Shark Week: Spotted Ratfish, the Unshark
Happy 25th anniversary of Shark Week! After you’ve celebrated by taking a self-guided tour with our new Ten Fun Finds: Sharks at Shedd map, mosey back to the Oceans gallery to meet the spotted ratfish, a distant relative of sharks—so distant that we’re calling him the unshark.
The oddly beautiful spotted ratfish gets its unlovely name from the tapering tail that accounts for half its body length. A better name might be airplane fish, based on its smooth, metallic-looking skin, tapering fuselage shape and large, triangular pectoral fins that stick out from the body at right angles like wings. Lacking much power in the tail section, the ratfish flaps these “wings” to achieve a modest cruising speed low over the ocean floor.
This species is also sometimes called a rabbit fish—not to be confused with the rabbitfishes of Indo-Pacific coral reefs. But the ratfish’s genus name is Hydrolagus, or water hare, inspired by its rabbitlike large head, well-developed, even limpid eyes and downward-facing mouth. While ratfish are missing anything analogous to long rabbit ears, they do have a tall dorsal fin tipped with a sharp, venomous spine.
The ratfish and its clan, the chimaeras (kih-MEER-uhz), are the oldest order of fishes alive today. They belong to the taxonomic class Chondrichthyes (kahn-drik-THEEZ, literally “cartilaginous fishes”), along with sharks and rays, but they split from the shark line early on, about 400 million years ago. (For context, at that time, the spot on the globe now occupied by Chicago was the reef-dotted bottom of a vast, tropical sea.)
Like sharks, the ratfish and other chimaeras have skeletons made of cartilage rather than bone, the males have claspers, and the females lay leathery egg cases that attach to the seafloor with a filament. Also like sharks, ratfish produce a small number of offspring—just two eggs during spawning. The embryos can take up to a year to develop, leaving them vulnerable to predation, but the 5½-inch young that hatch are miniature adults that will reach a foot long if they make through their first year.
And like most deep-sea fishes, including deep-dwelling shark species, ratfish cannot regulate the amount of light coming into their large eyes. You’ll notice our ratfish lives in a shaded environment. Too much light—whether from the sun or a camera’s flash—can hurt him.
Ratfish also share sharks’ underwater sense of smell and electroreception—through visible sensory pits around the mouth that pick up animals’ electrical fields—for detecting prey in dusky waters.
They don’t have sharks’ sandpapery, dermal denticle-covered skin, the mouthful of replaceable teeth and the muscular swimming ability.
Ratfish do have a few features characteristic of the modern bony fishes, plus some interesting adaptations all their own.
As in bony fishes, the upper jaw is fused with the skull (sharks can eject a detachable upper jaw to grab prey), and ratfish have a gill cover, or operculum, on either side of the head. They also have permanent teeth—two pairs of grinding plates in the upper jaw and one pair in the lower for dining on a seafloor smorgasbord of crunchy invertebrates such as crabs and clams.
But then ratfish go off into strange physiological territory. Not only does the male have two pairs of claspers, compared to sharks’ one, but the hindmost pair is retractable. He also has what is described as a “doorknocker-like” appendage called a tenaculum—again, retractable—on his forehead for securely grasping the female at a hollow on the back of her head during courtship.
But maybe the coolest thing about the remarkable ratfish is that it is one of our native North American fishes. It’s found along the Pacific coast from Southeast Alaska to Baja California. And while nearly all the chimaeras are deep-ocean fishes, the spotted ratfish seems equally at home in intertidal zones and at depths of nearly 3,000 feet. That adaptability makes it possible for us to delight in its mix of old and new, sharkiness and unsharkiness, at Shedd.
Karen Furnweger, web editor