Can Fish See In The Dark?
Can fish see in the dark?
This question might sound a lot like a shower-thought, but as you’re about to find out, the answer is a lot more complex than a “yes or no” option.
Scientists have put this issue on the table as well. But you might be thinking: “Why would it matter to us? Isn’t it just a type of trivia-curiosity?”.
As it turns out, finding out if fish can see in the dark has multiple advantages:
- It can help scientists determine the best ways to interact with deep-sea fish when studying them;
- For fishermen, it can guide the entire industry to develop more efficient bait (products & techniques);
- For aquarists, it is more of a curiosity than it is a necessity.
So, can fish see in the dark?
There are plenty of fish species that feed during both daylight and in the dark hours of the night. These species have an evolutionary mechanism that helps them go through a light-to-dark, dark-to-light adaptation when it comes to their eyesight.
Fish eyesight relies on the extension of the lens through the pupil, which means their eyes can’t adjust to fast changes in light levels by contracting and dilating the pupil.
How do fish adjust their eyesight from bright lights to darkness?
- A coordinated shift of the rods, cones, and dark pigment granules in the eye takes place;
- In bright light, the pigment granules will move to the outer sections of the rods and cones. The rods will retract into the pigment for protection against strong light rays, while the cones move forward to meet the light and allow color-vision in fish;
- In the dark (low-light), the rods move forward to meet any existing light, while the cones retract;
- The adjustment can take between 30-60 minutes to complete.
Can Fish See Colors In The Dark? What Colors Are Fish Able To See At Night?
To fully explore this possibility, scientists have studied the only category of fish that live their entire life spans away from sunlight: deep-sea fish.
The eyesight of deep-sea fish has an exceptional adaptation, which allows them to perceive light across the entire range of bioluminescence, from several shades of blue and green to yellow.
We actually found evidence that deep-sea fish can see colors in the dark after a group of scientists studied the Silver Spinyfin (Diretmus Argenteus).
Using a certain type of bacteria, they were able to replicate some of the rod pigment proteins found in the Silver Spinyfin’s eye in a petri dish.
They then exposed the proteins to light, hoping to see which parts of the color spectrum the pigment proteins could absorb. Various shades of blue, green, and yellow were the main takeaways of this experiment.
These results also allowed scientists to extrapolate their findings to determine which colors a deep-sea fish with multiple rod pigment proteins could see.
They were able to identify the colors by examining the shapes of the proteins. Each protein shape is sensitive to a different wavelength of light.
The Silver Spinyfin, the tube-eye fish, and the lanternfish are the deep-sea fish now considered to have the most rod pigment proteins, and implicitly the ability to see the most colors, of all deep-sea creatures.
What scientists could not determine is whether these deep-sea fish actually use their rods to see color.
Can fish see colors in the light?
The presence of rods, cones, and pigment granules makes it clear that fish have the ability to see colors.
But, if we were to explore which colors fish can see, we would have to take into account what wavelengths of light can reach a fishes’ eye.
Water is the medium that determines how much light a fishes’ vision can be exposed to. Also, in the wild, sunlight has multiple obstacles to pass through before reaching a fish’s eye.
Under the best water conditions, a clear body of water will absorb violet and red wavelengths easier, while blue wavelengths will reach greater depths.
That’s why the deep-sea looks saturated with blue in images captured by scientists.
How Do Fish See?
Vision in all fish species is a bit more complex, mostly because they live underwater. Water, be it freshwater or saltwater, can range from crystal clear to tea-colored, or even “cloudier” variations.
Open-water is an extremely unpredictable environment, which means water conditions can change in a flash. Even a quick rain can trigger a massive change in how well fish can see when exploring their habitats.
The clearest water with the least variables for fish in the wild is water covered by a layer of ice.
You can imagine the vision of fish to be permanently obstructed by a certain level of fog.
A fog that can sometimes be thinner (when fewer particles get displaced), and other times thicker (think of shallow lakes with plenty of dissolved organic matter).
How does fish vision look when compared to human vision?
Here’s a simplified comparison:
| Human Vision | Fish Vision |
| With a relatively similar eye structure, both humans and fish have an outer cornea, an adjustable iris, lens, and a retina that contains rods and cones. | |
| Our visual field can extend to miles away in the clear daylight. But below water, our max visual distance is limited to 100 feet, even in the best water conditions. | The lens of a fish eye is pulled back by specialized muscles through a process called accommodation.
This allows cruising-predators to see in the distance far better than we can. Sharks swim with their lens retracted, which optimizes their view of distant surroundings.
|
| Humans and other mammals have relatively flat lenses. | Fish eyes have outwardly protruding round lenses. Rounder and denser lenses are ideal for bending light, reaching a refractive index of up to 1.67. This gives fish a far superior peripheral vision to our own. |
The bug-eyed look of fish eyes is the exact result of those very useful protruding lenses. A wider field of vision allows fish to see significantly better than any human diver underwater.
Fish tend to swim with their lenses pushing forward, and that’s because this position allows for the best vision of close (or approaching) objects.
What Is The Lateral Line System, And How Do Fish Use It To Sense Their Surroundings?
The lateral line system, also known as a lateral line or lateral line organ, is a system of sensory organs found in all fish species.
Fish use the lateral line system to detect movement, vibrations, and changes in water pressure in their immediate vicinity.
The lateral line system uses pressure-sensitive organs aligned in rows of lateral lines running down each side of a fish’s body.
These organs are called neuromasts, and they allow the fish to recognize the presence of nearby animals/objects by sensing the shift in water pressure.
Fish use the lateral line system to accomplish some pretty basic/vital tasks:
- Lateral lines can give fish the ability to detect other animals underwater, even when light penetration is low or close to non-existent (at night);
- Fish can determine the direction and rate of the water’s movement using the hair cells of the lateral line system;
- The lateral line system can help fish accurately locate prey and escape from predators;
- Lateral lines are a crucial element in the ability of fish to form organized schooling patterns;
- Fish are able to develop a sense of their own movement thanks to the lateral line system.
All fish species have a form of the lateral line system, with some fish having a more advanced lateral line system than others.
You can visually inspect the lateral lines of most fish species. They look like faint lines running vertically down each side of the body, from the gill covers to the base of the tail.
A few fish species have seen an evolution of their lateral line system. Their line system organs have adapted to function as electroreceptors, sending information through electrical impulses.
Fish aren’t the only aquatic creatures that have a lateral line system. In fact, some amphibian larvae and adult amphibians have receptive mechanosensitive systems closely linked to the lateral line.
The lateral line system and the inner ear of fish are grouped together under a “parent-system” called the octavolateralis system (OLS). The lateral line and inner ear share many functions and are similar in structure and development.
Can Fish See Food In The Dark?
Aquarium fish don’t exactly see in pitch-black darkness. But they can, of course, use their lateral line system to search for prey or find food to nibble on even in the dark.
If your tank’s inhabitants are nocturnal feeders, you can safely feed them just after turning off your aquarium’s lights. This will allow them to feed in the environment they enjoy most, but it will also prevent food from becoming waste. Nocturnal fish like Plecos can be fed in the dark.
A fishes’ neuromasts (lateral line organs) will guide it through the aquarium in the dark, and signal it whether it’s getting too close to its tank mates, or whether food is moving/floating in close proximity.
Some aquarium tropical fish species, like the Peters’ Elephantnose fish, have a weak electrical organ on their caudal fin. It works the same way as electric eels navigate waters in the wild.
Some fish species have adapted to use their upwards facing eyes to detect prey. They can locate prey by distinguishing their silhouette against the dim light above. Examples that have adapted to this method of “seeing in the dark” are the Lantern fish, the Barreleye fish, and the Bigeye tuna.
So, the real answer to whether fish can see food in the dark is: “Yes, but not with their eyes.”
Can Fish See And/Or Recognize You?
Have you ever gotten the impression that your fish stare at you a lot?
Fish are food-driven fast learners. Also, they quickly associate seeing you with food. That’s why your mere presence will get most of the fish in your tank to come front and center, and they’ll wait for you to feed them.
In some anecdotally reported cases, fish were able to recognize their owners and even display signs of attachment.
Archerfish in particular have been studied and have shown signs of anxiety and nervousness when seeing strangers approaching the tank. These signs were a stark contrast to the archerfish’s engagement with a familiar observer.
To give you an even better understanding of just how well fish can recognize their surroundings, a veterinarian examining koi pond fish has recorded a significant difference in how the koi fish approached its owner for treats vs. how it retreated when seeing the vet in scrubs.
The koi fish was actually able to make the connection between blue scrubs and being taken out of water for investigations.
So, next time you pass by your tank, just know that you are seen!
Can Fish See And Recognize Their Tank Mates?
If fish can see/recognize humans, are they also able to recognize their tank mates?
There’s plenty of proof to support the notion that fish can recognize each other, but interestingly enough, fish have also shown researchers that they can form an attachment to other tank mates.
A study conducted with the exact purpose of examining whether fish can form an attachment with fellow fish in a tank has given us the answer, and it is: “Yes!”.
Given about 10 to 12 days, the tank mates formed a bond and preference towards each other. Also, the attachment displayed went as far as fish turning hostile towards new individual fish introduced to the same tank.
Not many fish species have been included in this type of study. Also, It’s been reported that guppies may have the ability to recognize up to 15 different individual fish.
A good indicator that fish might be a lot more socially-conscious than previously considered!
Should You Keep An Aquarium Light On? When Should You Turn Off The Aquarium Light?
Just like humans, fish need both periods of light and darkness. This stimulates and encourages fish to fall into a daytime-nighttime schedule.
Fish need to rest and replenish their energy levels after a day of exploring, searching for food, pursuing mates, and avoiding danger.
If you’re wondering how long you should keep your aquarium lights on, the answer is 10 to 12 hours per day.
To keep aquarium lights off your to-do list for each day, install a timer or get a lighting system with an integrated timer. With this timer, you can just set it for the intervals you decide on and leave it be.
Here’s a list of benefits of keeping your tank on a diurnal 12-hour light cycle:
- You can limit algae growth;
Algae love light, and too much light will cause algae to grow at a rapid rate. This can also make your tank look dirty. In addition, too much algae can negatively impact your aquarium’s water conditions.
- Fish do not need light at night;
If you were keeping your tank’s lights on 24/7 just because you’ve always thought aquarium fish need constant light, now’s the time to cut back on electricity usage.
- Light at nighttime can stress the fish;
It boils down to their need to rest. A daily extended period of darkness makes for the perfect incentive for fish to sleep and recharge.
- You sometimes have to train your fish to follow a diurnal cycle;
Aquarium fish will rest anytime they see fit. But a 12-hour period of darkness is sure to gradually put them on the same schedule you’re following. This way you can enjoy an active tank during daytime.
- Nocturnal feeders get a chance to feed in their preferred time of day.
Invertebrates (like snails) and some fish (like the Bristlenose Pleco) are significantly more active during nighttime/in the dark. Switching off the lights will let them know that it’s time to feed.
What do you do if for some reason you need/want to keep your aquarium’s lights on longer for the mesmerizing aesthetics we all know and love (while hosting parties for example)?
For such occasions, consider installing LED moonlight aquarium lights. They are bright enough to showcase your gorgeous tank, but dark enough to not disturb your fishes’ resting cycle.
Fun fact: While fish do not sleep like land mammals do, they certainly rest. In order to rest, studies have shown that fish will significantly reduce their activity. Also, they slow down their metabolism while remaining alert to danger. Most fish float in place, while some burrow themselves into a safe space (mud, coral, substrate, or a nest).
Conclusion
As it turns out, fish can see in the dark. But not in the ways we expect them to do it.
Adaptations of specialized organs make it possible for fish to do vital things. These include rest, feed, and avoid danger even in the dark.
Exploring this fun-fact has also revealed the fact that your tank-buddies might just like you back. You just keep those treats coming, and ignore the fact that you now know that there are plenty of eyes watching you daily!
See how a school of Flashlight fish manages to swim together in the dark:
