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In the summer of 2011, Matthew Bracken was on vacation, touring the coastline of southeastern Alaska with his father. They would hop in a boat, poke around the shoreline, and anchor in coves. One morning, the pair pulled into a cove and found themselves surrounded by a lush kelp bed. “And I said, wait a minute, this isn’t supposed to happen,” recalls Bracken, an ecologist at the University of California, Irvine. “You’re not supposed to find kelp growing on a mudflat.”
Kelp needs an anchor, a hard substrate at the bottom of the sea to grasp. Typically, that means a rocky seafloor. Without a strong anchoring point, kelp is quickly washed away by the surging power of coastal waves. The soft, muddy cove Bracken was exploring with his dad was wholly unsuitable for kelp—at least according to what he was taught as a student.
Then he took a closer look.
Since the water was so shallow, the ecologist could easily see that the ribbon kelp and sugar wrack were anchored to something—but it wasn’t the mud. The kelp’s rootlike holdfasts were gripping the tiny burrows formed by seafloor-dwelling northern feather duster worms.
The discovery “was really one of those examples of an accident meeting a prepared mind,” Bracken says. What he found was a previously unknown ecological relationship.
Tube worms like northern feather dusters are quite common along the Pacific coastline where they dwell on rocks and pilings and poke up from the muddy seafloor. As the tube worms tunnel through mud, they secrete a mucus that mixes with the mud to form a sort of slurry, which then hardens into a leathery, parchment-like substance. The resulting tube serves as a home for the worms and, as Bracken found, an anchoring point for kelp.
Kelp have long been recognized as crucial ecosystem engineers, and were it not for the tube worms Bracken suspects the kelp simply could not survive in this habitat, at least not for long. As he describes in a recent paper, this unexpected relationship is a facilitation cascade: one species facilitates the survival of a second, which in turn enhances the environment for a variety of other species—including the tube worms themselves, since the kelp environment ensures a steady supply of food.
Kerry Nickols, an ecologist at California State University, Northridge, who wasn’t involved in the study, says she’s probably seen baby kelp growing on invertebrates before. But those cases were likely accidents, she says, the result of young kelp settling on the wrong place to grow. What Bracken found is a proper ecological relationship. Tons of organisms grow on kelp, Nickols says, but this is the first time she’s heard of kelp consistently growing on another organism.
“Even smaller kelps like these provide an important three-dimensional habitat for juvenile fish that are of commercial importance,” says Bracken. “Understanding the factors that allow for their persistence is really important.”