{"id":232477,"date":"2026-04-19T08:00:30","date_gmt":"2026-04-19T18:00:30","guid":{"rendered":"https:\/\/www.hawaii.edu\/news\/?p=232477"},"modified":"2026-04-24T14:42:00","modified_gmt":"2026-04-25T00:42:00","slug":"honu-emerge-as-reef-defenders","status":"publish","type":"post","link":"https:\/\/www.hawaii.edu\/news\/2026\/04\/19\/honu-emerge-as-reef-defenders\/","title":{"rendered":"Honu emerge as reef defenders against invasive algae in Northwestern Hawaiian Islands"},"content":{"rendered":"
Reading time: <\/span> 3<\/span> minutes<\/span><\/span>[vc_row][vc_column][vc_video link=”https:\/\/www.youtube.com\/watch?v=Yn0EYguMK9s” el_aspect=”916″ align=”right” el_id=”wrap-video-right”][vc_column_text]\n

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 Â鶹´«Ã½<\/span> at Mānoa<\/span> and the U.S. Fish and Wildlife Service (USFWS<\/abbr>) have identified a potential ally in slowing its advance: sea turtles.<\/p>\n

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

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

“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<\/em> to spread to Oʻahu<\/span>,” said Celia Smith, UH<\/abbr> Mānoa<\/span>‘s School of Life Sciences<\/a> professor and senior author of the study.<\/p>\n

\"closeup
UH<\/abbr> Mānoa<\/span> researchers captured the first footage of green sea turtles helping control aggressive invasive algae.<\/figcaption><\/figure>\n

Dangers of C. tumulosa<\/em><\/h2>\n

First detected at Manawai (Pearl and Hermes Atoll) in 2016, C. tumulosa<\/em> has since expanded to more than 101 square kilometers of reef habitat (nearly the size of Kahoʻolawe<\/span>), including Kuaihelani (Midway Atoll) in 2021 and Hōlanikū<\/span> (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.<\/p>\n

Turtles take a bite<\/h2>\n
\"turtle
Native honu are acting as reef defenders by grazing on invasive algae that threaten Papahānaumokuākea<\/span> coral.<\/figcaption><\/figure>\n

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<\/em> 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<\/em> fragments throughout her digestive tract, accounting for roughly 25% of the material in her esophagus and crop.<\/p>\n

\"closeup
The invasive red alga Chondria tumulosa<\/em> forms thick, suffocating mats that smother native coral and threaten Â鶹´«Ã½<\/span>\u2019s reef ecosystems.<\/figcaption><\/figure>\n

“These turtles are consuming a meaningful amount of this alga in a single foraging session,” said Tammy Summers, USFWS<\/abbr> staff biologist and co-author of the study. “It\u2019s exciting because it points to honu as a native megaherbivore with the potential to suppress C. tumulosa<\/em> biomass—but it also raises important questions about whether fragments excreted during their migrations between atolls could accelerate the alga\u2019s spread.”<\/p>\n

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<\/abbr> monitoring at Lalo to track potential spread of C. tumulosa<\/em> beyond its known range.<\/p>\n

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

This work was accomplished under permit numbers PMNM<\/abbr>-2025-001, USFWS<\/abbr> 274 Recovery Permit TE72088A-3, Recovery Sub-permit TE163899-2, NOAA<\/abbr> Permit 21260, and 275 State of Â鶹´«Ã½<\/span>, Department of Land and Natural Resources Special Activity Permit 2026-01.<\/p>\n

The School of Life Sciences is housed in UH<\/abbr> Mānoa<\/span>‘s College of Natural Sciences<\/a>.<\/p>\n

Related UH<\/abbr> News<\/em> stories on UH<\/abbr> Mānoa<\/span> research on C. tumulosa<\/em>:<\/p>\n