In the turquoise waters of Palau, scientists recently retrieved underwater structures they had deployed nearly a decade prior, aiming to unlock the secrets of the ocean's poorly understood "deep reefs." These mesophotic coral ecosystems, located between 30 and 150 meters below the surface, exist in a twilight zone where light is diminished and temperatures drop, presenting unique challenges for research.
Luiz Rocha, a zoologist and curator at the California Academy of Sciences, has dedicated decades to studying these deep reefs. He noted that the area between 30 and 150 meters remains largely unknown, with researchers struggling to establish a baseline understanding of the ecosystems and the species they harbor. A key question driving this research is whether these deep reefs could serve as a refuge for species from shallower, more vulnerable coral reefs.
To gather more information, Rocha and his colleagues deployed Autonomous Reef Monitoring Structures (ARMS) between 2016 and 2018 in deep reef areas around Palau, Guam, the Marshall Islands, and French Polynesia. These structures, essentially stacks of PVC plates resembling small hotels, are designed to become encrusted with marine life over time. This method allows scientists to collect a diverse array of organisms in a standardized way, offering a more comprehensive view than brief, technical dives allow.
Exploring these depths poses significant challenges. Technical dives to 100 meters, for instance, allow for only about five to seven minutes of activity before divers must ascend slowly to avoid decompression sickness, a process that can take hours. While remote operated vehicles and submersibles can also be used, they often fail to capture the fine details needed to identify cryptic species. "Whenever we go somewhere, the first thing we find is new species," Rocha stated, highlighting the fundamental gaps in current knowledge.
"Prefab Housing" for Marine Life
The ARMS method, described by Chris Meyer, director of the Smithsonian Global ARMS program, as "prefab housing," provides a stable and diverse mini-reef. Larvae colonize the clean structures, and the spaces between the plates offer hiding spots for various marine creatures, creating a rich biodiversity hotspot.
After deploying the ARMS at depths of around 10, 50, and 100 meters to compare different zones, Rocha and his team faced the challenge of funding their retrieval. Once funding was secured, they returned to Palau to collect the structures. The retrieved ARMS, some sparse and others densely covered in organisms, were transported to the Palau International Coral Reef Center for analysis.
Scientists meticulously disassembled the structures, cataloging sea squirts, sponges, octocorals, and bryozoans. They aimed to identify morphospecies and collect tissue samples for DNA sequencing to confirm identities and discover new species. While some creatures, like nudibranchs, were more readily identifiable by experts like Cal Academy curator Terry Goslinger, many specimens required extensive genetic analysis. As of July, only a small fraction of the nearly 3,000 specimens had been identified at the species or even genus level, with much of the work still pending DNA sequencing and further data processing.
Beyond New Species: Understanding Ecosystems
Beyond cataloging new species, the researchers are particularly interested in observing the relationships between the organisms that colonized the ARMS. Identifying these interactions, such as a pompom crab holding anemones or skeleton shrimp camouflaged on hydroids, is crucial for tackling broader ecological questions. Susanne Bähr, a postdoc working on the project, is analyzing data from ARMS retrieved in Guam to understand species overlap at different depths within the deep reefs.
Rocha compares the current state of deep reef research to mid-20th-century mountain ecology studies, acknowledging how much further scientists have to go. Despite the complexity and long-term nature of the research, Rocha finds motivation in the potential rewards. "If it's a hard puzzle, at the end of it, the reward is always bigger," he said. While he does not expect the deep reef puzzle to be fully solved within his lifetime, he is committed to generating valuable pieces for future generations of scientists.