What Enables Deep-Sea Fish
to Glow In the Dark?

Answer:

Humpback anglerfish

Many fish inhabiting the deep sea carry their own lights. In a world where no sunlight penetrates, this luminescence is thought to serve such varied functions as attracting prey and enabling the fish to see nearby objects and recognize potential mates.

In some species specialized cells belonging to the fish itself are light producers. But more frequently it is bacteria living in the fish’s tissues that do the work — and derive nourishment from their host in return. Some fish actually activate their bacterial “guests” by pumping oxygenated blood to them; others mask and unmask the perpetually glowing bacteria by expanding or contracting cells in the skin.