It is a common thought that fish cannot see well, or are perhaps even blind. This idea stems from observing how light behaves in water compared to air. However, the truth is more intricate, as fish possess a diverse range of visual capabilities adapted to their specific environments. Most fish have functional vision, and some even have highly developed eyesight, allowing them to thrive in their aquatic worlds.
How Fish Perceive Their Surroundings
Fish eyes share basic similarities with human eyes, featuring a cornea, iris, pupil, lens, and retina. A significant difference lies in the lens; fish have a more spherical, highly dense lens with a very high refractive index. This spherical shape is well-suited for underwater vision, allowing them to focus on objects by moving the entire lens closer to or farther from the retina, similar to how a camera lens adjusts focus.
The retina of most fish contains both rod and cone cells, enabling them to see in various light conditions and perceive color. Rod cells provide high sensitivity in dim light, while cone cells offer higher spatial and temporal resolution, allowing for color vision. Some fish species can even detect ultraviolet (UV) light, which can be beneficial for foraging or communication, such as juvenile brown trout using UV vision to detect zooplankton in shallow waters.
Additionally, some fish are sensitive to polarized light. This allows fish to make fine-scale behavioral discriminations between stimuli that differ in e-vector orientation, independent of brightness, and can be used for spatial orientation and navigation.
Specialized Vision and Adaptations
While many fish have robust vision, some species exhibit remarkable adaptations or limitations based on their habitats. Predatory fish, for instance, often possess highly developed vision to spot prey from a distance. Their visual systems are fine-tuned to detect movement and subtle contrasts.
Conversely, fish living in environments with limited or no light have evolved different strategies. Deep-sea fish, residing in deep-sea environments, often have large, sometimes tubular, eyes that look upwards, giving them binocular vision and high sensitivity to faint light signals. Some deep-sea species even possess multiple rod opsin genes, allowing them to detect the faint bioluminescence emitted by other organisms and perceive color in near-total darkness.
The most striking examples are blind fish, such as cavefish like the Mexican tetra (Astyanax mexicanus), which have vestigial or no eyes. These fish have evolved in perpetually dark subterranean environments where vision offers no advantage. To compensate for their lack of sight, cavefish have significantly enhanced other sensory systems.
For example, their mechanosensory lateral line system, which detects pressure changes and vibrations in the water, is expanded, allowing them to sense prey and navigate their surroundings effectively. Cavefish also exhibit heightened chemoreception, meaning their sense of smell and taste is more acute, enabling them to locate food sources in the absence of visual cues.