Published May 2005
Dear Sea Scoop!
While visiting St. Thomas, I went on a night snorkel. When we turned off our flashlights we saw hundreds of glowing creatures. What were they and why do they glow?
Chagrin Falls, Ohio
What you saw were one or many different kinds of marine organisms that produce visible light. This shimmering lightshow is caused by a chemical reaction called bioluminescence (BY-oh-loom-ih-NESS-ense). On land there are very few
bioluminescent creatures, the firefly being the most commonly known. In the ocean, however, there are many different animals that produce their own light. Some of the marine organism that produce bioluminescence include dinoflagellates (dee-no-FLAJ-eh-lets), which are tiny, one-celled organisms; small crustaceans called copepods (KOH-puh-pods) and euphausiids (you-FAW-zee-ids); marine worms called polychaetes (PAH-lee-keets); sea jellies, and even some fish.
The chemicals involved in the process are called luciferin (the substrate) and luciferase (the enzyme). When these chemicals mix, the reaction produces a glowing light. Some animals need to replenish their supply of luciferin through food, but others are capable of synthesizing it on their own. In addition, there are some marine animals, like squid and cuttlefish, that have bioluminescent bacteria in their organs which allow them emit light. Most bioluminescent organisms produce a blue light, but some can emit green or yellow, and one family of fish can even emit red.
So why would an animals want to glow in the dark? Bioluminescent organisms employ their light to help them survive in the dark ocean. There are several different bioluminescent strategies for survival first, the light can help the organism find food just like a flashlight can help you find something in the dark. The deep-sea anglerfish uses its bioluminescent lure to help it find food – yes, this is the fish from "Finding Nemo" that nearly ate Marlin and Dory! Some marine organisms use their bioluminescence to find a mate, like the female fireworm Odontosyllis who swims up through the water in tight circles releasing a trail of bioluminescence to help the male find her. Glowing organisms also use bioluminescence for defense. Some copepods release a squirt of glowing fluid when confronted by a predator. This serves as a screen blocking the predator's view, and allowing the tiny crustacean to swim away.
There is a difference between bioluminescence, phosphorescence and fluorescence. Bioluminescence, as used here, is when a light-producing chemical reaction occurs inside of an organism. Fluorescence occurs when the energy from an external source of light is absorbed and almost immediately re-emitted. Phosphorescence is like fluorescence except that the product is more stable, allowing a glow long after the light source is gone. Most glow-in-the-dark products are phosphorescent.
For more information on bioluminescence:
Harbor Branch Oceanographic Institute's Bioluminescence Page
Scripps Institute of Oceanography
University of California at Santa Barbara's Bioluminescence Page
Bioluminescent Bay, Vieques, PR
Have a question about the world beneath the waves? Write it down and send it to Sea Scoop! Please remember to include your name and where you're from.
For more information on marine science in the Virgin Islands, visit the University of the Virgin Islands' Center for Marine & Environmental Studies.
Elizabeth Ban is the University of the Virgin Islands Marine Adviser for St. Thomas and St. John. She works to inform and educate citizens about ocean resources and promote coastal ecosystem health. She is based at UVI's Center for Marine and Environmental studies on the St. Thomas Campus. For more information about UVI's Marine Advisory programs, please call 340-693-1392.