Somewhere in the warm shallows of Papahānaumokuākea Marine National Monument, the largest marine protected area in the United States, Hawaiian green sea turtles are doing what they have always done: eating algae off the reef. What has changed is the algae. A fast-spreading red macroalga called Chondria tumulosa, first identified in the monument in 2016, has been smothering coral across hundreds of miles of remote reef. And new research from the University of Hawai’i at Mānoa, highlighted in an April 2026 news release republished by Phys.org, suggests that honu, as the turtles are known in Hawaiian culture, may be one of the reefs’ best natural defenses.
An invasion tracked from space
Chondria tumulosa arrived without warning. Researchers first documented the species at Manawai (Pearl and Hermes Atoll) within Papahānaumokuākea, and within a few years it had spread to Kuaihelani (Lisianski Island) and Holanikū (Kure Atoll). Because the monument stretches across some of the most isolated waters on Earth, scientists turned to high-resolution commercial satellite imagery to track the invasion. That analysis, published in peer-reviewed journals including Journal of Phycology and Coral Reefs, revealed that both the length and area of visible algal mats increased substantially between initial detection and 2021, with the satellite record showing mats expanding from localized patches to formations stretching across reef flats. According to the same NOAA-archived research, mats at Manawai were already extensive by August 2019, and the alga continued appearing at new sites through 2022.
The damage follows a familiar pattern. Invasive algae blanket living coral, blocking the sunlight and oxygen that reef organisms need. Over time, diverse coral ecosystems can degrade into monotonous algal fields that support far fewer fish and invertebrates. Hawai’i’s Department of Land and Natural Resources classifies invasive seaweeds as a primary reef threat statewide, running active removal and monitoring programs on the main islands. Species like Gracilaria salicornia, known locally as Gorilla Ogo, have already degraded nearshore reefs in places like Kāne’ohe Bay on O’ahu, where state Division of Aquatic Resources surveys in 2023 cataloged multiple invasive algae species smothering coral.
Turtles with an appetite for invaders
Green sea turtles are herbivores, and in Hawai’i they graze primarily on algae growing on reef flats and in shallow lagoons. A peer-reviewed study published in Aquatic Ecosystem Health and Management found that Hawaiian green turtles have increased their consumption of non-native algae species over time, incorporating multiple introduced seaweeds into their diets rather than simply tolerating them on the reef. That 2015 study provided some of the earliest direct evidence that honu actively forage on invasive algae in their primary grazing areas.
Building on that dietary research, a team led by senior author Celia Smith at the University of Hawai’i at Mānoa has framed the turtles as “reef defenders” whose grazing connects directly to reef health outcomes. In findings highlighted in an April 2026 news release republished by Phys.org, Smith’s group described the turtles as mobile lawnmowers capable of keeping algal growth in check, particularly in the shallow habitats where they spend most of their time. The team argues that protecting and rebuilding turtle populations could amplify this natural grazing pressure, and they have identified two management priorities: preventing the spread of invasive algae through ocean currents and human activity, and increasing turtle numbers to strengthen their ecological impact.
For Native Hawaiians, honu hold deep cultural significance as a symbol of wisdom, endurance, and safe passage. The idea that these animals might also serve as guardians of the reef resonates with Indigenous knowledge systems that have long recognized the interconnectedness of ocean life. Hawai’i’s green turtle population, listed as threatened under the Endangered Species Act, has been recovering steadily. NOAA data show nesting counts at French Frigate Shoals, the species’ primary nesting site in the archipelago, have trended upward over recent decades, meaning more turtles are returning to the reefs each year.
What science has not yet confirmed
The biggest gap in the evidence is specificity. No published study has yet documented green turtles eating Chondria tumulosa in Papahānaumokuākea or measured whether their grazing rate can keep pace with the alga’s rapid expansion. The dietary research confirms turtles eat non-native algae broadly, and Smith’s reef-defender framework speaks to general grazing behavior rather than consumption of a single species. Until field researchers can quantify how much Chondria tumulosa turtles actually consume, the connection between the two remains a well-supported hypothesis rather than a demonstrated outcome.
Scale is another open question. Chondria tumulosa has colonized reef across hundreds of miles in under a decade. Whether the monument’s turtle population is large enough to make a measurable dent in that coverage has not been assessed. The monument’s extreme remoteness complicates direct observation, which is precisely why researchers have relied on satellite imagery rather than frequent in-water surveys to track the invasion.
There is also no published comparison of turtle grazing rates against algae regrowth speeds at specific sites. It is possible that turtles reduce algal biomass locally without ever bringing it down to levels that allow corals to fully recover. And how turtle grazing interacts with other management tools, such as manual removal, suction dredging, and outplanting of native herbivores like sea urchins, has not been rigorously tested.
At the policy level, no official management plan from the Papahānaumokuākea monument administration has been published linking turtle protection to algae control. Smith’s recommendations, while grounded in research, have not yet translated into documented policy changes at the state or federal level. Turtles remain managed under existing endangered species and marine protected area frameworks, with invasive algae handled through separate programs.
A hopeful signal on reefs under pressure
Hawai’i’s coral reefs face compounding threats: warming ocean temperatures, coastal development, sedimentation, and now fast-moving algal invasions. Against that backdrop, the discovery that green turtles are actively eating invasive seaweeds offers something uncommon in reef conservation: a piece of good ecological news.
The evidence so far supports a clear, if cautious, path forward. The algal threat is real, fast-moving, and well documented. The turtle response is promising but not yet quantified at the scale of entire reef systems. Supporting turtle recovery, limiting the spread of invasive seaweeds, and investing in targeted monitoring can all move in the same direction without assuming more certainty than the data currently provide.
Honu may yet prove to be important allies in the fight to protect Hawai’i’s reefs. For now, their emerging role is best understood as a hopeful signal that science is still working to measure, and one that connects ancient cultural reverence for these animals with a very modern ecological need.
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*This article was researched with the help of AI, with human editors creating the final content.
