Most are smaller than a toothpick, though some can grow longer than a blue whale. Some of them come in an unassuming brown or beige color, while others stand out in varied palettes with everything from neon pinks to electric blues. They're called ribbon worms, and about 1,350 different species have been formally described so far. However, scientists estimate this is only about 10% of the total count, a point emphasized in a new study that documents the discovery of over 100 ribbon worm species off the coast of Oman, most of which are new to science.
"What we know is just the tip of the iceberg compared to what's really out there," said Svetlana Maslakova, a professor at the Oregon Institute of Marine Biology and lead author on the study published in PeerJ. Maslakova has studied ribbon worms, or nemerteans, for over 25 years and is one of only a handful of ribbon worm experts in the world.
Part of the reason many ribbon worms remain unknown is that scientists haven't been looking in the right places. About a third of the world's named ribbon worm species were found in temperate waters of Europe and North America. Meanwhile, the tropics, among the world's most biodiverse regions, remain relatively unstudied.
Oman is one of these overlooked places. "It's one of the most amazing parts of the ocean," said Gustav Paulay, curator of invertebrate zoology at the Florida Museum of Natural History. "There are really strange ecosystems there. You know about kelp forests, and you know about coral reefs — well, in Oman, you get the two together in one ecosystem. That's almost unheard of."
Surveying these uncharted waters has become a priority for scientists. Paulay spends much of his time conducting biodiversity surveys in oceans around the world. During these short surveys, Paulay and his colleagues work to sample as many species as possible. They've surveyed in the Middle East, Southeast Asia, Micronesia and Hawaii; uncovered invasive shrimp-sucking parasites off the coast of Canada; and in one survey collected over 6,000 specimens lurking in the coral reefs of the Red Sea.
This was the first survey accompanied by DNA sequencing of ribbon worms off the coast of the Arabian Peninsula and the first ever in Oman, where no nemertean species has previously been identified. Because many species look alike, DNA sequencing is critical to identifying these worms and understanding their distribution.
The blazing stripes and striking hues of some species make them visually appealing to humans but have the opposite effect on predators. The bright colors are a warning that ribbon worms are full of toxins.
They use these toxins to do their own hunting, too. When an unfortunate victim crosses a worm's path, it shoots out a flexible pipe called a proboscis that stuns its prey. In some species, the proboscis has a tiny stylet that punctures the prey's skin, so a cocktail of paralyzing venom and digestive juices can flow into the wound. In others, the proboscis branches out, not unlike the bronchi in human lungs, allowing the ribbon worm to ensnare the animal in a deadly embrace as the toxins seep into its body.
Once the prey is immobilized, the slender ribbon worm will contort its body, stretching to swallow the animal whole. In contrast, ribbon worms armed with stylets bide their time, allowing the digestive juices to liquefy the prey's tissues, at which point the ribbon worm slurps up the nutritious smoothie.
With these gruesome methods, ribbon worms can take down other worms, clams, crabs, amphipods and even fish. As carnivores, they play an important role in shaping local ecosystems, but they also offer insights on a larger scale. Paulay is working to close the gaps in our understanding of biodiversity across the oceans.
"Nemerteans turn out to be a really cool group for this kind of work," Paulay said. The large number of newly discovered and unnamed species offers "a window into biodiversity overall."
And peering through that window reveals just how limited our view is. Like with ribbon worms, scientists' understanding of global marine biodiversity is woefully incomplete. While there are currently about a quarter million known marine animal species, Maslakova said this represents an estimated 10% of what is living in the ocean.
"We went to Oman, and we found 107 species there. That instantly doubled the number of DNA-barcoded species of nemerteans known for the entire Indo-West Pacific region," Maslakova said. "It was stunning to see just how little we know of what diversity is actually out there." Of the species found, 93% appear to exist only in Arabia, which further underscores how rich ribbon worm diversity may be worldwide.
Maslakova and her team are making large strides in species discovery, but searching for ribbon worms can be challenging. If they're lucky, researchers may find ribbon worms by turning over boulders and rubble during low tide or while scuba diving. More often, it's like searching for a needle in a coral stack. The tiny worms often tuck themselves in crevices, burrow in soft mud or hide in mats of algae.
Instead of searching for individual worms, researchers will collect hunks of dead coral rubble. Back on land, they submerge them in containers of seawater and wait for the worms to come up to the surface as the water begins to stagnate.
While scientists work quickly to identify as many species as they can, climate change threatens to bring the animals' extinction. And with many species still unknown, we may not realize the true cost of losing ribbon worms before it is too late. In addition to the cascading effects of losing these predators in their ecosystems, humans may miss out on their biomedical potential.
"Many potent toxins also turn out to be potent medicine," Maslakova said. "Ribbon worm toxins have shown promise in treating conditions such as schizophrenia or Alzheimer's disease. And those are just a few of many that they produce. And that's just one ribbon worm out of the thousands of species that are out there. We're rapidly losing species that could potentially save our lives or treat our diseases. And we haven't even discovered them yet."
