UH: Honu emerge as reef defenders against invasive algae in Northwestern Hawaiian Islands

An invasive algae already well-established in the Northwestern Hawaiian Islands is raising concern among researchers as it threatens to spread into the main Hawaiian Islands. Scientists from the University of Hawaiʻi at Mānoa and the US Fish and Wildlife Service have identified a potential ally in slowing its advance: sea turtles.

Hawaiian green sea turtles (honu) have been documented for the first time actively grazing on Chondria tumulosa, an aggressive invasive red alga that has spread rapidly across reefs among three of the northernmost atolls in Papahānaumokuākea Marine National Monument (PMNM).

The findings, recently published in the journal Coral Reefs by researchers, suggest that honu may play a meaningful role in controlling this ecologically damaging species—while also potentially spreading it.

“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,” said Celia Smith, UH Mānoa‘s School of Life Sciences professor and senior author of the study.

Dangers of C. tumulosa

First detected at Manawai (Pearl and Hermes Atoll) in 2016, C. tumulosa has since expanded to more than 101 square kilometers of reef habitat (nearly the size of Kahoʻolawe), including Kuaihelani (Midway Atoll) in 2021 and Hōlanikū (Kure Atoll) in 2022. The alga forms dense mats more than 6 centimeters thick that can smother live coral and displace native reef species, making it one of the most pressing threats to the monument’s reef ecosystems.

Turtles take a bite

Using a stationary GoPro camera deployed on a reef at Midway Atoll in June and July 2025, the research team captured approximately 50 minutes of footage showing three honu grazing on C. tumulosa mats. One female took up to 18 bites in a 95-second burst, leaving disruptions 5–15 cm in diameter across the algal canopy—substantially larger than what urchins or fish could achieve. A complementary necropsy of a stranded adult female confirmed C. tumulosa fragments throughout her digestive tract, accounting for roughly 25% of the material in her esophagus and crop.

“These turtles are consuming a meaningful amount of this alga in a single foraging session,” said Tammy Summers, USFWS staff biologist and co-author of the study. “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.”

The findings carry immediate management implications. Because 96% of Hawaiian green sea turtles nesting occurs at Lalo (French Frigate Shoals) before individuals disperse to foraging grounds across the archipelago, the authors recommend eDNA monitoring at Lalo to track potential spread of C. tumulosa beyond its known range.

The study was a collaboration between UH Mānoa‘s School of Life Sciences and USFWS Midway Atoll National Wildlife Refuge, and was funded by USFWS Invasive Species Strike Team funds through a cooperative agreement with UH Mānoa. Smith leads the Limu Lab at UH Mānoa, where research on C. tumulosa physiology, distribution and ecology has been ongoing for several years. Other authors on the paper are Caroline Pott from USFWS and Angela Richards Donà from the School of Life Sciences.

This work was accomplished under permit numbers PMNM-2025-001, USFWS 274 Recovery Permit TE72088A-3, Recovery Sub-permit TE163899-2, NOAA Permit 21260, and 275 State of Hawaiʻi, Department of Land and Natural Resources Special Activity Permit 2026-01.

The School of Life Sciences is housed in UH Mānoa‘s College of Natural Sciences.