Friend Feature: Spotted Ratfish (Hydrolagus colliei)
A spotted ratfish, deep in the Salish Sea. Image by Brandon Cole, Salish Sea: Jewel of the Pacific Northwest
Baby sharks?
When you think of cartilaginous fishes (a.k.a. fishes with no bones), you might imagine sharks and rays gliding through the deep. While the Salish Sea is home to many of these incredible creatures, from sixgill sharks to sandpaper skates, our most common cartilaginous fish is actually something entirely different: the Spotted ratfish!
With their straight, flat pectoral and pelvic fins, spotted ratfish can look like they are flying. Image by Ed Bierman, Flickr Creative Commons
The spotted ratfish (Hydrolagus colliei) belongs to a group of fishes called the Holocephalans, or chimaeras. They are not baby sharks, but do share some characteristics with sharks, such as claspers, and with bony fishes (like goldfish and salmon), such as opercula, or gill covers.
Divers who encounter spotted ratfish often say they look like they’re flying. They make slow, sweeping motions with their pectoral fins, sometimes throwing in a barrel roll for style. These graceful movements, combined with shimmering skin that looks like patched-together scraps of precious metal, make the spotted ratfish a living work of art!
Does every spotted ratfish have the same pattern of spots? How many ratfish are in this photo? Image by Brian Gratwicke, Flickr Creative Commons
Hunting by electroreception and a sharp nose
Ratfish are happiest in the deep, typically between 80 and 160 meters, but they sometimes venture into shallower waters. Ever fans of the dark, they tend to be most active at night, when they use smell and electroreception to forage for small fishes and bottom-dwelling invertebrates. Ratfish have organs that can sense the weak electric fields emitted by living things.
Unlike sharks with rows of sharp, replaceable teeth, the ratfish has a mouthful of tooth plates to grind mollusk shells and tough crustaceans. They adopt the predator lifestyle very early in life, immediately beginning to hunt after hatching from the leathery egg cases that female ratfish deposit on the seafloor.
Who eats ratfish?
Because people don’t like to eat ratfish, we typically don’t catch them for sale or recreation. This is a major reason why ratfish are so abundant in the Salish Sea. Ratfish are occasionally eaten by harbor seals, California sea lions, and Steller sea lions, but they don’t make up a large proportion of these marine mammals’ diets.
This formidable spine has deterred Salish Sea seals from eating ratfish. Image from an article by Adrianne Akmajian and her team of scientists, 2012.
To defend itself from predators, the ratfish uses a venomous spine in front of its dorsal fin. This weapon can inflict serious damage, even after the ratfish has been swallowed. On multiple occasions, harbor seals in the Salish Sea have died from small ratfish spines piercing their esophagus or stomach!
From its tiny, rat-like tail to its enormous green eyes, the ratfish is an adorable and fascinating fish. If you want to meet one in real life, put night diving or snorkeling on your Salish Sea bucket list!
Special thanks to Lizzy Ashley, Veterinary Medicine Ph.D. candidate at UC Davis and former SeaDoc Society Research Assistant, for contributing this month’s Friend Feature. We love you, Lizzy!