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In the long days of summer, there is so much light that darkness holds a special magic. One of my favorite things to do on summer evenings is to be out in the darkness. And some of the most magical things about summer’s darkness are the unexpected lights. There are, of course, the moon and the stars that light up the sky. Their brightness makes some sense to me — the moon reflecting the light of the sun and the stars generating light from the burning gases that comprise them. But there are other lights that appear sporadically and elicit a different sense of wonder. These appear on both land and sea and, for a brief period at the start of summer, they show up at the same time.
Just as everything comes into bloom — the fields and the sea — the creatures within them respond with seemingly spontaneous flashes of light. The inconsistency of their illumination makes it seem like what you are seeing cannot possibly be real. This also makes it hard to understand how the light is being created.
When I was little, I caught fireflies on hot summer nights in the Midwest and put them in a jar on my dresser in hopes that I could have a bit of that outdoor magic in my room as I fell asleep. It wasn’t until much later in my life, when I landed in Maine and studied the marine environment, that I learned that animals didn’t just magically glow. They contain a specific combination of chemicals that, when mixed, create a light reaction known as bioluminescence.
A fun and simple experiment that simulates bioluminescence involves separating the components of a glow stick. To activate a glow stick, you have to bend it until it cracks. That crack is the breaking of a little glass tube inside the stick, releasing a chemical that, when mixed with the chemical in the rest of the stick, makes the whole thing glow. In fireflies, the two chemicals that make their little bellies glow are luciferin and luciferase. They are thought to do this flashing for a number of reasons, including to attract a mate and to scare off predators.
In the ocean, there are myriad creatures that bioluminesce, and they contain the same two chemicals as fireflies. In the beginning of the summer, during early plankton blooms, one of the most pronounced examples is a tiny, single-celled organism called a dinoflagellate. It is an odd, crossover type of plankton that is animal-like in its ability to move itself with tiny “flagella,” or tails, but plant-like in that it can photosynthesize. Dinoflagellates are so small that you can’t see them without a microscope — that is, until they glow. They glow in response to motion in the water, so all you have to do is wave your hand or a paddle around on a dark night when they are present and you will see sparkles in the water.
While these plankton are tiny, there are also really big creatures that glow. Another sign of early summer is the jellyfish that drift into shallow bays along the coast. The two most common types are moon jellies and lion’s mane jellies. My marine science professors instilled the fact that, because they are not fish, they should be called “jellies” rather than “jellyfish.”
Moon jellies have a clear or whitish bell and have such a low toxin level that it is rare for a human to feel any type of sting. Lion’s mane jellies are red or brownish and can give a mild sting if you touch their tentacles. In addition to stinging for defense, both of these jellies can bioluminesce when disturbed.
While the coincidence of fireflies and phosphorescent dinoflagellates has passed, there are opportunities to see glowing sea creatures well into the summer and fall. It’s often hard to predict when there will be little blooms, but they do recur through the end of the season. It is always a treat to see them and, though I now know the chemistry behind their glow, I still find it just as magical, if not more so, than when I saw my first firefly flash, unaware of the why.
