If you have ever wandered out onto Boundary Bay’s vast, sandy mudflats at low tide, you have been in a world dominated by a humble, rarely-seen, oddly-named creature. That mix of mud and sand and water, firm enough to walk upon, and so broad it looks as though you could scamper all the way from Tsawwassen to Blaine, is teeming with sub-surface marine life: clams, crustaceans, and worms. One of these, perhaps the most important, is the Pacific Lugworm.
The name, without even knowing what it means, seems to suit the creature. The average lugworm is about 8-10 cm long. It has a blobby prostomium, containing the mouth at the front end, and a segmented body, fleshier than that of an earthworm. Its colour ranges from yellow to brown, and there are several pairs of bright red, branched gills along the thicker forward half. I recommend an image search, guaranteed to provoke an “EWW.”
A lugworm lives in a U-shaped burrow, with its tail extended up one arm of the U, its head extended up the other. The worm pumps water through the burrow, over its gills, from back to front, using peristaltic contractions of its body. (There’s nothing about this creature that isn’t at least mildly revolting.) This helps liquefy the mud at the front end, which the worm draws into its mouth. Like an earthworm, it ingests the substrate and extracts organic material as it passes through its gut. As the sand-mud mix is swallowed, the surface around the “head”- end of the burrow subsides, leaving a small crater. This is a subtle sign of the worm’s whereabouts. The clearer sign that a lugworm lurks below comes from how the worm rids itself of the ingested mud. It squirms backward up its burrow and expels what remains, which is still mostly muddy sand, in an elaborate, heaped curlicue, known as a cast, onto the surface of the mudflat. When surrounded by these swirly little deposits, spaced a few inches apart and covering acres of ground, you know you are in the Kingdom of the Lugworm.
In Boundary Bay, it has been determined that there are as many as 200 lugworms per square meter of mud. They have a profound impact, making up almost 30 percent of the biomass (the total weight of living things) in the intertidal zone. And they process prodigious amounts of substrate. It has been estimated that at the population densities found at Boundary Bay, the actions of lugworms can completely rework the top 10 centimetres of mud every hundred days. This results in a constant mixing of nutrients and other materials throughout the mud. In addition, the worms’ respiratory currents continuously draw oxygen down to lower levels, which enhances the surroundings of other mud-dwellers.
Lugworms are a source of protein for shorebirds and small flatfish, and in a uniquely sustainable way. Usually it is just the last few body segments that get nipped off by a predator– when the worm sticks its posterior out into the world to defecate. This indignity does not kill the worm, which may then regenerate the lost segments.
The soil-circulating habit of lugworms can affect the distribution of pollutants introduced to mudflats. It can help mediate the effects of intertidal oil spills by carrying oil that has seeped down to oxygen-poor levels back to the surface where aerobic bacteria can break it down. On the other hand, lugworms can also transport harmful heavy metal pollutants that had been buried in the deeper mud to the surface and overlying water, which can be toxic to other worms, clams and crustaceans that live there.
Lugworms are an inestimably important component of intertidal mud. They are physically and functionally inseparable from it. In a sense they are the mud, which therefore, in many ways, is a living thing. This is something to keep in mind as you walk across the lugworm’s kingdom, before the tide chases you back to our world.
See also: The interpreter visits the Kingdom of the Lugworm.