water. Live observation is limited to whatever baited cameras and remotely operated vehicles can capture during brief deployments.
Taxonomy within the group is also in flux. The paraphyly of Ophidiidae means that the classification system most researchers grew up with does not accurately reflect evolutionary relationships, and revisions are ongoing. New species are still being described regularly, particularly from poorly sampled ocean basins and extreme depths. For a group that has been diversifying in the deep ocean for 90 million years, it would be surprising if we had found them all. The practical result is that basic life-history data, such as age at maturity, reproductive output, and natural mortality rates, remain unknown for the vast majority of cusk eel species. That makes conservation assessments difficult and means that if deep-sea fisheries expand into new areas, we may be harvesting species we know almost nothing about.