Many sea creatures make their own light, and the San Juan Islands are home to a surprising number of them. From microscopic plankton to translucent comb jellies, the nighttime waters around Friday Harbor host a living light show built by living organisms.
The Glowing Residents of the San Juan Islands
Bioluminescent organisms are animals and microbes that produce light through a chemical reaction inside their own bodies. In the cold, plankton-rich waters of the San Juans, several of them are common enough to encounter on a calm, dark night.
Each species uses its glow differently, but the chemistry is similar: a light-producing molecule reacts with oxygen to release a quick, cool flash. Here are the creatures most likely to put on a show.
Dinoflagellates: The Stars of the Surface
The brightest and most reliable glow comes from dinoflagellates, single-celled plankton smaller than a grain of sand. When the water moves, they flash blue-green, and millions flashing together can outline every paddle stroke and ripple.
Their light is a defense mechanism. A sudden flash startles tiny predators and can draw larger hunters to the area, turning the plankton attacker into prey. This is why the glow brightens exactly when something disturbs the water.
Comb Jellies: Shimmering Drifters
Comb jellies, or ctenophores, are among the most beautiful night residents of the San Juans. They move using rows of tiny hair-like cilia that scatter light into rainbow shimmer during the day. At night, many species add true bioluminescence, glowing softly when bumped.
Unlike true jellyfish, comb jellies do not sting. Their gentle, drifting glow is one of the quieter delights of a night on the water.
Comb jellies are also surprisingly ancient animals, with a body plan that has changed little over hundreds of millions of years. Watching one pulse past a kayak hull is a glimpse of a very old branch of life, lit from within. They are easy to miss unless the water is calm and dark, which is one more reason still, moonless nights reward patient paddlers.
Other Light-Makers You Might Encounter
The night ecosystem extends well beyond plankton and comb jellies. Depending on conditions, paddlers and naturalists in the region may notice or learn about a range of luminous life:
- Sea pens are feather-shaped colonial animals anchored to the seafloor that can glow when touched.
- Krill and copepods are tiny crustaceans that emit light and form the base of the marine food web.
- Certain jellyfish flash light along their bells as a startle defense.
- Marine bacteria produce a steady glow and sometimes light up decaying material on the seafloor.
- Brittle stars are relatives of sea stars whose arms can luminesce when disturbed.
Most of these are subtle compared with a dense bloom of dinoflagellates, but together they make the dark water a far busier, brighter place than it appears from shore.
Why So Many Creatures Glow
Bioluminescence serves several purposes in the marine world: defense, camouflage, attracting prey, and communication. According to the Monterey Bay Aquarium Research Institute, light production is one of the most widespread traits in the deep sea, used by a vast share of midwater animals. You can read more in their overview at MBARI: Bioluminescence.
In the San Juans, the most common reason for the glow is defense. The flash is a quick, energy-efficient alarm that helps small organisms survive in a world full of predators.
Seeing the Night Ecosystem for Yourself
The best way to meet these creatures is from the water itself, where a paddle stroke can trigger the glow and a naturalist can name what you are seeing. A guided bioluminescence kayak tour pairs the experience with the science, so the lights become a story rather than just a spectacle.
Conditions vary night to night with tide, temperature, and plankton density, so no two outings are identical. That unpredictability is part of why a strong night of glowing wildlife feels so memorable.
Frequently Asked Questions
What sea creatures glow in the San Juan Islands?
The most common are dinoflagellate plankton, which create the bright blue-green flashes. Comb jellies, krill, copepods, sea pens, certain jellyfish, brittle stars, and marine bacteria can also produce light in the region.
Do jellyfish glow in the dark?
Some do. A number of jellyfish and comb jellies produce bioluminescent light as a startle defense, flashing when disturbed. Comb jellies in particular are gentle, non-stinging, and a frequent sight on calm nights.
Why do so many ocean animals make light?
Light helps marine creatures defend themselves, hide, lure prey, find mates, and communicate. In cold coastal waters like the San Juans, defense is the most common reason, with a quick flash startling or exposing predators.
Are any of these glowing creatures harmful?
The plankton and comb jellies you are most likely to encounter are harmless. As always, paddlers should follow their guide instructions, avoid touching unfamiliar marine life, and let the wildlife move undisturbed.
Jellies keep earning entries in our field notes for good reason. For one memorable example, see our report of an unusual jellyfish found while paddling — one we had never before encountered on our San Juan Islands kayak tours.