The Mosquito Bay of Isla de Vieques is one of three bioluminescent bays in Puerto Rico, but the others -- Fajardo and La Parguera -- glow less brightly. Further, swimming is only permitted in Mosquito Bay. Vieques is approximately eight miles to the east of mainland Puerto Rico.

What Makes Mosquito Bay Glow in the Dark?

Dinoflagellates are freshwater and marine unicellular plankton. Referred to by guides as "dinos" but properly called pyrodinium bahamense -- which translates as "swirling fire" -- they produce a burst of light when disturbed by currents, or even when simply bumping into each other. Each measures approximately 1/500th of an inch but gives off a burst of light that spreads to more than 100 times its size.

Tour of Mosquito Bay

There is no public after-dark transportation to or from Vieques Island. One tour is offered that does not involve an overnight stay on Vieques as of 2011. The tour is operated by East Wind Excursions (, which runs a catamaran voyage from the mainland to Vieques, and Island Adventures (, which deals with the experiences on the island and the biobay tour itself. The week before the full moon the experience is slightly diminished, and the tour is not run on nights of the full moon or the nights that bracket it.

Meeting the Tour

Departure times vary according to the season; booking staff inform patrons of when to arrive at the marina when taking telephone reservations. Leave San Juan on Route 3 east and travel to Fajardo; at the merge with Route 53 -- immediately before the toll plaza -- follow exit signage to Puerto del Rey marina. The journey typically takes approximately one hour, but San Juan rush hour traffic can affect travel times. Puerto del Rey marina has gated parking for guests; guards refer to the tour operator as "East Wind" and direct drivers to the appropriate dock. Take a towel and a change of clothing when leaving your vehicle.

Voyage to Isla de Vieques

East Wind Excursions has two catamarans; the larger or smaller is used depending upon how many tickets have been sold. A menu is available at check-in, and clients make their choices of what they want for dinner. To enjoy traditional Puerto Rican food, the yucca mofongo or chuleta con tostones is recommended, followed by sweet fried plantains. The voyage takes approximately 45 minutes, passing some small islands. Unlimited rum drinks along with soft drinks and juices are included in the ticket price; pina coladas and rum punch are featured.

On Vieques

A 10-minute van or taxi ride -- depending on the size of the tour group -- takes patrons from the dock to the pre-booked restaurant; a guide travels with each party to talk about the island. Watch for wild horses on the beaches. The restaurant is owned and operated by Island Adventures, who also operate the biobay tour. Drinks at the restaurant are not included in the excursion price. The restaurant restrooms are the last private place to change into swimming gear. After a brief lecture on the bay, a 15-minute bus ride is taken to the pontoon boat that will make the last part of the outward journey. The pontoon is powered by electric motors.

What Makes Mosquito Bay So Special?

Seasonal bioluminescence is not unusual; in Mosquito Bay the phenomenon is year-round. Precipitous land above the bay is subject to special zoning regulations that mandate the forest cover not be disrupted, so sediment cannot be flushed into the water by heavy rains. The bay itself has a small mouth at its windward end, so there is a relatively small tidal exchange -- just large enough to prevent stagnancy -- and the ocean is prohibited from washing away the bioluminescent organisms. The water is the perfect depth to remain cool during the day, yet warmer than the ocean. The decomposing leaves and roots of the red mangroves on the bay shores release tannins that are rich in many nutrients, particularly vitamin B12, that provide the perfect diet for the dinos. Because of this confluence of circumstances the concentration of dinos is high, approximately 160,000 dinos per liter, making Mosquito Bay the brightest bioluminescent cove on the planet.

The Tour of Mosquito Bay

Bioluminescent glow is immediately evident: The wake of the boat becomes a glittering aquatic trail even before leaving the dockside. Fish can be seen darting away from the keel leaving incandescent blue-green-white trails in the water. Eventually, the boat moors in a dark section of the bay; the crew explain the nature and mechanics of the dinoflagellates, then help guests don flotation belts and climb in. The water glows when you move through it, and sparkles on your body if you stay still. For those choosing not to swim, guides are happy to dip a bucket over the side; the dinoflagellates in the bucket glow from simply being stirred. To take effective photographs with a digital camera it must be set to ISO 1200 or higher, flash off. The swim lasts approximately 30 minutes in water that is typically between 75 and 85 degrees F.

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