Ghostly white giant worms appear to be reproducing under the seafloor where tectonic plates meet

Giant worms appear to be breeding beneath the seafloor at hydrothermal vents in the eastern Pacific, near the Galápagos Islands, new research shows.

Scientists previously thought microbes and viruses were the only forms of life in this cramped and inhospitable subseafloor environment, but a Schmidt Ocean Institute exploration last year revealed that giant tube worms also make their home there.

"Here we report, to our knowledge for the first time, the discovery of animals excavated from fluid-filled, shallow cavities in the subseafloor of deep-sea hydrothermal vents," researchers wrote in a study published Tuesday (Oct. 15) in the journal Nature Communications. "While the subseafloor microbial and viral biosphere at deep-sea vents has been described, we show that animal life also exists in this shallow rocky subseafloor province."

The cavities hosted tube worms of "record sizes," with tubes reaching 8 inches (20 centimeters) long for one species (Oasisia alvinae) and more than 19 inches (50 cm) long for another (Riftia pachyptila), according to the study. Researchers found the ghostly white critters sandwiched beneath slabs of solidified lava along the East Pacific Rise, a fast-spreading ocean ridge where several tectonic plates meet off the coast of South America.

The researchers were originally searching for the larvae of tube worms and other hydrothermal vent creatures, which they hypothesized may colonize the vents from inside Earth's crust.

Related: 'Dragon' and 'tree of life' hydrothermal vents discovered in Arctic region scientists thought was geologically dead

Hydrothermal vents form in volcanically active areas when water percolating through cracks in the seafloor comes into contact with magma and erupts as boiling, mineral-rich fluid. This fluid attracts animals living in symbiosis with microbes that can convert minerals into energy — but how exactly these communities recruit larvae to persist over several generations remains unclear.

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"Rapid colonization by these animals suggests efficient larval dispersal, with larvae assumed to be transported through bottom, ridge and ocean currents before they settle at vents through downward swimming or sinking," the researchers wrote in the study.

But scientists have never observed the larvae in question in the water around hydrothermal vents, suggesting there is another mechanism for dispersal, according to the study. The researchers suggest larvae get sucked into the ocean crust with seawater and travel through the subseafloor before settling at vents.

To explore the possibility that the subseafloor could act as a reservoir for larvae, scientists deployed a remotely operated vehicle (ROV) to a vent site 8,250 feet (2,515 meters) beneath the ocean surface. The team used the ROV to drill holes into the seafloor and lift slabs of rock, revealing hydrothermal fluid-filled pockets within layers of solidified lava.

To their surprise, these pockets were teeming with adult tube worms. In one cavity, the team spotted two R. pachyptila males with sperm-filled testes and two females of the same species with egg-filled gonads, suggesting that reproduction may occur beneath the seafloor. Cracks leading from the cavities to the surface were also inhabited, the researchers noted.

"Apparently, there is a continuous habitat for tubeworms from … subseafloor cavities to the seafloor surface," they wrote. "These findings support our hypothesis that there is larval dispersal within the crustal subseafloor."

Larvae may enter the subseafloor by riding through cracks in the seafloor on bottom ocean currents, according to the study. While most of them probably find their way back out, the presence of adults in the cavities suggests some larvae make a permanent home within the ocean crust.