Exploring the Upper Twilight Zone: Discoveries and Threats in Guam's Deep Reefs

In November 2025, a team of scientific divers from the California Academy of Sciences embarked on a mission to explore the upper twilight zone of the ocean off the coast of Guam. This region, over 300 feet beneath the surface, is one of the least explored ecosystems on Earth due to its challenging conditions. The divers successfully retrieved 13 monitoring devices that had been collecting data on marine life and ocean temperatures for more than eight years. These devices, designed as artificial reefs, provide critical insights into the biodiversity and environmental changes occurring in these depths.

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Upon returning to the University of Guam Marine Laboratory, the team processed the retrieved monitors and discovered approximately 2,000 specimens, including 100 previously unrecorded species in the region. Among these were a potential new species of cardinalfish, an orange-clawed crab never before reported in Guam, and a unique sea slug. Luiz Rocha, an ichthyology curator at the California Academy of Sciences, highlighted a particularly intriguing hermit crab that utilizes clam shells for shelter, a notable deviation from the typical use of gastropod shells.

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Despite the excitement surrounding these discoveries, the expedition also raised significant concerns about the future of Guam's deep reefs. Rocha noted that over half of the species in this ecosystem remain unknown, yet they are already threatened by human activities such as fishing, pollution, and climate change. Research indicates that plastic pollution, primarily from the fishing industry, increases with depth, peaking in the upper twilight zone. “We are almost always the first humans to set eyes on these deeper reefs, and yet we see human-produced trash on every dive,” Rocha stated, emphasizing the pervasive impact of human activity.

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Initial temperature data from the monitoring devices suggests that deeper waters may not be the refuge from warming that scientists had previously assumed. Rocha remarked, “This contradicts assumptions that this depth would be a safe refuge, protected from warming.” The expedition marks the beginning of a larger two-year project aimed at collecting a total of 76 monitors from deep reefs across the Pacific, including locations in Palau, French Polynesia, and the Marshall Islands. This effort aims to enhance understanding of life in the twilight zone and inform conservation strategies.

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The November 2025 expedition to Guam's upper twilight zone has unveiled both remarkable biodiversity and alarming environmental threats. As scientists continue to gather data, the findings will be crucial for developing strategies to protect these unique ecosystems from the ongoing impacts of climate change and pollution.

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