Sporting a bright fuchsia hue, this new species of acorn worm was recently found some 8,850 feet (2,700 meters) deep near the Mid-Atlantic Ridge. The colorful creature is one of more than 12 new species and at least 4 new genera of worms discovered during two recent deep-sea expeditions, scientists have confirmed.
The research has shed new light on acorn worms, formerly known as a homogeneous group called shallow-water burrowers, said Karen Osborn, an evolutionary biologist at the Smithsonian Institution's National Museum of Natural History in Washington, D.C.
"The shallow-water worms pretty much all look the same," said Osborn, who co-authored a new study—published November 16 in Proceedings of the Royal Society B—showing the worms live in deep waters worldwide.
"But at some point, one of these shallow-water guys got into deep water, and they've changed their morphology to deal with the unique challenges of that deep-sea habitat."
For instance, the deepwater worms have extremely long "lips" that help them snag prey in a place where food is scarce.
"So now we have this group of deep-sea species that look[s] totally different and behave[s] differently [than the shallow-water worms]—leaving their burrows behind."
This new species of acorn worm appears to have a healthy appetite, as evidenced by the telltale pattern inscribed on seafloor sediments.
While shallow-water burrowing worms simply wait to feed on whatever passes by, deep-sea species must move around to find food.
"They feed outward, in a circle, eating the surface layers of sediment—the freshest layers of [nutrient-rich] sediment that have more recently sunk down through the water column," Osborn explained.
This newly discovered species of acorn worm splays its long "lips," which stretch nearly the length of its body. The lips are covered with tiny hairs that pick up particles of food off the seafloor and pass them to the worm's mouth.
"Most species just have lips on a narrow collar around the mouth. But in the deep sea, there is not a lot of high-quality food, so this way they can select more [floating] food faster," Osborn said.
"Think of a skinny push mower versus the kind of big, wide tractor you'd use to mow a giant field."
Technically called enteropneusts, these acorn worms have no bones, brains, or eyes-yet they're more closely related to humans than to other worms.
Lying like a flower on the seafloor, this newfound acorn worm has a color that poses an intriguing question for scientists.
"Why this brilliant purple, the dark reds, these amazing colors at depths where there is no light? I can't tell you," Osborn said.
"We see brilliant purples and reds [in other worm species] at depths of 500 or 1,000 meters [1,640 or 3,280 feet] where there is very little light, because [such colors] look black there, so those colors are great camouflage.
"But here, where there is no light at all except that produced by organisms, there seems no reason to spend energy making yourself these colors when nothing can see them.
"There is a reason," Osborn added. "We just have to figure out what it is."
A recently discovered species of acorn worm floats in the water column, one of the survival abilities the creatures have developed for life in the deep.
The worms' delicate, gelatinous bodies allow them to easily float up to 65 feet (20 meters) above the seafloor as well as to move around in search of food—an ability that initially shocked scientists.
"I wouldn't call it swimming but controlled floating. They use their gut contents as ballast," Osborn said.
"They empty their guts of sediment and sand and then make mucous balloons that help to lift them off the bottom. Once up, they very rigidly hold specific postures, the way they float best, and then to come down they release the mucous balloons and straighten up their bodies again."
In an evolutionary sense, acorn worms (pictured, a newfound species) are living fossils—part of a little-known group that may bridge the gap between invertebrates and the first vertebrate animals.
Acorn worms belong to a phylum that is considered the most closely related to the chordates—backboned animals—a group that includes birds, fish, humans, and many other vertebrate species, Osborn explained.
"Understanding the acorn worms' group better helps us understand what the earliest chordates probably looked like, because this group and the chordates probably came from the same ancestor."
Zigzag in the Sand
A feeding acorn worm leaves a distinctive zigzag trail in the seafloor sediment. Such patterns helped scientists discover the animals and realize their importance to the seafloor ecosystem, Osborn said.
"We've looked through a bunch of video from other underwater vehicles, and every ocean basin has these little trails that [the worms] leave," she said.
Scientists spotted acorn worms as early as the 1970s, but the gelatinous animals were too fragile to collect from the seafloor.
During the past decade, advances in remotely operated vehicles and imaging technology allowed scientists to not only identify new acorn worm species but also to observe how they behave.
One of Many
The new study confirms that the previously recorded acorn worm seen above is in fact a new species. Many more new acorn worm species are probably waiting to be found in the world's seas, Osborn said.
"It's a great example of how much we still need to learn about the oceans," she said.
"We're having major impacts on the oceans, from global warming to pollution to overfishing, and we don't even know what major groups of animals are living down there."