Britannica Sources used in this answer

Why do mid-water deep-sea fishes often have large eyes and huge mouths?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Mid-water deep-sea fishes possess enlarged eyes and huge mouths as evolutionary adaptations to the extreme darkness and specialized conditions of their environment.1

The enlarged eyes allow these fish to maximize their ability to detect light in the deep-sea habitat, where sunlight is minimal or absent. Many of these species also utilize bioluminescence—light produced chemically by the organism—to navigate, recognize mates, or attract prey.123

The huge mouths are a critical adaptation for survival in an environment where food may be scarce. These structures allow the fish to capture and engulf prey efficiently when it is encountered. For instance, some species use luminous organs as lures to draw prey within reach of their large jaws. These specialized traits are essential for the survival of mid-water species, enabling them to thrive in a habitat characterized by extreme pressure, cold, and darkness.13