Hawaiian Green Sea Turtles Rise as Natural Defenders of Coral Reefs Against Invasive Algae

Midway Atoll – Scientists have recorded the first instances of Hawaiian green sea turtles actively feeding on an invasive red alga that endangers coral reefs in this remote outpost of the Papahānaumokuākea Marine National Monument. The turtles, known locally as honu, targeted thick mats of Chondria tumulosa, a species that has overrun large swaths of seafloor habitat. This unexpected behavior raises hopes for natural controls on the invader while prompting questions about its broader ecological ripple effects.[1]

Footage Reveals Bold Grazing Action

Researchers deployed a stationary GoPro camera on a reef at Midway Atoll, now called Kuaihelani, during June and July of 2025. The device captured about 50 minutes of compelling footage showing three adult turtles interacting with the algal mats. One female honu delivered up to 18 bites in just 95 seconds, tearing away chunks and leaving clear disruptions measuring 5 to 15 centimeters across.[1]

These marks stood out as larger than typical damage from smaller grazers like urchins or fish. The observation marked the debut documentation of green sea turtles, or Chelonia mydas, consuming this particular alga. Lead author Caroline Pott and her team published the findings in the journal Coral Reefs, highlighting the turtles’ potential as native megaherbivores.[1]

The Alga’s Rapid Takeover

Chondria tumulosa first appeared at Manawai, or Pearl and Hermes Atoll, in 2016. It reached Kuaihelani by 2021 and Hōlanikū, or Kure Atoll, in 2022. Today, the alga blankets more than 101 square kilometers of reef – roughly the area of Kahoʻolawe – forming dense mats thicker than 6 centimeters.[1]

These mats smother live corals and crowd out native species, transforming vibrant ecosystems into barren expanses. The invasion threatens the delicate balance of the Northwestern Hawaiian Islands, a protected expanse managed as the Papahānaumokuākea Marine National Monument. Efforts to curb its spread now intensify amid fears it could reach the main Hawaiian Islands, including Oʻahu.

Confirmation from a Turtle’s Stomach

To bolster the video evidence, scientists conducted a necropsy on a stranded adult female turtle. Fragments of C. tumulosa made up about 25 percent of the material in her esophagus and crop. Traces appeared throughout her digestive tract, confirming regular consumption of the alga.[1]

This dual approach – field observation paired with anatomical analysis – provided robust proof. The study, involving experts from the University of Hawaiʻi at Mānoa and the U.S. Fish and Wildlife Service, underscored the turtles’ dietary habits. Nearly all Hawaiian green sea turtle nesting, 96 percent, occurs at Lalo, or French Frigate Shoals, from where juveniles disperse to foraging sites like Midway.

Potential Benefits and Risks

Senior author Celia Smith, a professor at UH Mānoa’s School of Life Sciences, emphasized the dual-edged discovery. “After these exciting finds, our multi-partner effort to prevent this seaweed from taking hold in the Main Hawaiian Islands must include a plan to increase numbers of threatened, native green sea turtles, as well as ramping up efforts to identify all routes that could allow Chondria to spread to Oʻahu,” she stated.[1]

Co-author Tammy Summers, a U.S. Fish and Wildlife Service biologist, added nuance. “These turtles consume a meaningful amount of this alga in a single foraging session. It’s exciting because it points to honu as a native megaherbivore with the potential to suppress C. tumulosa biomass – but it also raises important questions about whether fragments excreted during their migrations between atolls could accelerate the alga’s spread.”[1]

Thus, while turtles might reduce algal biomass locally, their long-distance travels could unwittingly aid dispersal. This tension defines the conservation challenge ahead.

Charting a Course for Reef Recovery

Experts now advocate targeted strategies. Boosting turtle populations could enhance grazing pressure on the alga. Environmental DNA, or eDNA, monitoring at key nesting sites like Lalo would detect early incursions.

Here is a timeline of the alga’s spread:

• 2016: First detection at Manawai (Pearl and Hermes Atoll).
• 2021: Arrival at Kuaihelani (Midway Atoll).
• 2022: Invasion of Hōlanikū (Kure Atoll).
• 2026: Covers over 101 square kilometers across northern atolls.

Multi-agency collaboration will prove essential to safeguard these reefs.

Key Takeaways

• Hawaiian green sea turtles graze invasive Chondria tumulosa, leaving significant disruptions on algal mats.
• The alga has spread rapidly, smothering over 101 square kilometers of reef habitat since 2016.
• Conservation must balance promoting turtle populations with monitoring for alga dispersal via migrations.

This breakthrough positions honu as vital players in reef defense, yet it demands vigilant management to prevent unintended consequences. As threats to marine ecosystems mount, such native solutions offer a glimmer of resilience. What steps should prioritize in protecting these waters? Share your thoughts in the comments.