When the tide has ebbed for some time, it’s time to mull over the hard-packed, yet moist, band of sand at the mid-tide level to look for surface pockmarks, the signature “footprint” of an insect unknown to most beachgoers. Segmented blood worms (Euzonus mucronata) are one of the most abundant sandy beach animals, yet they are clandestine critters because they live an underground life. Euzonus belong to the worm group known as polychetes, which have small, bristly paddles on each segment of their body. Blood worms may reach nearly 2 inches long, but their highly elastic bodies allow them to stretch well beyond that to slither between sand grains. Since they are air breathers entrenched in an environment that would otherwise suffocate the likes of us, their blood is crucial to letting them breathe easy. The blood’s bright-red color comes courtesy of hemoglobin, the same pigment that gives vertebrate blood its hue. And like vertebrate blood, hemoglobin carries oxygen, which blood worms need a store of to last through the long periods they are under water. Once the tide flows out, blood worms burrow upward toward the sand’s surface and hang upside down from their self-inflicted pinhole, where they use their rectum as a kind of lung to absorb oxygen. This window of time is also their chance to chow down, because once the tide rolls in, they must retreat to deeper sand to avoid getting carried away by the surf. During this time, blood worms may dive as deep as 8 inches but below that, there is not enough oxygen to sustain them. Sometimes I notice the holes look dug up, testimony that shorebirds have poked around for these iron-rich, easy pickings. Blood worms clean the beach just as their earthworm cousins clean our gardens. Called deposit feeders, they gulp sand but it’s really the organic material found between the grains on which they feed. As the grains move slowly forward through the body via successive waves of involuntary contractions (peristalsis), bacteria, plankton and other decaying life are digested. The cleaned sand is then ejected back onto the beach. In 1949, in what must have been a scintillating study, two researchers followed the plight of sand grains through an E. mucronatus and came up with two interesting results: The turnover rate is nearly 2 pounds of sand per year per worm (talk about a high-fiber diet!), with each sand grain cycling in and out in about 15 minutes. Variations in blood worms are not about A, B or O types but color types. Anywhere from light-pink to red to black, body coloration depends on the darkness of the sand grains that are ingested. I can see color differences along the length of one individual worm as well as diverse tints and tones in those of nearby blood brothers. If freed from their sand den to fresh air, blood worms rock their sharp-pointed heads back and forth as if searching for something. I know that something is the missing sand above them. When not found, they immediately start digging back down, knowing that without their protective barrier, the next tide will wash them away and they’ll drown. I don’t want to imagine that because they perform such an important service. I prefer imagining that while I’m leisurely walking on the sandy beach, thousands or millions of these hard-working worms are polishing the sand under my pounding feet. — Judith Lea Garfield, biologist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. www.judith.garfield.org. Questions, comments or suggestions? Email [email protected].
