A beam of white light cuts the winter’s heavy air, dividing the coast live oak forest into the comprehensible and the unknown. I click off my torch and let the darkness envelop me, prompting my senses to quickly create theories with the shadows. Click. Click. My ultraviolet (UV) torch fires up and suddenly a new world emerges—familiar shapes but in incongruent colors.
A species of flat-backed millipede, which I couldn’t see under white light, walks by fluorescent blue. Its rhythmic army of feet ripples beneath Himalayan blackberry leaves saturated in the red fluorescence UV light elicits from their chlorophyll. A euphoria emerges from this sort of pseudo-synesthesia and leads to more questions than I have answers for, but one thing is for sure: under UV light, a hidden Bay Area landscape comes alive, especially in the winter months.
UV fluorescence has been known about since at least the 1840s, but it has become far more accessible in the past few decades due to the widespread availability of inexpensive ultraviolet LEDs. For those just getting started, a 365-nanometer UV flashlight with a filter to remove blue and purple light, which costs less than $30, is all you need to turn an ordinary night hike into an otherworldly adventure.
There is some confusion between UV fluorescence and bioluminescence. UV fluorescence is caused when energetic light waves, invisible to our eyes in the UV spectrum, strike molecules, exciting their electrons to a higher energy state, which is unstable. The electrons then quickly fall back down to their ground state, releasing energy as a photon we can see in the visible spectrum. Depending on the molecule and its environment, the fluorescence can be almost any color.
In contrast, bioluminescence is when an organism produces its own light through a chemical reaction. In the Bay Area only a handful of organisms do this—western jack-o’-lantern mushrooms, California pink glowworms, and the microscopic plankton dinoflagellates—compared to the dizzying number of organisms that UV-fluoresce.
Scorpions are the UV-fluorescent organism most people are familiar with. Much to the dismay of many people, scorpions are widespread in the Bay Area—and you may only become aware of this fact after walking with a UV light at night. I often see the western forest scorpion (Uroctonus mordax) on rocky slopes that face the setting sun. A fascinating thing about these arachnids is that they not only fluoresce, but change their behavior due to seeing their own lime-green fluorescence. Scorpions have extraocular light sensors in their tails and lateral eyes that allow them to see their own fluorescence, and research suggests this helps them gauge how exposed they are during moonlit nights.
Among the arthropods in the Bay Area that fluoresce, crab spiders are some of the brightest. Hanging out in flowers, they often ambush pollinators that can see in the UV spectrum. With most of the ground spiders, orb weavers, and harvestmen, fluorescence is usually noticeable at their joints and in their eyes, which have a spooky blue color to them. Another bright arthropod in moist winter forests of the Bay Area is the millipede Xystocheir dissecta. You can easily see them from 10 to 20 meters away with a UV light, and they have no behavioral change to their fluorescence, which, coupled with their hydrogen cyanide chemical defense, makes me wonder if their fluorescence is more of a warning than anything else.
More than 100 mushrooms in the Bay Area are UV fluorescent, and nighttime forays highlighting their various colors have become very popular with participants in local mushroom gatherings. After more than 10 years of looking at fungi, the local mushroom that fluoresces most to me is also one that is prevalent in our forests—the sulphur tuft (Hypholoma fasciculare). It has a striking lime-green fluorescence that rivals the intensity of a scorpion’s and can be found near decaying wood in closely growing groups called troops. Russula mushrooms also often have strong fluorescence, with blue blushes on their caps and very bright green on their gills and stem. Locally, these mycorrhizal mushrooms predominantly grow in association with oaks, madrone, pines, and Douglas fir. One of the more unusual fluorescent displays in fungi are the red gills of Deconica, which is a helpful way to identify this genus of little brown mushroom. The wet winter months are the best time to see our local mushrooms and ponder if their fluorescence is happenstance or whether it’s an evolutionary adaptation.
The winter months are also a good time to see many of our larger nocturnal animals as they are a little more active while looking for food. Many birds have UV fluorescence, including owls. Their fluorescence increases throughout their lifetime, so some researchers are using this to gauge their age. One of the brightest-fluorescing nighttime animals is the opossum. These denizens of the urban-wild interface are often hot pink to lavender, with blue around their nose, hands, and ears. Another common nocturnal critter, raccoons, fluoresce blue all over and can occasionally be found foraging in a wonderful place to bring a UV light: tidepools.
The intertidal zone has many organisms that fluoresce strongly. The showstoppers are sea anemones, which can be fluorescent green to highlighter orange depending on the species. They are in the same phylum as the crystal jellyfish (Aequorea victoria), from which green fluorescent protein (GFP) is derived—a commonly used tool in cell and developmental biology research that has led to many Nobel Prizes. In the intertidal zone, starfish, sea urchins, and some seaweeds can fluoresce vividly. A UV flashlight can also make it easier to find camouflaged or very small organisms like sea spiders, which aren’t true arachnids but have a similar number of appendages and give off a very bright green glow.
As more amateurs and academics pick up UV torches to look at the world under new light, the questions are expanding faster than the answers. To be part of this process, you can take photographs of organisms under UV light and upload them to the iNaturalist project UV Fluorescent Organisms, which I created and which can be a useful resource for finding things that fluoresce near you.
Editor’s Note: Park and open space hours in the Bay Area vary—check before heading out.
