Although they remain firmly attached to rocks on the sea floor and other surfaces for most of their lives, ocean-dwelling tubeworms spend their early days as free-swimming larvae, looking for a place to call home. A key factor in their decision to latch on somewhere is the presence of certain bacteria. Now, researchers have discovered that these bacteria prepare a microscopic welcome mat, which apparently invites the larvae to settle down and grow up—triggering the animal’s dramatic metamorphosis into its adult, tubelike form. The finding might lead to new ways of discouraging tubeworms from taking up residence on ship hulls and other humanmade objects, where they cause billions of dollars of damage each year.
Scientists knew that the presence of certain bacteria helps trigger the larvae of the tubeworm Hydroides elegans to attach to a surface and metamorphose. But no one could explain how the microbes prompt this dramatic transformation, one of the most decisive steps in the animal’s life cycle. While studying one of those triggers, the bacterium Pseudoalteromonas luteoviolacea, Nicholas Shikuma, a microbiologist at the California Institute of Technology (Caltech) in Pasadena, and his colleagues found that it carries a set of genes for making elaborate structures that resemble bits of phages, viruses that infect bacteria.
Similar structures are well known to microbiologists; many bacteria use them as weapons to kill other bacteria or to harm animal hosts. Yet no one had found any example in which they were beneficial to an animal. But when Shikuma and his colleagues knocked out the genes and cultured the mutant bacteria together with tubeworm larvae, they did not trigger a metamorphosis. The team calls the structures metamorphosis-associated contractile structures, or MACs for short.
The researchers teamed up with Caltech structural biologist Grant Jensen and his colleagues to image the bacteria. They found that about 2% of the cells in a P. luteoviolacea colony produce the MACs. Once a bacterium fills with MACs, it breaks open and releases hundreds of the structures. The tails of the MACs join together to form a net, while their heads resemble tiny, spring-loaded hypodermic needles, the team reports online today in Science. When a larva touches the net, the needles release.
Dianne Newman, the microbiologist at Caltech in whose lab Shikuma did the research, says that the needles firing might help signal to the larvae that the surface is a promising place to live, although the exact mechanism isn’t yet known. It’s not clear why the bacteria make the arrays, she says. It’s possible that the microbes somehow benefit from the tubeworm’s presence. But it may simply be a random phenomenon in the bacterial cells that the tubeworm has adapted to take advantage of. “What is really striking is that for the first time, these structures are doing something that benefits the animal,” she says.
It’s doubtful that the bacteria evolved the signal to attract tubeworms, says microbiologist Edward Ruby of the University of Wisconsin, Madison. More likely it’s simply an environmental cue the larvae recognize. “It’s like the sun coming up,” he says. “Every time I see it, I know it’s a good surface to land on.”
Similar interactions probably determine where many marine invertebrates settle, says marine biologist and co-author Michael Hadfield of the Kewalo Marine Laboratory at the University of Hawaii, Manoa. (Shikuma started the project as a graduate student in Hadfield’s lab.) A type of coral and a species of sea urchin also settle in response to P. luteoviolacea, he notes. “It’s very likely the same mechanism,” he predicts. Genes similar to the ones for MACs have also been found in other marine bacteria.
The work “is cracking the doors on an entire field,” Ruby says. Scientists have recognized that bacteria were important, he says, “but now it becomes a science, not just an observation.” Understanding the MACs will likely be crucial to marine conservation efforts—both to discourage the growth of invasive species and encourage endangered ones, he says. The ship industry would certainly welcome any new way to discourage tubeworm growth, although Newman cautions that such practical application is a long way off. “The first step is identifying how [the MACs] interact with the animal before we could do anything to rationally block it.”