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The research submersible ''Alvin.''

Invasive species have stowed away on the remotely operated vehicle Alvin (file picture).

Photograph by Emory Kristof, National Geographic

Limpets on an instrument on Alvin.

L. fucensis limpets on one of Alvin's instruments. Photograph courtesy Ray Lee, WSU.

Dave Mosher

for National Geographic News

Published May 29, 2012

Invasive deep-sea species can hitchhike from one extreme environment to another as stowaways on submersibles, according to a new study of mollusks called limpets.

The findings underscore why scientists should be extra careful when collecting the ocean's deepest animals.

"These limpets," study co-author Janet Voight said, "are like Energizer bunnies."

In 2006 the deep-sea submersible Alvin dived to hydrothermal vents 1.7 miles (2.7 kilometers) underwater off California.

After sucking up sediment and animals for study, the remotely operated vehicle (ROV) surfaced, traveled for two days on a research ship, then dived again, this time to a Washington State site 1.38 miles (2.2 kilometers) underwater.

Raising Alvin after the Washington expedition, the researchers were surprised to find some very familiar mollusks in one of the ROV's sample baskets.

"This was exciting and surprising at first, but then things stopped adding up," said expedition member Voight, an invertebrate zoologist at the Field Museum of Natural History in Chicago.

(Related pictures: "New Volcanic Sea Vents, Crawling With Creatures.")

Too Good to Be True

After years of probing the limpets' radiation signatures, genetic sequences, and physical traits, Voight's team realized that the supposed Washington State limpets, of the Lepetodrilus gordensis species, were perfect matches to those from the California hydrothermal vents.

The team thought they'd found a previously unknown population of the mollusks, but the critters turned out to be ROV hitchhikers and by now may have colonized the Washington State site.

In hindsight, Voight said, she should have known better. After all, other deep-sea organisms are known to survive punishing trips to the surface and return to normal once they're back on the ocean bottom.

But the idea that the limpets could survive such tremendous changes in pressure and temperature—not to mention prolonged exposure to air—just didn't occur to the researchers, she said.

(Also see "Invasive-Species 'Stowaways' May Lose Rides on Ships.")

Alvin's protocols require scientists to flush and clean all the ROV's equipment before moving from one ocean-bottom site to another. But after the California expedition, Voight said, the team neglected to clean the chamber in question.

"It's really embarrassing that we did this," she said. "We should have known better, but it didn't seem like a clear and present danger."

"Recipe for Disaster"

Failing to flush out deep-sea subs can contaminate more than just study data, Voight said—especially when the species involved are born survivors.

Across the planet, underwater volcanic ridges create hydrothermal vents that spew piping-hot, mineral-rich water. So-called extremophile bacteria thrive in tall chimneys around the vents, and their existence supports oases of strange creatures, including types of tube worms, snails, crabs, and even octopuses.

(Watch video: What are hydrothermal vents?)

"The faster ridge spreads, the more often these habitats change," Voight said. "So the animals have to cope when chimneys fall or a vent shuts down."

Often that means the species evolve to grow fast and breed like mad—traits that make them formidable as invasive species.

"They're primed to exploit any open habitat. It's kind of a recipe for disaster," she said.

(Related pictures: "'Supergiant' Shrimp-Like Beasts Found in Deep Sea.")

The problem with the limpets, in particular, is that they've evolved to harbor bacteria in their gills, in some cases, disease-spreading bacteria. The mollusks can also host minute crustaceans called copepods that feed on limpet gonads—and could similarly afflict mollusks at a new site.

As such, stowaway limpets could ruin pristine habitats, Voight said.

"Each vent site is like its own island of life," Voight said. "So we're now saying to our colleagues: You've got to be extra careful. Please learn from our mistake."

The submersible-hitchhikers study appears in the May 24 issue of the journal Conservation Biology.

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