Last month during a low tide on the day of the full moon, Faylla Chapman and a few of our friends visited the Morro Bay mudflats. We came upon an area in the mudflats that was scattered with red polychaetes, which are commonly known as “bristle worms”. (Refer to photo 1) I was so enthralled with the presence of these bristle worms that I decided to do some research on their natural history and share my results with you all.
Polychaetes are worms that are assigned to the Phylum Annelida, which are known as segmented worms, in the Class Polychaeta. Over 9000 species of annelids have been described thus far and fall into three classes: the Polychaetes, earthworms and their allies, and leeches. Polychaetes are scare or absent in most freshwater habitats, whereas they are dominant in the sea. A diverse group, some are brilliantly colored, light up with a bioluminescent glow, create tubes of sand or shell material, or can tolerate seawater temperatures up to 140 degrees Celsius.
When one looks carefully at the bristle worm, one will notice the head and tail ends are trunk-like segments, varying in number with different species from a few to several hundred. Many segments bear bristles projecting laterally. Polychaeta is Greek for “with much hair or many setae”. The bristle like setae is generally both numerous and conspicuous, projecting from the body wall on either side of most trunk segments. Along the shore, polychaetes are particularly abundant in situations where they receive some protection from the surf by going under rocks or burrowing in sand or mud.
Polychaetes and their relatives have been around for a very long time and fossil species were discovered in the Burgess Shale, dating back to the Cambian period, some 505 million years ago. Like today’s polychaetes, both fossil creatures had many parapodia with feather-like bristles and sensory tentacles extending from their heads.
In the years to come, Earth witnessed five mass extinction events, one of which killed some 96 percent of all marine species. The hardy and adaptable polychaetes made it through all these die-offs to give rise to the abundance of species we see today
Upon more examination of the bristle worms, we picked a few from the mud and placed them in a plastic tray, added water, and observed a milky cloud emitting from one of the worms that looked like sperm. I later read in Intertidal Invertebrates of California by Morris and Abbott, that members of the class Polychaeta are mostly sexual with the females producing a pheromone that attracts and signals the males to shed sperm. This in turn stimulates females to shed eggs. This behavior is known as swarming (refer to photo 2). After fertilization, most eggs become planktonic and develop into larva, which later mesomorph into juvenile stage (body lengthened), and later develop into adults.
Faylla, who is a frequent visitor to the mudflats, mentioned that she had observed this phenomenon last year. We both were curious about the relationship of the full moon and the spawning of these bristle worms. Our lunar clock has been known to typically impact the reproduction of organisms. Everything that has a monthly period length that oscillates is by definition a lunar clock. (Ref.). These cycles typically impact the reproduction of organisms. This is mainly about getting the eggs and sperm ready in many species and impacts the behavior of species during mating. ‘For marine organisms the full moon often serves as a signal and many organisms sync to it. There are several explanations for the widespread occurrence of lunar-controlled rhythms in the marine environment. One that might be quite plausible is that many of these species are external fertilizers, which expel their eggs and sperm into the open water. In order for eggs and sperm to meet, and hence successful reproduction to occur, they have to be ready and released together.
I was aware of edible polychaetes that are known as palolo worms, several species of which occur in various parts of the world. A common feature of these worms is that they swarm to the surface in very large numbers at predictable times of the year. The large swarms are primarily for reproduction and have a unique breeding behavior: During the breeding season, always at the same time of year and at a particular phase of the moon. These worms break in half; the tail section bearing reproductive cells, swims to the surface, where it releases eggs and sperm. High numbers of palolo worms swarm “tens of thousands,” and release gametes simultaneously.
Interestingly, in the bristle worms, the lunar clock has an effect on their daily behavior too. Normally, the worms are nocturnal (active during the night), but during the full moon phase, even if they don’t see the moon, they will be a lot less nocturnal and more diurnal (active during the day).
Prof. Kristin Teßmar-Raible, University of Vienna, Austria stated:
“For marine organisms the full moon serves as the signal for reproduction, they all sync to it and reset their clocks.” For humans, and land animals in general, it’s not clear what role the moon plays, if any.”
I had a personal experience with the lunar clock one full moon night in Tulum with my younger daughter. Off our front porch of our beach hotel, we saw many green sea turtles laying eggs. Some adults had laid eggs there earlier and the freshly hatched turtles were heading out to sea. A biologist who worked in Tulum, told us that we could help the young turtles on their voyage, by shutting off our porch light and carrying them out to sea, as there were many predators consuming them on their journey. I grabbed my flashlight and placed the turtles in the water, but they were very disoriented and kept heading up the shore. It took me a while to realize that they thought my flashlight was the full moon. They kept coming towards me and I kept re directing them out to sea. Finally, when I realized that my flashlight was a form of light pollution, they scurried out to sea.
‘As our dependance on electricity expands with the growth of our population, we must also think about light pollution and how this could have an impact on mass spawning events which are very important for species to regenerate. If they don’t happen it can severely impact the ecosystem. The second factor one might want to consider is how these monthly cycles impact on food chains in the marine environment.’
A group of scientists from James Cook University in Queensland, Australia, determined that light pollution could have serious impacts on coral clock-driven processes. The absence of moonlight also disrupts normal transcription cycles, and changng weather patterns with increased cloud cover may have similarly detrimental effects on coral clock -driven systems, potentially included spawn timing.
So much to ponder….
‘ISLING K. BRADY1,†, BETTE L. WILLIS2, LAWRENCE D. HARDER1, AND PETER D. VIZE1,*
Lunar Phase Modulates Circadian Gene Expression Cycles in the Broadcast Spawning Coral
Reference: Biol. Bull. 230: 130–142. (April 2016)
© 2016 Marine Biological Laboratory
Morris, Robert H., Donald P. Abbott, and Eugene C. Haderlie. Intertidal invertebrates of California
