The green sea turtle, Chelonia mydas, is a globally recognized marine reptile navigating tropical and subtropical seas. Its diet is a remarkable feature, setting it apart from most other sea turtle species. As it matures, the species undergoes a profound dietary shift, transitioning from an omnivorous diet to one based almost entirely on vegetation. Understanding what these animals consume across their lifespan clarifies their unique biology and role in the marine environment.
The Omnivorous Juvenile Stage
The lives of green sea turtles begin with an oceanic, or pelagic, phase where hatchlings and young juveniles drift in the open ocean. During this stage, their diet is broad and focused on achieving rapid growth to escape predation.
These small turtles are omnivores, consuming a variety of organisms floating in the water column. Their meals include small invertebrates, such as crabs and shrimp, fish eggs, sponges, and gelatinous organisms like jellyfish.
They also graze on floating algae and seaweeds that accumulate in ocean currents. This protein-rich, mixed diet provides the necessary fuel for their initial development before they settle into shallower coastal foraging grounds.
Specialized Adult Herbivory
As green sea turtles grow into sub-adults and adults, they undergo a dramatic dietary transformation, becoming the only predominantly herbivorous sea turtle species. They abandon the open ocean for shallow, sheltered coastal habitats, such as bays and lagoons, where lush vegetation thrives.
The adult diet centers almost exclusively on consuming marine plant matter, mainly seagrasses and various types of algae. The primary food source is often seagrass, such as Thalassia testudinum (turtle grass), which can make up over 85% of their total diet in some regions.
These turtles selectively graze on specific plants, often choosing young, nutrient-dense blades over older vegetation. They also consume various forms of marine algae, including green, red, and brown macroalgae, which they scrape off rocks and other substrates.
This specialization is the source of the species’ common name. The immense quantity of plant matter stains the turtle’s body fat a greenish hue due to chlorophyll and carotenoids. This green fat, not the color of their shell, is the defining characteristic separating them from other sea turtle species.
Foraging Habits and Digestive Adaptations
Green sea turtles possess anatomical features that allow them to effectively harvest and process their tough, fibrous diet. Their jaw structure is highly adapted for an herbivorous lifestyle, lacking sharp teeth but instead forming a finely serrated beak.
This serrated edge functions like shears, enabling the turtles to clip, tear, and scrape vegetation from the seabed and hard surfaces. Processing plant cellulose requires a specialized internal system.
Green sea turtles have a notably long and complex digestive tract, which provides maximum time for plant matter breakdown. This process is facilitated by a rich community of cellulolytic microbes in the gut, particularly in the cecum and colon.
These microorganisms ferment the tough cellulose and hemicellulose, allowing the turtle to efficiently extract nutrients. This fermentation produces volatile fatty acids, such as acetate, butyrate, and propionate, which contribute a significant portion of the turtle’s daily energy requirement. The high efficiency of this digestive process is comparable to that found in mammalian herbivores like ruminants.
The Ecological Importance of Green Sea Turtle Grazing
The feeding activity of green sea turtles is a powerful ecological force that shapes the coastal environments they inhabit. By consuming large amounts of vegetation, they act as primary consumers that structure the entire seagrass ecosystem. This process is often described as “grazing” or the “lawnmowing” effect.
When green sea turtles selectively crop the tops of seagrass blades, they prevent the beds from becoming overgrown. This rotational grazing stimulates new, more nutritious growth and increases the productivity of the seagrass.
The maintenance of healthy, cropped seagrass beds is beneficial for numerous other marine species. These beds provide essential habitat, nursery grounds, and food sources for fish, shellfish, and crustaceans. Without the grazing pressure from these turtles, seagrass meadows can decline in health, leading to reduced biodiversity and a less stable marine environment.