Quick answer: Bioluminescent bays are shallow, sheltered bays where the water glows blue-green when it’s disturbed, by a paddle, a fish or your hand. The light comes from huge numbers of tiny single-celled organisms called dinoflagellates, which flash using a chemical reaction between luciferin and the enzyme luciferase. Puerto Rico has three of the best-known: Mosquito Bay on Vieques, Laguna Grande in Fajardo and La Parguera in Lajas. The best time to visit is on a dark night near the new moon.
Imagine dipping a paddle into black water and watching it explode with sparks of blue light. Bioluminescent bays are one of the most magical natural experiences on Earth, and the science behind them is just as fascinating as the glow.
What causes bioluminescent bays to glow?
The glow is made by microscopic plankton, most often dinoflagellates such as Pyrodinium bahamense in Puerto Rico. Each one is a single cell, far too small to see on its own. When the water around them moves, the pressure changes the cell’s membrane, triggering a chemical reaction:
- Movement in the water squeezes or bends the cell.
- An electrical signal releases acidic protons inside the cell.
- The change in acidity activates the enzyme luciferase.
- Luciferase makes the molecule luciferin react with oxygen, releasing a flash of blue light lasting a fraction of a second.
With hundreds of thousands of dinoflagellates in each liter of water, every movement creates a sparkling trail. Read more about glowing living things in our deep sea guide.
Why do plankton glow? The “burglar alarm” theory
Scientists think the flashes help dinoflagellates survive. When a small predator such as a copepod starts eating them, the flash may attract larger predators that eat the copepod. This idea is called the burglar alarm hypothesis. The sudden light may also startle predators directly.
Why are some bays brighter than others?
Most coastlines have some bioluminescent plankton, but only a few places have enough to glow brightly all year. The best bioluminescent bays share several features:
| Feature | Why it matters |
|---|---|
| Narrow entrance to the sea | Keeps the plankton inside instead of washing out |
| Mangrove forests | Supply nutrients such as vitamin B12 that the plankton need |
| Shallow, warm water | Supports fast plankton growth |
| Little light pollution | Makes the glow visible |
| Low pollution and runoff | Prevents other organisms from taking over |
Puerto Rico’s three bioluminescent bays
| Bay | Location | Highlights |
|---|---|---|
| Mosquito Bay (Puerto Mosquito) | Vieques Island | Recognized by Guinness World Records in 2006 as the brightest bioluminescent bay; kayak and electric-boat tours |
| Laguna Grande | Fajardo, northeast Puerto Rico | Reached by kayaking through a mangrove channel; easy to visit from San Juan |
| La Parguera | Lajas, southwest Puerto Rico | Larger and more open; boat tours, and one of the few places where swimming has been allowed |
Mosquito Bay dimmed sharply after Hurricane Maria in 2017, but its glow recovered over the following months, showing how sensitive these ecosystems are.
Other places to see bioluminescent water
- Luminous Lagoon, Jamaica: near Falmouth, where freshwater and seawater meet.
- Vaadhoo Island, Maldives: famous for glowing waves on the beach, often called the “Sea of Stars”.
- Toyama Bay, Japan: firefly squid light up the bay in spring.
- Gippsland Lakes, Australia: occasional spectacular blooms of glowing plankton.
- Holbox Island, Mexico: glowing water along the shore in the warmer months.
- Tomales Bay, California: seasonal glow, usually late summer to autumn.
For other light shows in nature, see natural light phenomena.
When is the best time to visit a bioluminescent bay?
- Near the new moon. Moonlight drowns out the glow. Plan your trip for the darkest nights of the month.
- After sunset, on clear nights. Clouds don’t hurt, but bright city lights do.
- Puerto Rico’s bays glow all year, though brightness varies with rain and weather.
- Check local conditions. Heavy rain and storms can temporarily reduce the glow.
How to protect bioluminescent bays
- Choose licensed, eco-certified tours.
- Don’t wear sunscreen or insect repellent in bays where tours ask you not to, as chemicals can harm plankton.
- Don’t use flash photography; it ruins the experience and doesn’t capture the glow well.
- Stay in kayaks or boats unless swimming is clearly allowed.
Frequently asked questions
Can you swim in a bioluminescent bay?
In most protected bays, including Mosquito Bay, swimming is not allowed, to protect the plankton. Rules vary by place, so check before you go.
Is bioluminescent water safe?
The dinoflagellates in Puerto Rico’s bays are harmless to touch. Some other glowing plankton blooms, such as certain red tides, can be harmful, so follow local advice.
Can you photograph a bioluminescent bay?
It’s difficult. You need a camera with manual settings, a long exposure, a wide aperture and a steady base. Phone cameras usually struggle.
How many bioluminescent bays are there in the world?
Glowing plankton occur in many places, but only a handful of bays glow brightly and consistently. Puerto Rico’s three are among the best-known.
Related articles
- Natural light phenomena
- Natural phenomena that will leave you in awe
- Mysteries of the deep sea
- Hidden wonders of the world
Sources
- Haddock, S. H. D., Moline, M. A. & Case, J. F. (2010). “Bioluminescence in the sea.” Annual Review of Marine Science. doi:10.1146/annurev-marine-120308-081028
- Valiadi, M. & Iglesias-Rodriguez, D. (2013). “Understanding bioluminescence in dinoflagellates: how far have we come?” Microorganisms.
- NOAA Ocean Service: “What is bioluminescence?” oceanservice.noaa.gov
- Puerto Rico Department of Natural and Environmental Resources: Natural reserves of Puerto Mosquito, Laguna Grande and La Parguera.