What Do Hawksbill Turtles Eat? Sponges on Coral Reefs

Hawksbills are sponge specialists

The hawksbill sea turtle is an omnivore, but on most reefs sponges are by far its most important food. Studies of what hawksbills eat, especially in the Caribbean, have found sponges making up 70–95% of the diet. In a classic 1988 paper titled “Spongivory in hawksbill turtles: a diet of glass,” biologist Anne Meylan showed that Caribbean hawksbills feed overwhelmingly on sponges.

Hawksbills are also picky eaters. Out of the hundreds of sponge species on a Caribbean reef, they concentrate on a fairly small number, mostly in the large group called demosponges. Favorite foods include the chicken liver sponge, Chondrilla, and sponges of the genera Geodia, Ancorina, Placospongia and Spheciospongia. Many common reef sponges are ignored completely.

A diet of glass and toxins

Sponges are a tough meal. Many are strengthened by spicules, tiny needles made of silica, the same material as glass. Hawksbills seem to prefer some of the sponges with the most silica, and their digestive tracts can contain a large amount of these glassy fragments.

Sponges also defend themselves with chemicals. Several species eaten by hawksbills, such as Aaptos, Chondrilla, Tethya and Suberites, are highly toxic to many other animals. How hawksbills tolerate these toxins is still not fully understood. The chemicals can build up in the turtle’s tissues, which is why eating hawksbill meat has sometimes poisoned people.

Because so few animals can deal with spicules and toxins, sponges are an abundant food with little competition, and the hawksbill has the reef’s sponge supply largely to itself among large animals.

How the hawksbill’s beak works

The hawksbill gets its name from its feeding tool. The head is long and narrow, and the upper jaw ends in a sharp, hooked beak, a little like the bill of a hawk. This shape lets the turtle push its head into small holes, cracks and crevices in the reef to reach sponges and other prey that a broad-headed turtle could not reach.

Strong jaws with sharp cutting edges tear chunks of sponge tissue away, and the turtle can also wedge its sturdy shell into tight spaces as it forages. The head is covered in thick scales, which protects it from stinging prey. Hawksbills even eat the Portuguese man o’ war, closing their unprotected eyes while they feed.

What else do hawksbills eat?

Beyond sponges, hawksbills eat a wide range of other foods, which become more important where sponges are scarce. In Hawaii, for example, sponges are less available and hawksbills feed more opportunistically.

In the eastern Pacific, many hawksbills live in mangrove estuaries rather than on coral reefs, and their diet there includes more invertebrates and plant material from the mangroves.

Do young hawksbills eat the same food?

Not at first. After hatching, young hawksbills swim out to sea and spend roughly 1–5 years drifting with ocean currents, sheltering in floating mats of seaweed and debris. Little is known about their diet at this stage, but like other young sea turtles they probably eat small floating animals, fish eggs and bits of algae.

Once they reach about 20–30 cm (8–12 in) in shell length, juveniles settle on shallow reefs and rocky areas and gradually switch to the adult diet of sponges and other reef animals.

Why the hawksbill diet matters for coral reefs

Sponges compete with reef-building corals for space on the seabed. Because hawksbills eat large amounts of a few sponge species, they are thought to help keep sponges from overgrowing slower-growing corals, which makes them an important part of reef ecosystems. Corals themselves are animals, as explained in is coral a plant or an animal?

The relationship works both ways. When reefs are damaged by coral bleaching, disease or pollution, hawksbills lose both food and shelter. Reef decline is one of the threats listed in why is the hawksbill critically endangered?

How the hawksbill diet compares with other sea turtles

Each of the world’s seven sea turtles has its own diet, which reduces competition between them.

The green sea turtle often shares the same reefs as the hawksbill but grazes on plants instead. For a full guide, see what do sea turtles eat?