Now at the Aquarium are six egg cases from several female spotted ratfish, Hydrolagus colliei. Each tough egg case contains one embryo which may take a year to mature. Like sharks, ratfish produce a small number of young, typically just two egg cases at a time; each female may extrude 20 or more of these single-embryo egg cases per year. The egg cases may stay attached to the female’s body by long filaments for three to six days.
Hydrolagus, the first part of their scientific name, means “water hare” and is inspired by their large, rabbit-like heads. Their tapering tails, which account for about half their body length, inspired their common name, ratfish. A venomous spine tops the tall dorsal fin of both females and males.
Although ratfish belong to the same class as sharks and rays (Chondrichthyes), these cartilaginous fish split from the shark line about 400 million years ago. They share sharks’ acute sense of smell and electroreception through visible sensory pits around the mouth that detect prey in murky water. Unlike sharks, their teeth are permanent and made for grinding up crunchy critters such as crabs, clams, worms, small bottom fish, and even their own egg cases. It’s estimated that there are about 200 million ratfish in Puget Sound, and uncounted numbers in nearby states—their current listing by the World Conservation Union (IUCN) is as a species of “least concern.” Nevertheless, trawling can seriously affect numbers quickly, so in some states such as Oregon, trawlers use gear that avoids by-catch to reduce pressure on local populations.