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Why do some organisms contain bacteria that produce light for them?

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time-lapse photo of fireflies — Bioluminescence Time-lapse image of fireflies in the Catskill Mountains, New York. Firefly light is a cold light, with approximately 100 percent of the energy given off as light and only a minute amount as heat. Only light in the visible spectrum is emitted.

Some organisms, particularly certain species of deep-sea fish and squids, harbor symbiotic bacteria within specialized light organs to produce light. These organs are typically supplied with a rich blood supply that nourishes and maintains the bacterial population, allowing the host to utilize the light they generate.1

This relationship provides significant survival advantages for the host. The light produced by these bacteria can be used for various purposes, such as:

Attracting prey: Luminous lures help some organisms draw in food.23
Defense: Light can be used to frighten predators or to disguise the host's form.2
Communication: Distinctive light patterns may assist in species recognition or help individuals maintain schools.23

The host maintains control over this light through anatomical features, such as black membranes or pigment granules (melanophores) that can be drawn over the light organ to extinguish or reveal the glow as needed. While the bacteria provide the light, the host provides a stable environment, creating a mutually beneficial partnership.1