The Remarkable Spotted Ratfish
The spotted ratfish (Hydrolagus colliei) is a small fish from the Pacific Ocean along the North American coast. These fish are a member of the Chimera group meaning that they have cartilage in the place of bones, and they are closely related to both rays and sharks. Because of its unique morphology, the spotted ratfish is fairly easy to identify. In an uncommon reversal of sexual dimorphism, females are larger than the males at 97 cm long; approximately a half of this length is composed of the fish’s long, pointed tail. In contrast, the head is flat, bulbous, and sports a duckbill-like snout. Like sharks and rays, these fish are smooth and scale-less, and usually iridescent silvery-bronze with speckled white dots.
Because of their thinner tail, spotted ratfish are relatively poor swimmers, so to defend themselves they use a poisonous spine on their dorsal fin. When they can, sharks, bass, rockfish, halibut, and marine mammals all readily consume spotted ratfish. In turn, H. colliei hunt slowly on the sandy ocean floor at night, using a combination of chemical and electrical receptors to find their prey; primarily crunchy animals like crustaceans, mollusks, and polychaete worms, as well as small fish. Interestingly, spotted ratfish share a trait with bony fish: the upper jaw is fused with the skull. This gives spotted ratfish a large biting force, which they use in combination with their upper two pairs of teeth to break the shells of of their prey.
H. colliei, like their prey, are almost exclusively benthic fish, and prefer muddy or rocky habitats. They also tend to stay in cool, shallow waters around 7-9 °C and up to 1000m deep, moving steadily deeper as their range moves south. While populations are found as far as the southern United States, they are most commonly found in the Pacific Northwest, where they are occasionally attracted to shallower waters by dock or boat lights. They also move with the seasons; swimming in more shallow waters in the cooler spring and fall, and deeper again in the summer and winter.
Alongside some sharks, spotted ratfish reproduce by laying eggs in the spring or fall. These fish have a unique copulation process that lasts up to 30 hours, in which the male uses a club-like appendage on the head called a cephalic clasper to grip the female’s pectoral fin. After mating, the female releases one or two eggs over a period of several days. The eggs are attached to the ocean floor by thin tendrils, and the female will guard them for a few weeks to ensure they are not eaten by predators or mistaken by inanimate objects by unsuspecting divers. The eggs themselves take over a year to hatch, and young emerge already 14 cm long and fully independent.
Conservation Status: The spotted ratfish is currently listed as Least Concern by the International Union for the Conservation of Nature. It is occasionally caught as by-catch in the Pacific Northwest, but it is not harvested for food or materials.
