Despite their serpentine appearance, cusk-eels are not true eels at all.

Nature · Published 3 October 2026 Swipe more like this →

In short: With bodies that mimic the slithering form of an eel, these marine fish are actually distant relatives of tuna and seahorses. Thriving from shallow coastal waters to the crushing depths of the hadal zone, they use specialized, barbel-like fins to navigate the ocean floor in search of prey.

The cusk-eel family, Ophidiidae, belongs to the order Ophidiiformes, a group that diverged from the lineage of true eels during the Jurassic period. While they share a long, slender body shape—often twelve to thirteen times as long as they are deep—they are taxonomically closer to perch and tuna within the Percomorpha clade. Their most distinct anatomical feature is a pair of forked, barbel-like pelvic fins located beneath the mouth, which act as sensory organs for foraging along the seafloor. Unlike true eels, which lack well-developed ventral fins, cusk-eels rely on these structures to navigate their dark, benthic environments.

These fish are masters of extreme habitats. One species, Abyssobrotula galatheae, holds the record for the deepest-dwelling fish ever recorded, found at 8,370 meters in the Puerto Rico Trench. While some species are solitary, others have evolved complex behaviors, such as hiding in coral crevices or living as parasites inside sea cucumbers and clams. Their feeding habits are primarily nocturnal, targeting small invertebrates and crustaceans. Unlike their close relatives, the viviparous brotulas, cusk-eels are oviparous, releasing eggs that drift in the plankton layer before the larvae eventually descend to the bottom.

The diversity within the family is significant, comprising approximately 240 species across 50 genera. Classification remains a challenge for scientists due to inconsistent physical traits, leading researchers to rely on reproductive strategies—specifically whether a species practices viviparity—to define groups. While some species like the pink cusk-eel are commercially fished, the conservation status of the family remains largely unknown due to a lack of comprehensive data. From the 2-meter-long Lamprogrammus shcherbachevi to smaller, bottom-dwelling varieties, these elusive fish remain one of the most successful, yet mysterious, lineages in the global ocean.

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