Scientists describe sea turtles as keystone species in recognition of the roles they play to support ecosystem function and balance. Leatherbacks keep jellyfish populations in check and hawksbills prevent sponges from outcompeting corals.

Leatherbacks are the largest sea turtles. Males can reach up to 8.5 ft (2.6m) and 2,000 lbs (900 kg). Unlike other sea turtles, leatherbacks have a flexible, rubbery carapace that allows them to dive deeper than 4,100 feet (1,250 meters). Leatherbacks nest on tropical beaches and are found in all the world’s oceans except the Arctic and Antarctic. The leatherback’s esophagus is extra-long and lined with spines to facilitate digestion of jellyfish.

Green turtles have small, rounded heads and smooth carapaces. Ranging across the tropical and subtropical Atlantic, Pacific, and Indian Oceans, they feed mainly on seagrasses and algae, a diet that gives their fat its telltale green color and the species its name. The East Pacific subspecies, sometimes called the black turtle, has a notably darker shell.

Hawksbills have narrow heads and hooked, bird-like beaks built for reaching sponges tucked into coral reefs. They nest on tropical beaches and are most often found near reef habitats. Their strikingly patterned, overlapping shell scutes once made them a prime target of the tortoiseshell trade.

Olive ridleys are considered the most abundant sea turtle species, though today’s global population is only a fraction, about 0.2%, of its historical size. Females nest together in massive synchronized aggregations called arribadas, sometimes up to 200,000 turtles at once. They forage along drift lines for crabs, jellyfish, clams, snails, and algae.

Beyond the four turtles above, three more species round out the world’s seven: loggerheads, with powerful crushing jaws built for crabs and mollusks; Kemp’s ridleys, the rarest sea turtle, found mainly in the Gulf of Mexico and known for daytime arribadas; and flatbacks, found only in Australia’s shallow coastal waters, feeding on jellyfish and soft-bodied invertebrates.