The Green Sea Turtle (Chelonia mydas) is a globally distributed marine reptile found across tropical and subtropical oceans. This species is unique among sea turtles because its diet undergoes a dramatic shift as the animal matures. The turtle’s common name does not reference its shell color, which is typically olive or brown, but the greenish tint of the fat layer beneath its carapace. This coloration is a direct result of the plant matter that dominates the adult animal’s feeding habits.
The Omnivorous Juvenile Diet
The life of a Green Sea Turtle begins with a prolonged “pelagic” phase where hatchlings drift in open ocean currents. During this stage, young turtles are omnivorous, relying on a protein-rich diet to fuel rapid early growth. They forage in floating communities, such as mats of Sargassum seaweed, where they find small prey.
Specific food items include small invertebrates like jellyfish, sea snails, and sponges, along with fish eggs and plankton. This diet is an adaptation for survival in the nutrient-scarce environment of the open sea. As juveniles grow larger, they move closer to shore, marking the end of their omnivorous lifestyle and signaling a fundamental change in their food source.
The Defining Herbivorous Adult Diet
Once Green Sea Turtles reach maturity and settle in nearshore habitats, they become the only sea turtle species to be almost exclusively herbivorous. Their diet consists primarily of seagrasses and marine algae found in shallow coastal waters.
The turtles possess specialized anatomical features to manage this fibrous vegetation. Their jaws are finely serrated, forming a beak structure adapted for tearing tough seagrass blades and scraping algae off hard surfaces. Processing cellulose, the main component of plant cell walls, requires assistance.
This digestive challenge is met by a specialized hindgut that houses a complex community of microorganisms. Gut bacteria, including phyla such as Firmicutes and Bacteroidetes, ferment the plant material. This fermentation allows the turtle to extract sufficient nutrients from its low-energy food source, defining their ecological niche.
Feeding Behavior and Habitat Impact
Adult Green Sea Turtles are important “grazers” in tropical and subtropical seagrass meadows. They exhibit an effective grazing strategy, often returning to the same areas to maintain distinct feeding plots.
The animals crop the tops of the seagrass blades rather than pulling up the entire plant. This technique promotes compensatory growth, where the grazed seagrass responds by producing new, more nutritious shoots. Continuous cropping prevents the seagrass from becoming overgrown and encourages nutrient cycling within the meadow.
By acting as ecological engineers, Green Sea Turtles regulate the health and productivity of these underwater pastures. Their grazing behavior ensures the structural integrity and biodiversity of the seagrass habitat, making their presence a significant factor in the coastal environment.
Dietary Links to Health and Conservation
The Green Sea Turtle’s reliance on coastal vegetation makes it susceptible to human-induced environmental changes. Habitat degradation, caused by coastal runoff and pollution, directly impacts the health of the seagrass and algae they consume. This vulnerability is compounded by the threat of marine debris.
Juveniles in the pelagic phase are especially at risk of ingesting plastic bags or fragments, which they mistake for prey like jellyfish. Plastic consumption can lead to blockages and starvation. Another significant threat connected to their diet is the tumor-forming disease known as Fibropapillomatosis (FP).
While FP is associated with a herpesvirus, its widespread prevalence is linked to environmental stressors. Turtles foraging in nearshore waters affected by nutrient pollution may consume arginine-enriched macroalgae. The ingestion of these algae is hypothesized to exacerbate the viral condition, resulting in debilitating external and internal tumors.