Photo: Brian Greene
August 26, 2025
Beneath American Samoa's waves lies a hidden world of “twilight reefs” — mesophotic coral ecosystems found 100–500 feet down, where sunlight barely reaches but life still thrives. Scientists are now diving and deploying technology to unlock their secrets, learning how these habitats may help sustain coral reefs in a changing ocean. Just as important, researchers are sharing these discoveries with local students and communities, inspiring the next generation of ocean stewards. Together, exploration and education are helping protect one of American Samoa's most mysterious and vital marine treasures.
Dive Into the Hidden Depths of National Marine Sanctuary of American Samoa
In the waters of National Marine Sanctuary of American Samoa, there lies a habitat that few people have ever seen, much less understand. Because of this, scientists and researchers are going deeper to understand the importance of mesophotic coral ecosystems. These twilight reefs stretch along the island slopes and offshore banks, offering a refuge for species that might be struggling in shallower reefs affected by changing ocean conditions, overfishing, and pollution.
The ocean's twilight reefs have been hard to study because of their depth. While most recreational scuba divers can only safely dive to about 130 feet (40 meters), exploring mesophotic reefs requires specialized training and advanced gear—closed-circuit rebreathers, mixed gas diving, and extensive dive safety planning. Today, thanks to advances in underwater technology like remotely operated vehicles and autonomous systems, scientists are exploring these hard-to-reach areas more than ever. As interest grows, research is expanding in both tropical and temperate mesophotic coral ecosystems.
Scientists are diving deeper than ever before to understand how mesophotic coral ecosystems function, how they differ from shallow reefs, and how they might serve as refuges or sources of recovery following disturbances.
Dr. Anthony Montgomery, a marine biologist with the U.S. Fish and Wildlife Service, is the principal investigator and lead technical diver in a project to explore the mesophotic reefs in American Samoa. He shared, "The deep reefs of American Samoa are vast unexplored areas that serve as a resource of undiscovered biodiversity. I have been diving the deep reefs of American Samoa for almost 10 years and every dive teaches us more about this unique place."
These efforts are particularly vital in American Samoa, where nearly 80% of nearshore habitat is at mesophotic depths and steep island slopes create ideal conditions for twilight reef ecosystems just offshore.
These twilight zone reefs are home to unique and distinct coral communities that are specially adapted to low light, cooler temperatures, and have slower growth rates. Mesophotic coral ecosystems provide safe places for fish to breed, feed, and grow. Protecting these deeper reefs helps keep our coral reef ecosystems healthy and strong for the future.
Some scientists believe mesophotic coral ecosystems may serve as refuges for coral and reef fish displaced from warmer, degraded shallow reefs. Though not a guaranteed safe haven, these deeper ecosystems may help support long-term reef recovery when connected to shallower habitats.
Mesophotic reefs also play a broader role in ocean health. They act as stepping stones for larval dispersal and feeding grounds for deep-diving species. Despite being more remote and less impacted by local human activities, mesophotic coral ecosystems remain vulnerable to global threats such as ocean warming, acidification, and invasive species. As our understanding of their ecological importance grows, so does the urgency to protect them.
Working Together to Understand the Deep
The Deep Coral Reef Ecosystem Studies, or “Deep-CRES project,” is a collaborative initiative aimed at advancing our understanding of mesophotic coral reef ecosystems in Fagatele Bay, and diving into new areas like Fagalua/Fogamaʻa, Aunuʻu, Taʻū, and Swains Island.
The Deep-CRES project focuses on advancing scientific understanding of mesophotic coral ecosystems by exploring and documenting them, assessing biodiversity, and investigating their ecological roles and connectivity to shallow reefs. This research aims to inform conservation efforts and foster ocean stewardship through community engagement.
Funded by NOAA's National Centers for Coastal Ocean Science from 2019 to 2026, this effort brings together experts from NOAA's Office of National Marine Sanctuaries, U.S. Fish and Wildlife Service, Old Dominion University, Bishop Museum, University of Hawaiʻi, and Papahānaumokuākea Marine National Monument, with additional support from NOAA National Marine Sanctuary of American Samoa and National Park of American Samoa.
Sharing Science with Students
A vital component of the Deep-CRES project is engaging with communities in American Samoa to share discoveries and inspire future generations of marine scientists and conservationists. The National Marine Sanctuary of American Samoa and the National Park of American Samoa partnered to bring outreach to local youth. Led by Hawai‘i Institute of Marine Biology researchers, the team engaged over 300 students and teachers from 11 Title I schools.
Students participated in a hands-on reef exploration activity, using microscopes and tweezers to examine and classify tiny organisms found in reef rubble. This marine taxonomy exercise introduced them to biodiversity and ecology while encouraging curiosity, scientific thinking, and a deeper connection to marine science. The research team also welcomed teachers, students, and parents into their temporary lab space, offering a behind-the-scenes look at their work. Five teachers, four students, and two parents took part in the visit, gaining valuable insight and expressing enthusiasm for future learning opportunities in their own backyard.
Carlos Alberto Tramonte, a graduate researcher with the Toonen-Bowen Lab at the University of Hawaiʻi involved with Deep-CRES, shared that “outreach isn't just an extra part of the research, but an essential component. Without it, we risk treating the ecosystems we study as disconnected from the people who rely on and steward them, and this visit reaffirmed my commitment to ensuring that our work not only advances scientific knowledge but also brings students to the forefront of what we do and feel empowered to engage with and protect their reefs.”
A Commitment to Deep Reef Conservation
The mesophotic zone is a special and fragile place. By learning more about it, we can help protect the ocean's deep reefs and the creatures that call it home. As the Deep-CRES project continues to explore and promote protection of mesophotic coral reef ecosystems, scientists remain committed to scientific discovery, conservation, and community engagement. Through these collaborative efforts, we move closer to understanding the secrets of the twilight zone and ensuring the long-term sustainability of these vital marine habitats.
Nerelle Que Moffitt is the communications and outreach coordinator for National Marine Sanctuary of American Samoa
Rachel Plunkett is the content manager and senior writer-editor for NOAA's Office of National Marine Sanctuaries