Reefing After Dark – Bioluminescence in the Reef Aquarium

Reefing After Dark – Bioluminescence in the Reef Aquarium
By Alex Gawura


In all the years that I have been an aquarium hobbyist, there have been a few moments that have put me in awe of nature and the wonders contained within our oceans. Recently I have experienced one of these moments, and I feel it should be shared with the reefing community. I want to draw some positive attention to one of the natural wonders in our aquariums that often goes unnoticed. Since I started building my 1600-gallon system over two years ago, I have spent a lot of time researching diverse topics related to marine biology and aquariums. One topic that has always interested me is bioluminescence which is defined by Haddock, Moline, Case as “The emission of visible light by an organism as a result of a natural chemical reaction.” (2010, pg. 444). I wondered if I could find some bioluminescent animals that could thrive in my system. I have had the experience of witnessing bioluminescence in the ocean once during a fishing trip and in a public aquarium display, but I have never seen bioluminescence in a home aquarium. After watching a documentary on bioluminescence and hearing of the considerable number of animals that can create bioluminescent light, I decided maybe I should take a closer look at my aquarium at night. With much skepticism, I decided that I would try to look for bioluminescence in my aquarium that night. When the lights went out, I decided the best place to look for bioluminescence in my system would be the 150-gallon refugium tank since I had 60lbs of live rock from the Gulf of Mexico in that tank. As night set in, I started to prepare for what I figured would be a disappointment.


150-gallon refugium tank

First Sighting

After lights out in my basement I sat down in my fish room and started to stare into my refugium tank, but it quickly became apparent that it was not dark enough in that room. I wanted total or near total darkness, so I grabbed the electrical tape to block out the light emissions from the equipment in my fish room. It was not perfect, but I achieved near total darkness and began watching the refugium tank. After 5-10 minutes, my eyes began to adjust, and I saw what I thought was a reflection of light on the glass. The light was dim, but I decided to try and locate its source in the fish room. I quickly realized that light was not coming from any equipment in the fish room but had to be from another source. Bioluminescence was staring me in the face as an organism was glowing and presenting me with my first experience of bioluminescence in a home aquarium. I could not believe my eyes, and I was awestruck that my assumed night of disappointment was quickly becoming one of the best experiences I have had as a hobbyist. I sat back in my chair to take in the moment, and then suddenly there was a flash of bright light from another part of the refugium tank and then another shortly after that. I started to realize that this world of darkness and bioluminescence that I did not think was possible to witness in my home aquarium was now very much a reality and a new way to experience the reefing hobby.


Isopod presenting a constant glow in the same way I first experienced bioluminescence

Functions of Bioluminescence

Bioluminescence has many functions depending on the organism. The main functions of bioluminescence are for offensive, defensive, communication, and reproductive purposes. Bioluminescence is important to organisms in the darkness of the ocean because it helps to find food, attract a mate, or to avoid being eaten which are all critical to survival (Widder, 2010). Bioluminescence serves as a means of communication or attraction in the darkness where light can be visible over great distances even when the size of the organisms producing this light can be relatively small. During the observations of my home aquarium, I can say with certainty that I have observed defensive bioluminescence behavior. Other bioluminescence I have observed is probable to be used for communication or reproduction. I believe that it is probable that the use is for communication or reproduction as these organisms live in large numbers in my system with bioluminescence taking place throughout the refugium tank. These organisms don’t seem to create light on the trigger of being disturbed or threatened which is also why I feel that their light emissions are not for defensive purposes. I find that as humans we to use light for many of the same functions as these organisms. Light is important to our everyday lives as when we see light in the darkness we are drawn to the light for several reasons depending on the circumstance. Bioluminescence has evolved over 40 separate times in evolutionary history making it clear that this trait is critical to survival (Widder, 2010). No matter what the purpose of bioluminescence is for an organism in the ocean, it seems clear that it is important to life in the oceans otherwise it would not be seen in such a wide variety of organisms.


The presence of bioluminescent light is occurring due to a chemical reaction that is being controlled by the organism emitting the light. As stated by Haddock, Moline, Case “Bioluminescence is typically produced by the oxidation of a light-emitting molecule—generically called the luciferin—in conjunction with a catalyzing enzyme—either a luciferase or photoprotein.” (2010, pg. 446). The chemistry behind bioluminescence is remarkable to me as it is a natural chemical that is being produced by these organisms. As I am not a chemistry expert, I will not attempt to explain the chemistry further as it is well beyond my level of understanding.


Observation conditions

The key to observing bioluminescence in your home aquarium based on my observations is that total darkness or near total darkness is necessary to have a meaningful observation experience. Most of our aquarium life support equipment has lots of bright LED status lights or “LBLs” telecom acronym humor for “little blinking lights”. In some cases, these LBLs will help to attract consumers to make product purchases. These light sources though ultimately must be blacked out as much as possible. I don’t recommend shutting down your aquarium life support equipment during a bioluminescence observation session. Instead, use some electrical tape or a blackout barrier around you and the aquarium to block these light sources. Remember that some electrical equipment can get pretty warm, so I don’t suggest covering this equipment with a thermal insulator or flammable objects such as a towel or shirt. Once light sources have been eliminated its time to start the viewing session. If I can no longer see my hand in front of my face or barely see it after my eyes have adjusted to the darkness I feel the conditions are right to start viewing.

Are my eyes are playing tricks on me?


This photo does not show any real bioluminescence

When a viewing session is started, it can take up to 30+ minutes for your eyes to fully adjust to the darkness, but normally around the 10-minute mark, I can start to notice the dim flashes of light. If you turn on lights or check your smartphone, these bright lights will ruin your night vision, and your eyes will need to readjust to the darkness again. A red flashlight can be used see your way around in the darkness while not significantly impacting your night vision. Bright bioluminescent flashes should be visible easily as soon as you start observing but at least in my aquarium, they are infrequent compared to the dimer flashes which I have observed at a much higher frequency. The color of the visible light is likely to be in the blue or green range. It’s important to remember that your eyes can play tricks on you in this level of darkness so you might think you see bioluminescence when in fact nothing is happening. An effective way to check that you are not seeing things is to look at the floor or a wall as they should not be producing flashes of bioluminescent light. I know that this has happened to me a few times, but when you see an actual flash of light or a glow from an organism, you will know it’s real. I have also had some issues with eye strain as there is no depth perception my eyes will start to feel uncomfortable at times when trying to focus on tiny points of light. I found that closing my eyes for a minute or two while relaxing can help to relieve this eye strain and allow me to continue an observation session. Another observation I have made is the consumption of alcoholic beverages can impact your ability to see the bioluminescent light, so I suggest forgoing the adult beverages before a viewing session. On a final note, I have also found that using my peripheral vision to pick up bioluminescent light has also been useful in making observations. If you have ever looked through a telescope or a microscope and have been told not to look directly at an object but allow your peripheral vision to see the object, it becomes brighter. I never understood this when I was younger, but I now understand that it can help to view objects that are very faint. I cannot guarantee that everyone will observe bioluminescence in their home reef aquariums, but I hope these few tips I have provided help during an observation session.

Observations of Bioluminescence

The preliminary observations of bioluminescence consisted of trying to understand what I was witnessing. I have observed organisms perform constant glowing, quick flashes, long flashes, and pulsing flashes. During my first observation sessions, I was very interested in trying and determining what organisms were producing the light. Much of the light generated is so faint that it is difficult to tell what exactly was generating the bioluminescent light. I have identified two organisms to date that are producing bioluminescent light. The first is a brittle starfish pictured below. I don’t know the exact species of this brittle starfish, but it was living on my live rock that originated from the Gulf of Mexico. The second organism is an isopod pictured below that was also a resident on the live rock from the Gulf of Mexico. I believe there are other organisms in my aquarium system that are also bioluminescent, but due to the nature of this activity in the darkness and the small size of these organisms, I have been unable to make any identification.


Unidentified bioluminescent Brittle Starfish


Isopods on the front glass of the refugium tank which sometimes produce a constant glow of bioluminescence

When I first observed bioluminescence, I mainly saw faint flashes of light or a constant glow of faint light with some random bright flashes of light throughout the refugium tank. The first observation of the brittle starfish bioluminescent light was startling as I was not directly looking where the brittle starfish was located when the bioluminescent display occurred. It was over a week into bioluminescent observations before I determined that some of these random bright flashes of light were originating from the same location. I had seen in a documentary that some brittle starfish could be bioluminescent, but I did not think it was possible that one of these brittle starfish was living in my aquarium. I could see the arms of a brittle starfish were extending out of the rock from the area where the bright flashes occurred. I used a feeding stick to touch the arms of the brittle starfish with the lights out and when touched the arms of the brittle starfish would produce bioluminescent light which points to a defensive function of bioluminescence. During later bioluminescent observations I also found that vibrations and irregular water movement near the brittle starfish can also trigger a bioluminescent response. I will state that while I can stimulate the brittle starfish to create bioluminescent light through touch, vibration, or irregular water movement, it is on a limited basis that I take these actions when trying to capture bioluminescent activity on camera. Based on current observations it seems the brittle starfish is naturally stimulated to create bioluminescent light about 1-2 times per hour. I have counted a total of three of these brittle starfish living in my refugium tank.


Arms of the bioluminescent brittle starfish extending from the rock after dark


One of the first images captured showing the bioluminescent display of the brittle starfish

The bioluminescent light created from the isopods and other small organisms in my refugium tank thus far have been the most common to see during any given observation session. The primary purpose of having total or near total darkness allows the observation of the faint bioluminescent light created by these small organisms. At times, I have witnessed isopods on the front glass constantly glowing or flashing which allowed me to make an identification of the isopods for the first time. The isopods live in large numbers in my refugium tank with hundreds on the front glass at night. Despite the number of isopods on the front glass they do not always present bioluminescent behavior. It seems that a small number of the isopods will flash at random and even less common is the constant glow of an isopod. I believe this is some form of communication between these isopods which could be related to reproduction.


Isopod on the front glass in a constant glow with another isopod flash captured on camera below and to the right


Isopod on the left was in a constant glow and the isopod image on the right was the flashes of either one or two isopods


Long exposures showing three isopod flashes

I still have no idea what organisms are generating some of the bioluminescent displays I have observed. There have been bright flashes in the refugium tank which are brighter than usual but have not yet been tied to a specific organism. The bioluminescence is not limited to the refugium tank and has also been observed in the display tanks which appears to most likely be from isopods, but again there are some bright flashes with an unknown origin. The brightest and most interesting unknown display of bioluminescence I have observed is pictured below.


Unidentified bioluminescent display

I was about to end an observation session when a brilliant glow starts coming out of the rock in the refugium tank. I was lucky that the camera was next to me, so I quickly set up the camera and started taking some pictures. The unidentified organism was glowing for a minute or two with enough light being generated to create to create some small shadows in the tank. The light slowly dimed but I did manage to get some additional photos of the unidentified organism.


The five images above in order from left to right were the same unidentified bioluminescent display that slowly dimed over time

Visual Media Failure to Success

After my first observations, I was determined to document the experience on camera using video or still photographs. My first attempts to record an image of bioluminescence were a complete failure. I made several attempts using my DSLR and failed to capture an image that was not a bunch of noise. I am by no means a professional or even an amateur photographer which means there was a lot of trial and error required before I was successful. I started out with an exposure time 30 seconds ISO100 F3.5. I worked the ISO higher, but after ISO800 the image was grey with noise. When I discovered that I could stimulate the brittle starfish to emit light, I attempted an 8-second exposure at ISO100 F3.5 and then stimulated the brittle starfish during that exposure, but it was just an empty image full of noise. I then shifted gears towards a low light camera solution which might be picking up some bioluminescence, but I am not confident that what it is capturing can be considered useful, but I will continue to experiment with this camera or just use it to watch the displays tanks during the day. I started to research more on using the DSLR as it should be able to capture bioluminescence. The best information I found was on the photography of fireflies. I went back to my DSLR and revamped my camera settings turning noise reduction on to the highest setting with a 5-second exposure ISO3200 F5.6. I then stimulated the brittle starfish and finally captured my first image of bioluminescence seen below.


The first image of bioluminescent light I captured on camera

With my first successful image of bioluminescence, I started to increase my ISO to 6400 then 12800. I have now been able to capture several images successfully of the brittle starfish displaying bioluminescence. While these images are exposures lasting several seconds, and show the movement of the brittle starfish making the image blurry, it does show the color and brilliance of their bioluminescent display. I will continue to try photographing this wonder of nature to see what images I can capture and share with the community. My camera is Canon Rebel T5i using an 18-135mm lens. The camera settings that I have been using are the manual mode, manual focus, exposure time 1-30 seconds, F3.5-5.6, ISO12800, and noise reduction set to the highest level. Here are some of my favorite images of bioluminescence that I have been able to capture. All of the still images in this article are jpegs have not been enhanced with photo software and have only been cropped.


One of my favorite pictures of the bioluminescent display put on by the brittle starfish


On the left side of the image the segments of the brittle starfish arm can be seen



Pictures above are bioluminescent displays put on by the brittle starfish

Capturing the faint flashing light from the smaller isopods and other organisms is also possible using the same settings on my DSLR, but the visual results are far less impressive with individual images. However, when you take a series of images looking at the same part of the tank and stitch them together as a movie, it becomes a little more interesting to observe. The YouTube video below contains the video I stitched together. The only edits I have done to the photos before stitching them together in the video was to increase the brightness and turn up the contrast to remove most of the noise from the images. The little dots of light appearing and disappearing are the bioluminescence displays the camera was able to pick up.

More questions

Now that I have observed bioluminescence in my aquarium there are several questions to consider as a hobbyist.
  • How common is bioluminescence in our reef aquariums?
  • What role if any does bioluminescence play in the health or success of our reef aquariums?
  • What other organisms in our reef aquariums produce bioluminescent displays?
  • Are there better ways to capture pictures or video of bioluminescent light?
I hope as a reefing community we can come together to further discuss the topic of bioluminescence in our reef aquariums and consider the questions above. I will continue to observe and document bioluminescence in my reef aquarium. I encourage others to try out an observation session or two for bioluminescence in their reef aquariums as I am interested to know how many reef aquariums are putting on a light show after dark. I find bioluminescence to be a fascinating natural wonder which has made reefing after dark something I look forward to every night.


Haddock, S., Moline, M., Case, J. (2010). Bioluminescence in the Sea. Annual Review of Marine Science 2010 2:1, 443-493, Retrieved from:

Widder, E. (2010, April). Edith Widder: Glowing life in an underwater world [Video file]. Retrieved from:
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About author
Alex Gawura has been keeping aquariums since the early 1990’s starting with a 5-gallon aquarium. Since starting with that 5-gallon aquarium Alex has kept multiple aquariums focusing on African cichlids, fresh water plants, marine fish, LPS corals, soft corals, tridacna clams, and stomatopods (Mantis Shrimp). During high school and through college he also worked at an LFS along with a maintenance company that maintained aquariums at homes and businesses. In the last few years Alex has dedicated his time towards building a DIY saltwater aquarium system which holds over 1600 gallons. While focused on maintaining his aquarium system Alex has also been researching bioluminescence.

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