The Hawaiian Green Sea Turtle, known locally as Honu, is a marine species within the Hawaiian archipelago. These large reptiles are unique among sea turtles because their diet transforms significantly as they mature, changing from an opportunistic omnivore to a nearly exclusive herbivore. This specialized feeding ecology is central to the Honu’s life cycle and impacts the coastal habitats they inhabit.
The Dietary Shift of Juvenile Honu
The feeding habits of a Honu are directly tied to its life stage and habitat location. Newly hatched turtles spend their first several years in the open ocean, or pelagic zone, a period often referred to as “the lost years.” During this time, they have an omnivorous diet, taking advantage of whatever food is available in the water column.
Their diet in the open ocean consists of small, soft-bodied invertebrates, crustaceans, fish eggs, and floating organisms like sea jellies. This opportunistic feeding strategy supports rapid growth in a food-scarce environment. The transition to a plant-based diet begins as juvenile turtles move from the pelagic zone to near-shore, benthic habitats, which usually happens between the ages of five and seven. This shift allows them to settle into their adult coastal foraging grounds.
The Staple Diet of Adult Green Sea Turtles
The adult Honu diet is almost entirely herbivorous, consisting primarily of marine algae and seagrasses. In the Hawaiian Islands, a major component of this diet is benthic marine algae, known locally as limu. Honu feed on a wide variety of marine vegetation, sometimes consuming over 130 species.
The bulk of their nutrition often comes from specific species of red and green algae. A common food source is the red seaweed Pterocladiella capillacea. In the Kāneʻohe Bay area of Oʻahu, Honu have adapted to consume non-native algae, including species like Acanthophora spicifera, Hypnea musciformis, and Gracilaria salicornia.
The native Hawaiian seagrass, Halophila hawaiiana, is not widespread enough to be a primary food source for the Honu population. The turtles’ specialized jaws, which feature serrated ridges, are adapted for tearing and clipping plant material. To process this high-fiber diet, adult Honu possess a long digestive tract and rely on a specialized gut microflora to ferment the plant matter and extract nutrients from the cellulose.
Grazing Behavior and Ecological Impact
Adult turtles are active grazers, shaping the health and appearance of the nearshore ecosystem. They are often seen methodically scraping algae from the surface of rocks and coral or cropping the blades of seagrass. This behavior creates distinct “grazing plots” where the vegetation is kept short and young, which is more digestible and higher in protein than older, tougher plant growth.
By grazing on marine algae, Honu function as ecological engineers, acting as the “lawnmowers” of the reef. Their feeding helps prevent fast-growing algae from overgrowing and smothering corals. Without this consistent cropping, algal blooms could dominate the shallow-water environment, negatively affecting reef diversity.
Honu are known to return consistently to preferred foraging areas along the main Hawaiian Islands, such as the coasts of Oʻahu, Maui, and Molokaʻi. They spend time foraging each day, with feeding activity often beginning a couple of hours after dawn.