The Bulbfish is a small [Exotic] fish that glows yellow light. They are found in deep waters. Hooking one allows it to act as a lightbulb. It's used as bait for the Maw. They can be caught with the small bait, at night and day, At night they are closer to the surface, in the day, they are founds closer to the Caverns. The amount of money that you receive after selling an uneaten Bulbfish is $4,200. They are related to the Angel.
In-game description: Fields of Bulbfish light up the dark depths. Catching one intact allows you to harness their light.