Can Fish See in the Dark?

Can fish see in the dark?

Can fish see in the dark? Here’s the surprising answer: kind of, but not in the way you might think.

Before we dig into the interesting facts about fish vision, it’s important to understand why this question needs to be asked and addressed.

Scientists find it useful to study these types of questions because it helps them better understand other species like mysterious, deep-sea fish. Recently a team of scientists found a new species of deep-sea fish which opens up new questions and new possibilities to explore.

As an aquarist, how can knowing this question be useful to you? By knowing how your aquarium fish live their lives underwater, you can create the perfect and most suitable environment and enjoy your fish for many years to come.

So how do fish see in the dark? Well, there fish eyes help a little bit. But life in water is different than life in air and so they must have different sensory organs. Vision in water is slightly less useful due to the physics of light refraction in different mediums. In other words, aquatic fish, and other animals, must supplement with other sensory organs.

Lateral lines of fish are the main sensory organ that a fish uses to see in the dark. The fish is actually not seeing, but rather feeling in the dark (and in light too). It is feeling movement in the water and detecting water displacement. Lateral lines help fish detect the minutest movements in the water.

A fish’s eye is made up of similar proteins and structures like human eyes. All vertebrates – which include fish – have rods and cones that work together to respond to light. Fish have cone cells that are sensitive to different wavelengths of light. Fish also have rod cells that are sensitive to a wide range of wavelengths. These work in unison to capture light, but how much light is captured depends on where the fish lives in the water column.  

In clear water, with ample light, water can absorb longer wavelengths, like red or blue, and fish with specific proteins in their rod and cone cells will be able to capture and interpret color. So, fish can also see color! It just depends on their location in the water column and their habitat in the wild.

If fish are in muddy waters or dark waters, including deep water, then light becomes harder to interpret in a fish’s eye. So, they have to rely on other sensory organs to help them survive, like the lateral lines we previously mentioned! Open waters often change rapidly, and so fish always have to have reliable ways of surviving in the wild.

Lateral lines on fish make up for weak vision in fish because it’s a very sensitive organ. It runs along the side of the body in fish and found in all species of fish. In fact, it’s found in all aquatic vertebrates and some amphibians that have primarily aquatic adult lives. These sensory organs are lined with neuromast cells which are as sensitive as tiny hair cells. This helps fish species feel changes in water pressure and helps them to hunt, feed, and detect changes in swimming direction of other fish which is useful for fish that like to swim in groups.

Are there fish species with night vision? All fish have some sort of night vision through how they use their lateral lines.  Their lateral lines help them navigate even inside total darkness. But their eyes do help because of the rod and cone cells. It’s a fun fact that fish don’t have any eyelids. They can’t actually blink because they have no eyelids. Instead, fish have a protective covering over their eyes that helps them see clearly. This monocular vision also includes color vision. Because fish have rod and cone cells, they are able to interpret the color spectrum. They are able to see red light and green light as well as ultraviolet light. For fish, this becomes easier to see as they get closer to the surface of the water, but if they are in deep water, they rely on other special sense organs like lateral lines.

Deep-sea fish live in low light conditions and so vision is of little use to them. They have evolved specific light organs beyond their lateral lines, like photophores that actually emit blue light and is useful for identifying potential mates or during feeding. These photoreceptor cells are useful for capturing light and helping fish make sense of their environment.

So what does all this mean for your fish tank? It means that your aquarium fish can see in the dark primarily through their sensitive lateral line, and they use their eyes whenever there is sufficient light.

Here are some cool species of fish you might own that use their lateral line to “see” in the dark:

• Goldfish use their lateral lines as a sixth sense to orient themselves in their environment. It’s often a darker colored than the scales or skin
• Betta fish have lateral lines that you can see. It forms a gentle curve from the side of the head towards the caudal fin.
• Guppies have been documented showing changes in their lateral line shape when in the presence of predators.
• Oscar fish are prone to developing Hole in the Head disease, an often fatal disease that attacks the lateral line through lesions. 

Is it OK for fish to be in the dark?

Yes it is okay and absolutely necessary for your fish to be in the dark. Like other species of animals, fish live their lives by the light cycle and dark cycle. Changes in the hours of light dictate how fish behave and generally live their lives. So fish need to be in the dark, so they can rest and recuperate and build up their energy reserves. Changes in the hours of light also initiate specific behavioral cues, or chemical and physiological responses such as finding mates.

What do fish do when it’s dark? Fish sleep! They might not sleep just like us with their eyes closed. As mentioned earlier, fish do not have any eyelids. You may actually catch your fish sleeping during the day. A sleeping fish will usually be extremely still and may not respond to movement. It might be just floating in the water. Some species of fish actually find cover, such as patches of planted vegetation in your aquarium, and they will use that cover to rest. Using their lateral lines, fish can be sleeping and still detect any pressure changes in the water which would alert the sleeping fish to any potential danger such as a predator.

Do fish need complete darkness to sleep?

Yes fish need periods of complete darkness and light. In order to sleep, fish need total darkness. It helps regulate their internal functions and behaviors. Some nocturnal fish take their cues from environmental light, for example, they hide under rocks in the presence of a lot of light. Different species of fish may be more energetic and interact more with other schools of fish or their closest neighbors. Using their sense of smell, fish can learn to recognize one and other and have been known to form bonds with tank mates. These types of behaviors can happen if fish are allowed to follow a consistent day and total darkness schedule.

Having complete darkness is also important because too much light can cause excessive algae growth. Too much algae in your tank may contribute to unhealthy and polluted waters. This would eventually harm your fish community. 

Do fish like the light on at night? Fish do not like the light on for long periods of time and prefer to have total darkness for certain periods of time. For your aquarium, it is important to adjust your aquarium lights to reflect a period of light followed by a period of darkness. This is important because it mimics the natural habitat that your fish is found. Fish sleep and it is important for them to do so or they can be more prone to diseases and stress which eventually leads to an early death.

You can follow a simple schedule like this one: leave aquarium lights, like white light or blue light, on during the day, then turn them off at night time. Just remember that, like humans, many fish species follow the same day and night cycles. Lighting is important not only for plants that depend on light to make their own food, but also for aquarium fish in order to manage their daily lives. Too much light can cause fish to stress and make it harder to fight off diseases and can cause excessive algae growth.

It is best to mimic the same natural environment your fish is from. Provide your fish tank with adequate lighting during the daytime and blue light can be used in addition to other lights like white lighting. However, it is important to have dark periods otherwise you risk having too much light enter the tank which can disrupt a healthy environment. Aim to make each photoperiod, or the light cycle, be 8-12 hours on and 12-16 hours off.  

Different species of fish found in the tropics, like guppies and betta fish, live under conditions that provide roughly 12 hours of light a day, which means you should strive to mimic that in your aquarium as close as possible. Other cold-water species such as goldfish are from temperate climate zones and the hours of light vary according to the season. In your aquarium, you should vary the amount of light in your tank over the year. A good in between is 8 to 12 hours of light depending on how your fish acclimate.

Can fish eat in the dark?

Yes, some nocturnal fish can eat in the dark. They use special sense organs like lateral lines, in combination with their sense of smell, to locate prey in the darkness.

But many fish species aren’t as active in the night and enter of period of energy recovery, so they won’t be actively searching for food.

Nocturnal fish like catfish would use their lateral line system to navigate and locate prey in total darkness. They are use their sense of smell and hearing too. Most tank/aquarium fish do not have any special form of night vision so they prefer to eat when there is light, even if it’s dim light.

Can fish see humans?

Yes, your fish can definitely see you! In fact, fish can actually recognize each other and their tank mates.

There has been research that shows fish can recognize each other through their species sense organs as well as their sense of smell. 

On average, it takes less than two weeks for your fish to start forming bonds with one another or specific tank mates. There has been some research that shows fish form special attachments and can become hostile towards new fish. 

Your fish may actually learn to recognize you as it associates you with food. Try forming a bond with your aquarium fish by admiring their beauty and caring for their home. It could be good for your fish and your health too!

Summary

As a general rule, fish can see in the dark, but they rely primarily on their lateral line system. Lateral lines are sensitive organs that help detect changes in pressure and movement in the water. Fish eyes are evolved to have rod cells and cone cells that help them interpret light and see the color spectrum.

Fish love to be in the dark and it is healthy for fish to be in the dark. Fish need periods of light followed by periods of total darkness. Fish mainly sleep in the dark, but some nocturnal fish may be active and hunt for food, which means fish can also eat in total darkness.

Fish have been known to form bonds with their owners, so keep visiting and caring for your aquarium and get all the good benefits for your health!