Can fish see in complete darkness? I’m curious specifically about fish that live in the deep ocean, where sunlight doesn’t penetrate. Do these fish have any visual capabilities at all, or do they rely solely on other senses like pressure, smell, or electrical fields? Are there different adaptations in different species of deep-sea fish that allow for varying degrees of "seeing" in the absence of light? And if they can "see" in some way, is it true vision, or is it more like detecting bioluminescence from other creatures?

Answer

No, fish cannot see in complete darkness.

While some fish species have evolved remarkable adaptations to see in low-light conditions, true vision requires light. No animal, including fish, can perceive images in the complete absence of photons.

Here’s a more detailed breakdown:

• Vision Fundamentals: Vision relies on photoreceptor cells (rods and cones) in the retina that detect light. These cells contain pigments that undergo a chemical change when struck by photons, triggering a signal that the brain interprets as an image. Without light, this process cannot occur.

• Adaptations for Low Light: Many deep-sea fish and fish that live in murky waters have evolved adaptations to maximize their ability to see in dim light:

  • Large Eyes: Larger eyes collect more available light.
  • High Density of Rods: Rods are more sensitive to light than cones and are responsible for black and white vision in low light. These fish often have a higher proportion of rods in their retina.
  • Tapetum Lucidum: This reflective layer behind the retina reflects light back through the photoreceptors, giving them a second chance to detect the photons and enhancing light sensitivity. It’s responsible for the "eye shine" observed in many animals.
  • Specialized Photopigments: Some fish have photopigments that are sensitive to specific wavelengths of light that are more prevalent at certain depths.

• Alternative Sensory Systems: In completely dark environments, fish rely on other senses to navigate, find food, and avoid predators:

  • Lateral Line System: This system detects vibrations and pressure changes in the water, allowing fish to sense the movement of other animals and objects around them.
  • Electroreception: Some fish, like sharks and rays, can detect the weak electrical fields generated by the muscles of other animals.
  • Chemoreception (Smell and Taste): Fish use their sense of smell and taste to detect chemicals in the water that indicate the presence of food or other organisms.
  • Sound: Sound travels well in water. Fish use their inner ears and, in some cases, swim bladders to detect sound vibrations and orient themselves.
• Bioluminescence: Some deep-sea fish produce their own light through a process called bioluminescence. This light can be used for attracting prey, communication, or camouflage (counterillumination). However, even in these cases, the fish are still "seeing" light, albeit generated by themselves or other organisms. They are not seeing in the absence of light altogether.