Can Goldfish See in the Dark?

Like us, fish rely on their senses to navigate through their habitats, find ideal sources of nutrients, and stay away from potential dangers. Some have remarkably keen sense organs, allowing them to thrive in areas where most animals would be unable to survive through a single night. In the dark depths of large lakes and in waters where moonlight hardly pierces through the surface, night vision would definitely be an evolutionary edge.

“Seeing”, however, is not always a function of eyesight. The humble goldfish, perhaps the most popular fish due to its historical spread and its docility, relies on its vision to see through the day. With sharp eyes, it locates its food, its tank companions, and its favorite points of exploration with uncanny precision. In tanks and ponds with night lights, it can continue to rely on its eyes to find adequate shelter and nocturnal treats.

Darkness is another matter entirely as goldfish cannot see well in low light! Although they are tetrachromats, able to see four primary colors (i.e. red, blue, yellow, and ultraviolet) instead of just three, they are far from adept at visual navigation in the dark. Instead, they rely on other means of “sensing” where they are and staying away from predators.

Some Facts on Goldfish Vision

Among all types of freshwater fish, the goldfish has some of the most well-studied sensory organs. There is proof that its eye movements and its specialized lenses do indeed aid in its perception of its surroundings and its acquisition of basic needs. Just like us, it relies on its optic adaptations for navigation. It can even see beyond the walls of a glass tank or process the color of the sky from within a pond.

Goldfish arguably have a more advanced retina compared to humans, who can only perceive three primary colors. They can see ultraviolet light due to the addition of a class of cones and a UV receptor in their retina. Yet, compared to species that have evolved to hunt and remain active in darkness, they have fewer UV receptors and fewer adaptations for maximizing low light levels. Keep in mind that even the most advanced UV vision would not be useful in the total absence of light.

A well-developed retina, despite being characterized by advanced photoreceptors, does not allow goldfish to see as sharply as we do. While they can see more colors and can evaluate the adequacy of their environments based on light intensity, their lenses do not have the most fantastic resolution. Essentially near-sighted, they perceive objects located more than 15 feet (4.6 meters) away from them as blurry blobs of color.

A Keen Sense of Smell

In the wild, it’s crucial for animals to have other well-developed senses apart from sight. Hidden dangers are often first detected by subtle changes in pressure, vibrations in the air or water, and traces of unusual smells. Similarly, basic survival needs like food and water are often detected by wafts of appetizing smells and by sound.

The olfactory processes of a goldfish are regulated by a set of nares or specialized nostrils, which have their own scent receptors located inside of them. Unlike the human nose, which is connected to other sense organs, a goldfish’s nares are blind-ended. In darkness, goldfish “see” their surroundings by relying on their sense of smell and on their lateral line organ system (discussed below). In tanks and ponds, these senses keep them from bumping into one another and colliding with physical structures.

As scent travels differently in water, goldfish smell by detecting chemical cues. Cues are often released, in the form of pheromones, by their sexually mature conspecifics, by potential predators, and by smaller prey items. Even artificial feeds, vegetation, and other components of aquatic habitats have unique scent signatures that goldfish may pick up on. This way, goldfish can maneuver (for a limited amount of time) in total darkness without being completely lost.

Sensing Vibrations in Darkness – The Lateral Line Organ System

When chemical cues and light levels are scarce, goldfish can rely on their lateral line organ (LLO) system for some awareness of their surroundings. In dark and deep water, where it is seemingly impossible to differentiate up from down and east from west, the LLO plays a crucial role in navigation and escape. Considered a primitive sensory system in vertebrates, it is present in both young and adult fishes. Permanently aquatic amphibians are also equipped with an LLO.

Essentially, the LLO is a network of water flow detectors (neuromasts) along the grooves or just underneath the skin of the head, trunk, and tail of fish. It appeared early in the evolutionary timeline of vertebrates. Its primitive nature highlights just how important it is for animals to sense vibrations, changes in pressure, and directional flow in a marine or freshwater environment.

By detecting these “disturbances”, goldfish can distance themselves from dangers that lurk in the darkness. Nonetheless, having a functional LLO and well-developed sense of sight does not guarantee their survival, even during daylight hours. Many predators with an even more advanced set of senses can catch a goldfish off guard.

Survival in Low-Light Conditions

Goldfish can still navigate and search for food in low-light conditions, though they will need to rely more on their sense of smell and their LLO to move in an efficient manner. While they can stay alive in perpetually shaded ponds and in poorly lit tanks (as long as water parameters are optimized), they may develop dull colors and behavioral abnormalities.

The lack of sunlight naturally reduces the amount of pigmentation in the chromatophores of goldfish. Even fish that are brightly colored can lose their skin pigments over time. The loss of color is sometimes a sign of inadequate lighting, which can lead to stress, a weaker immune response, and reduced skin protection. To keep them happy and healthy, make sure your goldfish are exposed to ample light levels every single day.

Can a Goldfish Live in Total Darkness?

Many bodily processes are regulated by natural shifts in light levels over a day-night cycle. This establishes a rhythm that regulates daily routines. Metabolism, one of the most important aspects of fish survival, is drastically affected by a lack of change in light levels. As goldfish are a diurnal type of freshwater fish, an increase in light urges them to eat and digest their food. In contrast, the eventual decrease in light is a signal to slow down and search for a place to rest.

Although goldfish have adaptations for searching for food and maintaining a safe distance from harm in low light, they will suffer under prolonged conditions of darkness. Unable to rely on their eyesight, their chances of survival may be compromised. Moreover, as a light-dependent routine cannot take place in continuous darkness, they are prone to becoming extremely stressed. Goldfish need light to stay active; they also need as many as 8 – 12 hours of darkness to rest at night.

Similarly, because goldfish do not have eyelids to shield their eyes, they will suffer in endless periods of light. Thus, darkness is essential for a regulated sleep cycle. This ornamental fish’s visual adaptations are truly suited to outdoor ponds where it can spend its nights under cover of darkness and its days in the warmth of sunlight.