A ratfish is a cartilaginous fish closely related to sharks and rays, belonging to a group called chimaeras (or Holocephali). With a lineage stretching back roughly 400 million years into the Paleozoic Era, ratfish are among the oldest groups of jawed vertebrates on the planet. Despite their ancient origins, they remain relatively obscure because most species live in deep water, far from the casual observer.
How Ratfish Relate to Sharks and Rays
Ratfish belong to the class Chondrichthyes, the same group that includes sharks and rays. All of these animals have skeletons made of cartilage rather than bone. Within that class, ratfish split off onto their own evolutionary branch (the subclass Holocephali) very early, while sharks and rays occupy the other branch (Elasmobranchii). Genetic and fossil evidence confirms these two branches are distinct lineages, meaning ratfish are cousins of sharks rather than direct ancestors or descendants.
The family Chimaeridae is the most species-rich group of ratfish, with the genus Hydrolagus alone containing more than 20 described species. You’ll sometimes hear ratfish called chimaeras, ghost sharks, or rabbitfish, depending on the species and region. The most familiar species in North American waters is the spotted ratfish (Hydrolagus colliei), common along the Pacific coast.
What a Ratfish Looks Like
Ratfish have a distinctive, slightly alien appearance. Their large, reflective eyes dominate a blunt, rounded head, and their bodies taper into a long, whip-like tail, which is where the “rat” in their name comes from. Most species have smooth, scaleless skin with a silvery or iridescent sheen, often marked with spots or mottled patterns.
Instead of the rows of replaceable teeth that sharks have, ratfish possess permanent mineralized grinding plates. The spotted ratfish has one pair of these plates in the lower jaw and two pairs in the upper jaw. These plates function like built-in nutcrackers, letting them crush hard-shelled prey with ease. A prominent dorsal spine sits just in front of the first dorsal fin. This spine is venomous and serves as the animal’s primary defense, a feature that matters if you ever handle one.
An Ancient Lineage, Mostly Unchanged
Fossil chimaeroids have been found in deposits dating to the Devonian period, roughly 400 million years ago. Both paleontological and genomic evidence shows that chimaeras and their close relatives have remained relatively unchanged for millions of years. Their cartilaginous skeletons don’t preserve as well as bone, so the fossil record is patchy, but fossilized egg capsules and tooth plates confirm their deep history. In evolutionary terms, ratfish are living relics, occupying a body plan that has worked well enough to persist through multiple mass extinctions.
Where Ratfish Live
Most ratfish species favor deep water. The large-eyed rabbitfish (Hydrolagus mirabilis), for example, is widely distributed across the Atlantic Ocean and Mediterranean Sea, generally occupying depths beyond 800 meters (about 2,600 feet). The spotted ratfish is somewhat more accessible, found along the northeastern Pacific from Alaska to Baja California, where it inhabits continental shelves and slopes. It sometimes ventures into shallower bays and estuaries at night, making it the species divers and shore fishermen are most likely to encounter.
Ratfish are slow, deliberate swimmers. They propel themselves primarily with large, wing-like pectoral fins, gliding over the seafloor rather than darting through open water. This unhurried lifestyle suits their role as bottom-dwelling predators in cold, dark environments.
How They Hunt in the Dark
Living in deep or murky water means vision isn’t always useful, so ratfish rely heavily on two other senses: electroreception and smell. Like sharks, they have ampullary organs, specialized pores concentrated on the head and snout that detect the faint electrical fields generated by the muscles and nerves of nearby animals. These organs essentially let ratfish “feel” hidden prey buried in sediment. They also have a lateral line system, a series of pressure-sensitive cells running along the body that detect changes in local water flow, helping them sense movement from predators or prey.
The spotted ratfish swims slowly along the bottom substrate, searching for clams, crabs, shrimp, polychaete worms, and small bottom-dwelling fishes. Those grinding tooth plates make quick work of hard shells. Spotted ratfish are also documented cannibals, feeding on their own egg cases as well as other free-swimming ratfish.
Reproduction and Egg Cases
Ratfish reproduce by laying eggs, a strategy they share with some sharks and rays. Females produce leathery, elongated egg cases, sometimes called “mermaid’s purses,” that they deposit on the seafloor. Each case contains a single developing embryo nourished by a large yolk. Incubation is slow, taking many months depending on species and water temperature. This slow reproductive rate is a common trait among deep-water cartilaginous fish and makes ratfish populations vulnerable to overfishing, since they can’t replace lost numbers quickly.
Male ratfish have a unique feature: a club-shaped clasping organ on the forehead called a tenaculum, covered in small, tooth-like projections. Males use it to grip the female during mating, a trait found in no other fish group.
The Venomous Dorsal Spine
The sharp spine in front of the first dorsal fin is the one part of a ratfish that demands respect. Deep-sea trawler fishermen in the northeast Atlantic frequently encounter chimaera species and can suffer dangerous penetrating wounds from this spine. The spine delivers venom that causes immediate, intense burning pain.
In one documented case, a fisherman stung on the calf by a ratfish spine developed a swelling with bluish discoloration on the first day, followed by spreading numbness through the calf and back of the thigh. He needed crutches for three weeks, and symptoms persisted for nine weeks. In another case, a spine lodged adjacent to the femoral artery, highlighting the risk of serious bleeding from deep puncture wounds. The venom is heat-sensitive, so the standard first-aid approach is to rinse the wound, remove any embedded fragments, and submerge the affected area in water as hot as you can comfortably tolerate for at least 30 minutes. The heat helps break down the venom proteins and reduces pain.
Ratfish are not aggressive. These injuries happen when fishermen handle them carelessly after pulling them from nets. If you catch one or encounter one while diving, simply avoid the dorsal spine and you’ll be fine.
Why They’re Rarely Eaten
Despite being caught regularly as bycatch in commercial trawl fisheries, ratfish have almost no market value. Their flesh is soft and oily, and historically most cultures have found it unpalatable. In some regions, ratfish liver oil was once extracted for use as a machine lubricant and lamp fuel, but that practice has largely disappeared. Today, most ratfish caught in nets are simply discarded. Because they’re slow to reproduce and increasingly caught as bycatch in expanding deep-water fisheries, some conservation biologists have raised concerns about population sustainability, though comprehensive population data for most species remains limited.