The Hawaiian green sea turtle, known as honu in Hawaiian, is a significant marine species in the islands. Their feeding habits are distinctive and play an important role in their biology and the marine ecosystem. Understanding their diet provides insight into their life cycle and their interactions with the vibrant Hawaiian waters.
The Herbivorous Diet of Adults
Adult Hawaiian green sea turtles primarily eat marine plants like algae (limu) and seagrasses. They graze on algae found on coral rocks and consume seagrasses.
Common red macroalgae species such as Acanthophora spicifera, Gracilaria salicornia, and Hypnea musciformis are frequently part of their diet, along with seagrasses like Halophila decipiens and H. hawaiiana. Consuming these plant materials gives their fat layer a green color, which is how they get their common name.
Their digestive system is adapted for processing fibrous plant matter. Green turtles possess a hindgut fermentation digestive tract that uses cellulolytic microbes to break down plant material in the cecum and proximal colon. Their serrated jaw structure aids in chewing and processing tough green and red algae and seagrasses. While their diet is overwhelmingly plant-based, adult turtles may occasionally supplement it with small amounts of animal prey, such as invertebrates like sponges or jellyfish.
Dietary Changes from Hatchling to Adult
The diet of Hawaiian green sea turtles transforms significantly as they mature. Newly hatched honu are carnivorous and pelagic, meaning they live in the open ocean and are opportunistic feeders.
During this early stage, their diet includes small fish, fish eggs, sea jellies, crustaceans, and other small invertebrates. This varied diet supports their rapid growth and development where they spend their “lost years.”
As juvenile turtles grow, typically between ages 5 and 7, they gradually transition to a more herbivorous diet. This shift is influenced by changing metabolic needs and a move from the open ocean to coastal grazing areas.
Upon reaching coastal habitats, usually when their carapace length is approximately 40 cm, they adopt a diet predominantly based on macroalgae. This allows them to utilize the abundant plant resources in their adult foraging grounds.
The Role of Their Diet in Marine Ecosystems
The herbivorous feeding habits of adult Hawaiian green sea turtles play a significant role in maintaining the health of marine ecosystems, particularly coral reefs and seagrass beds. As grazers, they help prevent algae overgrowth on coral reefs.
Their consumption of algae ensures corals receive sufficient light for photosynthesis, supporting coral growth and reef health. This grazing activity contributes to reef resilience, helping them recover from disturbances.
Green sea turtles also contribute to the health of seagrass meadows by grazing on seagrass, which prevents overgrowth that could shade out and harm the beds. Their feeding behavior can enhance the nutritional value of seagrasses by removing older growth.
They also act as nutrient cyclers, digesting plant matter and excreting nutrient-rich waste near coral reefs. This supplies important nutrients like nitrogen and phosphorus, supporting coral polyp growth. This activity also supports the overall biodiversity and balance of their marine habitats.