Green sea turtles are the only herbivorous sea turtle species, feeding primarily on seagrasses and algae as adults. But their diet isn’t exclusively plant-based, and it changes dramatically over their lifetime. Hatchlings and juveniles start out eating mostly animal prey before gradually shifting to a plant-heavy diet as they grow.
Why They’re Called “Green” Turtles
The name has nothing to do with the color of their shell. Green turtles get their name from the greenish color of their body fat, which is thought to come from the pigments in the seagrasses and algae they eat. Their shell is actually brown, gray, or olive.
What Juveniles Eat
Young green turtles living in open ocean habitats eat a primarily carnivorous diet. During this early stage, they feed on small invertebrates and other animal matter they encounter while drifting in ocean currents. Once they settle into shallow coastal waters, their diet begins shifting toward plants, though many juveniles remain omnivorous for a while.
Research tracking green turtles across different body sizes found that smaller juveniles (under about 60 cm in shell length) ate a mix of macroalgae, seagrass, and jellyfish. These younger turtles tended to stay close to shore with small home ranges of less than 1.5 square kilometers. As they grew past the 60 cm mark, their diet became predominantly seagrass and jellyfish, and they ranged farther from the coastline.
The Adult Diet: Seagrass and Algae
Fully grown green turtles graze on seagrass beds and algae-covered reefs, eating roughly 1 to 3 percent of their body weight each day. For a 150-kilogram adult, that works out to about 1.5 to 4.5 kilograms of plant material daily. The specific plants they eat depend heavily on where they live.
In Hawaii, green turtles feed mainly on red algae species. One of the most commonly eaten is an introduced species called Acanthophora spicifera, along with several other red and green algae. In the Caribbean and along Florida’s coast, seagrass tends to dominate the diet. Across all populations worldwide, the pattern holds: seagrass and macroalgae make up the bulk, with small amounts of animal material mixed in.
Animal Foods They Also Eat
While plants are the foundation, green turtles aren’t strictly vegetarian. Adults occasionally eat sponges, other invertebrates, and discarded fish or bait when the opportunity arises. Jellyfish consumption is particularly interesting. Research from the University of Tokyo compared green turtles in two Japanese regions: one group in Okinawa that ate a typical herbivorous diet, and another along the Sanriku coast that regularly ate jellyfish. The jellyfish-eating turtles showed surprisingly good nutritional health, suggesting this protein source can be a viable supplement.
The Frontiers in Ecology and Evolution study found that jellyfish consumption actually increased steadily with body size. Larger adults ate proportionally more jellyfish than smaller turtles, which challenges the simple image of green turtles as pure herbivores. Many researchers now consider true omnivory to be the norm rather than the exception.
How They Digest Tough Plant Material
Seagrass and algae are full of complex carbohydrates that most animals can’t break down on their own. Green turtles solve this problem the same way cows and horses do: they rely on specialized gut bacteria to ferment plant material for them. This fermentation happens in the cecum and the first section of the large intestine, making green turtles hindgut fermenters.
Their digestive tracts are home to communities of bacteria dominated by two major groups. One group specializes in producing enzymes that break down cellulose, the tough structural fiber in plants, converting it into short-chain fatty acids the turtle can absorb as energy. Another group can break down specific sugars found in brown algae, fermenting them into fatty acids that provide real nutritional value. These bacterial communities closely resemble those found in other marine herbivores like marine iguanas and manatees.
Several physical traits support this digestive strategy: large body size, a slow metabolic rate, a long digestive transit time, a large colon, and scissor-like jaw structures that help them crop seagrass blades efficiently.
How Grazing Shapes Seagrass Meadows
Green turtles don’t just eat seagrass. They maintain it. They practice a form of rotational grazing, returning to the same patches repeatedly rather than stripping an area bare and moving on. This pattern removes old, overgrown blades and reduces the overall leaf biomass, which actually benefits the meadow. Trimmed seagrass can capture light more efficiently and produces new growth with higher nutritional content. Other species living in seagrass beds, from small fish to invertebrates, benefit from this increased productivity as well.
This ecological role makes green turtles something like underwater lawn mowers. Healthy turtle populations help keep seagrass meadows in a productive, balanced state. Where turtle numbers have declined sharply, seagrass beds can become overgrown and less diverse, a pattern that underscores how tightly linked these animals are to the ecosystems they feed in.