Deep-sea corals are a vital part of Alaska’s productive ecosystems and provide important habitat for commercially important fish. Understanding what these corals need to reproduce and survive is essential to protect critical habitat and predict climate effects. However, because their remote habitat has made them difficult to study, the early life of Alaska’s deep-sea corals remains largely mysterious.
This summer an international team of scientists set the stage for a multiyear study that will shed light on the reproductive ecology of deep-living red tree corals. The team deployed a video camera system and remotely operated vehicle (ROV) to place Autonomous Reef Monitoring Structures in deep-sea coral habitat off southeast Alaska. They will be retrieved over subsequent years to observe coral settlement and growth. The findings will inform ecosystem-based fisheries management to help maintain productive, climate-resilient Alaska fisheries.
“Deep and cold coral ecosystems form a foundation of seafloor ecosystems around the globe. They create these unique and amazing habitats for thousands of associated species, including those that are important food sources for humans,” said Rhian Waller, University of Gothenburg, Sweden, who is co-leading the project with Jerry Hoff, NOAA Fisheries biologist at the Alaska Fisheries Science Center. “This project will give us insight into how the larvae of keystone coral species develop, and how they travel and settle throughout the Alaska region.”
“Our efforts will help us understand the factors influencing coral and sponge settlement and growth, and their importance as essential habitats,” Hoff said.
Coral Life Cycle: from Sea Surface to Seafloor
Corals have complex life cycles. Bottom-living adults are either males or females, and release eggs or sperm into the water column. There the eggs are fertilized and develop into embryos, progressing to larvae that then drift among the zooplankton. Larvae eventually settle to the bottom where they metamorphose into coral polyps. They eventually grow into the large colonies that can be found around the Alaska region.
Corals have different habitats and needs during different stages of their life cycle. To predict how climate change may affect deep sea coral habitat, we need to understand each stage.
In a previous study the team found that red tree corals reproduce throughout the year. However, shallow water populations were unable to produce viable offspring—possibly due to recent warming in their glacial fjord environment. This means that the more accessible colonies in shallow water could not be used to study larval development and recruitment.
To study the larvae of red tree corals, they would have to go deep—more than 820 feet—to reach cold deep water. That would require a new approach using innovative technology.
To study larval transport and settlement of deep-sea red tree corals, the team deployed Artificial Reef Monitoring Structures (ARMS). These are stacks of plastic plates that provide surfaces where coral larvae and other sea life can settle. Scientists place them on the seafloor and leave them for a period of time. Then they recover them to see what creatures moved in. They are globally standardized so that settlement by marine life can be measured the same way and compared around the world.
During summer 2022, the team set out three pairs of ARMS plates at three locations. Each one was equipped with a temperature recorder and a bucket lid, which makes them more visible for retrieval.
The team used a MiniCam video camera developed at the Alaska Fisheries Science Center to position the ARMS in red tree coral habitat. Then they sent down an ROV to determine the exact position for later retrieval and to make sure they landed upright and in appropriate places.
“We could not have done this without the Alaska Department of Fish and Game crew and ROV team aboard the research vesselSolstice. They use the ROV for rockfish surveys and they know what they’re doing,” said Pam Goddard, NOAA Fisheries affiliate, Alaska Fisheries Science Center, who led field operations this summer. “Our partnership with them was crucial to our success.”
2023 and Beyond: Larval Settlement Results and Coral Spawning Experiments
In summer 2023 the team will retrieve one ARMS from each location, and put down two new ones. In 2024, they will retrieve the second ARMS of the original pair.
“The first will show 1 year of settlement and temperature data, the second will show two. The team hopes to continue beyond 2024,” said Goddard.
In 2024 and 2025, the team plans to collect live corals to culture for studies of spawning, fertilization, and larval ecology. In preparation, they will develop a coral collection arm for the ROV. This will be used to collect red tree coral sprigs (about 10 inches) from the deep-water colonies surrounding the ARMS. The live sprigs will be brought to the Alaska Fisheries Science Center Auke Bay Laboratories, where they can be maintained for experiments.
Together these projects will greatly advance our knowledge of deep-sea, cold-water coral ecosystems.
“We have begun to understand how these ecosystems function in Alaska,” Waller said. “But there is still so much more to know.”