Five of the world's seven sea turtle species nest somewhere on Costa Rican shores — green, hawksbill, olive ridley, loggerhead and leatherback.1 Not all of them turn up on an actual dive off Tamarindo, though, and the ones that do behave nothing alike. Knowing which species you're looking at changes what to expect: whether it's grazing, hunting sponges, or simply passing through on its way to somewhere synchronized with the moon.
Olive ridley: the one you're most likely to see
The olive ridley (Lepidochelys olivacea) is the sea turtle divers on the Pacific coast encounter most often, and the smallest of the group.2 It's an omnivorous forager, eating crabs, shrimp, lobster, sea urchins, jellyfish, algae and fish, and it will dive to considerable depths — over 150 metres in some recorded cases — chasing bottom-dwelling prey.2
What makes the species remarkable, though, happens on the beach rather than underwater. Olive ridleys are one of only two sea turtle species on Earth that nest in arribadas — mass, synchronized nestings where thousands of females come ashore together over a few nights, timed to the lunar cycle and concentrated around the last-quarter moon and the darkest nights before a new moon.3 Guanacaste's Ostional Wildlife Refuge, protected since 1984, is one of the most significant arribada beaches on the planet: regular arribadas run from August through December, with the largest events — sometimes exceeding 500,000 nesting females — concentrated in September and October.3
"Only olive ridleys and their close relative, Kemp's ridley, synchronize their nesting in these mass emergences — the Spanish word for it, arribada, simply means 'arrival.'"
Green turtles: grazers that outlive most divers who spot them
Green turtles (Chelonia mydas) are seen less often on Pacific-coast dives than olive ridleys, though new nesting activity for the species continues to be documented along this coastline.4 They're named for a layer of green fat beneath the shell, not their coloring, and they undergo one of the more distinct diet changes of any sea turtle: juveniles are omnivorous, ranging into open water and feeding on invertebrates, before adults settle into coastal seagrass beds, mangroves and reefs and switch almost entirely to seagrass, algae and mangrove leaves.5 They're also long-lived even by turtle standards — up to 80 years in the wild, with some individuals reaching 90 — and the IUCN currently lists the species as Endangered.5
Hawksbill: rare, solitary, and essential to the reef
The least frequently seen of the three on a Pacific dive is the hawksbill (Eretmochelys imbricata), usually spotted alone rather than in numbers.4 Its narrow, tapered beak is built for one job: working sponges out of coral crevices. In parts of its range, an adult hawksbill can eat over 500 kg of sponge a year, and by keeping fast-growing sponge colonies in check, the species plays an outsized role in maintaining healthy reef structure.6
Hawksbill hatchlings spend their first one to five years adrift in open-ocean algae mats before migrating to the coastal reefs where they'll spend their adult lives, and mature females return to nest every two to three years — typically on the very same beach where they themselves hatched.6 Nesting season across most of the species' range runs roughly from April to November.6
Leatherback: a story that happens mostly on the sand
The fourth species with a real Guanacaste presence, the leatherback, is more a beach story than a dive-site one — nesting at Playa Grande, Ventanas and Langosta inside Las Baulas National Marine Park from late October to mid-February, as we covered in a previous piece on what to do in Tamarindo beyond diving. It's a reminder that "seeing a sea turtle" in this part of Costa Rica can mean two entirely different experiences: a guided night walk on a protected beach, or an unplanned pass by a grazing olive ridley thirty feet down.
Either way, the same principle applies: let the animal keep doing what it was doing before you arrived. None of these encounters require getting closer than the turtle is comfortable with.