Decade-Long Experiment Sheds Light on the Ocean's Mysterious 'Deep Reefs'
Scientists in Palau have retrieved research structures left in the ocean's little-understood 'mesophotic' zone almost a decade ago, hoping the encrusted samples will reveal new insights into deep reef ecosystems.
Nearly a decade ago, zoologist and curator Luiz Rocha of the California Academy of Sciences and colleagues lowered devices called autonomous reef monitoring structures (ARMS) — stacks of PVC plates — into the waters off Palau, Guam, the Marshall Islands, and French Polynesia. The structures were placed at several depths, roughly 10, 50, and 100 meters, so researchers could later compare shallow reefs with their deeper, far less studied counterparts.
Last May, the team returned to Palau to haul the structures back up. Retrieval had been delayed for years because pulling the ARMS from such depths turned out to be costlier than deploying them, and the project needed additional private funding before divers could go back down.
The target ecosystems, found roughly 30 to 150 meters below the surface, are known as mesophotic coral ecosystems, or simply 'deep reefs.' They sit in a dim zone where sunlight is too weak to fully support the photosynthetic algae that sustain shallow coral reefs, and where water temperatures are cooler. Studying them directly is difficult: divers working at these depths can safely stay for only five to seven minutes before having to make a slow, hours-long ascent to avoid decompression sickness.
Once pulled up, the ARMS were brought to the Palau International Coral Reef Center, where a team including researchers from the Smithsonian and the California Academy of Sciences began taking the plates apart, photographing them, and sorting through the organisms that had colonized them over nearly a decade. The haul included sea squirts, sponges, soft corals, bryozoans, and other invertebrates — almost 3,000 specimens in total. Nudibranch specialist Terry Gosliner identified several likely new species on the spot, pending genetic confirmation.
Rocha and his collaborators hope the samples will help answer basic questions that remain unresolved about deep reefs: what energy sources sustain life there, what the food web looks like, how species evolved to survive in low light, and whether these ecosystems could serve as a refuge for species from shallower reefs threatened by climate change. Only a small fraction of the collected specimens have so far been identified to species level, and much of the analysis is still ongoing.
