Traces of the epic sea voyage are inscribed in their shells
The ocean reveals one of its secrets: how members of the same species can find each other in deep and remote hydrothermal sources.
An epic migration to – and across – the ocean surface could be the answer. Chemical traces on the shells of three species of limpets 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 transform into their adult forms and colonize new habitats, researchers at the University of Tokyo report July 15 in Scientific advances.
Host of hydrothermal vents some of the strangest ecosystems on Earth. These isolated oases, with their crushing pressures, extreme temperatures and waters filled with heavy metals seeping from underground, have become home to a retinue of strange animals. Marine biologists have long suspected that eggs or larvae drift with currents, perhaps even reaching the surface, on their journey to vents. But researchers lacked definitive evidence.
The study “provides compelling documentation” to confirm this extreme journey, while adding new clues about how these isolated communities remain interconnected, even though they are sometimes separated by thousands of miles 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 the millimeter-sized larval shells that sometimes remain attached to the bodies of adult limpets. Analysis of these youthful remains revealed chemical signatures characteristic of warmer waters near the surface, explain marine biologist Takuya Yahagi and his 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 Tu’i Malila site, 1,845 meters deep, in the southwest Pacific, and 33 from a vent field located approximately 440 meters deep on the Kaikata seamount in the northwest Pacific. All limpets shared the same shallow-water signatures. This suggests that this surface migration is an integral part of their life cycle, and not just isolated cases, the researchers say. By migrating upward, limpet larvae access more food than they would in the dark, deep ocean, while surface currents carry them vast distances to new habitats.
But the chances of success after such a pilgrimage are minimal. “Almost all larvae are probably eaten or lost before finding a suitable hydrothermal vent,” says Yahagi. Even if they survive, the chances of landing in a small, active seafloor vent are extremely low. Vent animals increase their chances of success by producing enormous numbers of eggs, Yahagi says. While the team only studied limpets, the same strategy was suggested for other animals, like mussels and shrimp.
Hydrothermal vent ecosystems are often considered isolated worlds powered entirely by geothermal energy. “Our results suggest that they are actually much more connected to the sunny ocean than we thought,” says marine biologist Yasunori Kano of the University of Tokyo. “This is an important step toward understanding both the evolution of vent animals and how deep-sea ecosystems function within the context of a larger ocean.”
