Fireworms at Mission Bay

I don’t often get emails with the subject “glowing fireworms.” Even “fireworms” would have been strange enough to catch my attention, but the suggestion that these worms were not only fiery but glowing prompted me to open the email immediately.

It turned out to be an open invitation from a naturalist who works in the Exhibit Department of the Birch Aquarium at Scripps Institution of Oceanography. She was going to view the luminescent mating ritual of Odontosyllis phosphorea, the “glowing fireworm” and she was hoping to introduce some friends and colleagues to this spectacular and relatively unknown event. The fiery glow that the worm is named for is in fact a glowing blue-green mucus produced by the female worms as they release their gametes into the water. This glow signals to the males the position of the females and their freshly released eggs, so they can swim there to release their own gametes (gamete is a generic term for a cell that contains half the genetic information of an individual in organisms that reproduce sexually, e.g. sperm and eggs are gametes). The opportunity to see such an underwater light show doesn’t come along every day, as the worms mate only one to two days before each quarter moon, 30 to 40 minutes after sunset and the glowing ritual lasts only 20 to 30 minutes. A worm mating or “swarming” event was coming up the next day, and I knew I couldn’t miss it.

The following evening found me waiting for dark to arrive and the fireworm spectacle to appear at Mission Bay along San Diego’s coastline. I strolled up and down the beach between bonfires, searching for the other fireworm enthusiasts. A borrowed camera around my neck, I practiced taking pictures of small, faraway objects so I would be ready to document the fireworms. I was equally anxious and determined to capture the luminescence on film—this was my first experience as a nature photographer, but worms seemed to be a good beginner’s subject. After all, they were just worms—how fast could they possibly be moving?

Thirty minutes after my arrival, it was getting dark and there was no sign of the viewing group and the naturalist. I was on a long and crowded beach, and it was difficult to find someone I’d never met there, especially in the dark! I gave up on finding the others, and decided I could view the worms on my own. I stepped out into the murky waters of Mission Bay, and after a few minutes, I began to notice some winking blue-green sparks in the water, just below the surface. I waded further out, and soon the lights were all around me. It was hard to believe these tiny sparks were worms—they “flew” around in the water, swimming in circles and spirals and then disappearing. The light displays were more enchanting than I could have imagined…but also much more difficult to photograph!

The worms zoomed around my ankles as I clumsily tried to follow them with my lens. The mud squelched below my feet as I followed them further out in the water. I was up to my knees now, and I was sure I felt something crawling up my legs…could it be the worms? I hoped I wasn’t interrupting their mating.

Eventually the blue-green lights appeared only every few seconds, and after twenty minutes or so the water was still and dark once again—except for me, splashing around with my camera in the water as I trudged up onto the beach. No pictures…but what an incredible wildlife experience! I felt so lucky to have witnessed this precisely timed event.

Impressed by the beauty of the spectacle I had seen, I set out to learn more about the science behind it. Conveniently, two scientific experts on these worms are located here at Scripps. Dimitri Deheyn and Michael Latz are interested in the biochemical and molecular basis of the light produced by Odontosyllis phosphorea, as the worm is called. Last year they made a groundbreaking discovery: The green glowing mucus is not only used for mating purposes, but also as a defense mechanism. Drs. Latz and Deheyn were both kind enough to meet with me, and they gave me a much more in-depth perspective on the worms and their mating ritual.

Their research group has been to Mission Bay during the swarming events to collect the fireworms a number of times. I mentioned that in my own observations at Mission Bay each individual worm did not seem to glow for very long. Dr. Deheyn responded, “Well, it’s long enough. [The swarming] happens at twilight, so the daytime predators have gone to bed and the nighttime predators haven’t become active yet, but they still have to balance making enough light for their sexual partner to find them without making too much light, which could attract predators. To us it doesn’t seem like much light, but for them, it is enough.”

Female worms also stay safe during these events by swimming to the surface, glowing, and breaking off their glowing tail at the surface. The tail spews gametes while the rest of the worm keeps swimming. The female then has to regenerate her tail, but in the long run, this is a useful trick because the glowing light could attract predators, and this way the females stay safe but males have a chance to find and fertilize the glowing water full of the precious gametes.

The worms spend most of their lives in parchment-like tubes attached to a substrate—a solid surface they can “anchor” to—on the ocean floor, only surfacing when the time is right for the periodic swarming events. “The interesting thing is that they go away in the winter and [we] have no idea where they go!” Dr. Deheyn remarked. Drs. Latz and Deheyn have sifted through the sediment where the worms are normally found in the summer while not mating, and during the winter months there are absolutely no traces of the fireworms. “Do they dig down? Die off? Hibernate? …There is still a big mystery there,” Dr. Deheyn added.

Apparently, in tropical places such as Panama the worms are observed year round, mating with lunar cycles. But here in the sub-tropical climate of San Diego we only see these events through the summer, and then the worms are nowhere to be found.

This is not the only mystery associated with the fireworms—according to Dr. Latz the worms may be part of America’s history. When Christopher Columbus was discovering the new world, near the Bahamas they saw some incredible light displays that they thought were on land. A popular hypothesis is that what Columbus actually saw were the mating displays of fireworms!

The worms know when the quarter moon is approaching because they have four, extra-sensitive eyes to detect changes in light. This light sensitivity also allows them to find the light from their mates. For us, even with our two much more sophisticated eyes, we now must rely on calendars to tell us when the next quarter moon is coming up.

The next quarter moon and accompanying fireworm “swarming” event will be on October 21. The same naturalist will be at the DeAnza Cove dock just after sunset at 6:45 p.m.; she will also demonstrate dinoflagellate bioluminescence. I recommend this opportunity to everyone; go see the fireworms before they pull their disappearing act!
 

– Mallory Pickett is a first-year master's student in the lab of chemical oceanographer Andreas Andersson at Scripps Institution of Oceanography