Green sea turtles are marine reptiles inhabiting temperate and tropical coastal waters across approximately 140 countries. Among the seven species of sea turtles, they are the only ones predominantly herbivorous as adults. Their name comes from the greenish hue of their fat and cartilage, a characteristic influenced by their plant-based diet, not their shell color.
The Predominantly Herbivorous Adult Diet
Adult green sea turtles primarily graze on marine vegetation, with seagrasses and algae forming the vast majority of their diet. They actively seek these plants in shallow coastal areas where meadows of seagrass thrive. This specialized diet is sustained by specific anatomical adaptations.
Green turtles possess finely serrated jaws, resembling a saw or beak, well-suited for efficiently tearing tough plant material and scraping algae from hard surfaces. Their digestive system is also uniquely adapted for processing fibrous plant matter. They utilize a hindgut fermentation process, relying on cellulolytic microbes within their gut to break down complex carbohydrates found in seagrass and algae.
This internal microbial community allows them to extract essential nutrients from their plant-based food sources. While their diet is almost exclusively herbivorous, adult green sea turtles might occasionally consume small amounts of sponges or invertebrates. This occurs infrequently as their primary nutritional needs are met through plant consumption.
Dietary Shifts from Hatchling to Adult
The diet of green sea turtles undergoes a significant transformation as they mature from hatchlings to adults. Newly hatched green turtles and young juveniles begin their lives with a more varied, omnivorous diet. During this early pelagic (open ocean) phase, they feed on a range of small marine organisms.
Their initial diet includes worms, small crustaceans, aquatic insects, fish eggs, and jellyfish. These young turtles often associate with floating mats of Sargassum seaweed, which provide both shelter and a rich foraging ground. As juveniles grow, typically reaching a carapace length of about 20 to 25 centimeters, they begin to transition.
This transition involves a gradual shift from the open ocean to coastal habitats and a corresponding change in their food sources. They progressively adopt a more herbivorous diet, moving towards the seagrass and algal consumption characteristic of adults. This dietary shift is supported by changes in their gut microbiome, which adapts to efficiently digest plant polysaccharides.
Ecological Role of Green Turtle Foraging
Green sea turtles play a significant role in maintaining the health of marine ecosystems through their foraging activities. They are often referred to as “lawnmowers of the sea” due to their grazing habits on seagrass beds. By continuously cropping the seagrass, they prevent it from becoming overgrown and dense.
This regular grazing stimulates new growth, increasing the productivity and nutritional value of the seagrass meadows. Overgrown seagrass can obstruct water currents and lead to decomposition, potentially fostering the growth of undesirable organisms like slime molds. The turtles’ foraging helps to keep these underwater pastures healthy and vibrant.
The maintenance of healthy seagrass beds by green turtles benefits numerous other marine species that rely on these environments for shelter, breeding, and food. Their grazing also contributes to nutrient cycling within these ecosystems.