The ocean is giving up one of its secrets: how members of the same species can turn up at far-flung deep-sea hydrothermal vents.
An epic migration up to — and across — the ocean’s surface could be the answer. Chemical traces on the shells of three limpet species endemic to these extreme seafloor environments suggest that the animals spent their youth feeding on sun-loving phytoplankton. Eventually, the larvae of these flattened snails returned to the depths to turn into their adult forms and colonize new habitats, researchers at the University of Tokyo report July 15 in Science Advances.
Hydrothermal vents host some of the strangest ecosystems on Earth. These isolated oases, with their crushing pressures, extreme temperatures and heavy metal–filled waters escaping from underground, have become home to a suite of strange animals. Marine biologists have long suspected that eggs or larvae drift with the currents, maybe even reaching the surface, on their journey to the vents. But researchers lacked definitive proof.
The study “provides convincing documentation” to confirm this extreme journey, while adding new clues about how these remote communities remain interconnected, even if they are sometimes separated by thousands of kilometers of barren ocean floor, says Lisa Levin, a marine ecologist at the Scripps Institution of Oceanography at the University of California, San Diego.
A key clue to this epic journey was hidden in millimeter-sized larval shells that sometimes remain attached to the bodies of adult limpets. Analysis of these vestiges of youth revealed chemical signatures characteristic from warmer, near-surface waters, say marine biologist Takuya Yahagi and colleagues. The shells also lacked heavy metal elements, such as manganese and barium, which are abundant in hydrothermal water.
The team analyzed 39 limpets — six from the 1,845-meter-deep Tu’i Malila site in Southwest Pacific, and 33 from a vent field at about 440 meters deep on the Kaikata Seamount in the northwestern Pacific. All the limpets shared the same shallow-water signatures. That suggests that this surface migration is an integral part of their life cycle, not a few isolated cases, the researchers say. By migrating upward, limpet larvae access more food than they would in the dark, deep ocean, while surface currents transport them across vast distances to new habitats.
But the odds of success after such pilgrimage are minimal. “Almost all larvae are probably eaten or lost before finding a suitable hydrothermal vent,” Yahagi says. Even if they survive, the chances of landing at a small, active seafloor vent are extremely small. Vent animals increase their chances of success by producing enormous numbers of eggs, Yahagi says. While the team studied only limpets, the same strategy has been suggested for other animals, such as mussels and shrimp.
Hydrothermal vent ecosystems are often thought of as isolated worlds powered entirely by geothermal energy. “Our results suggest that they’re actually much more connected to the sunlit ocean than we used to think,” says marine biologist Yasunori Kano of the University of Tokyo. “That’s an important step toward understanding both the evolution of vent animals and how deep-sea ecosystems function as part of the larger ocean.”
