Green turtle
Chelonia mydas

The name "green sea turtle" almost systematically causes confusion among beginner observers. The carapace is not green. It is olive-brown, often marbled with yellow and dark brown, with reflections that vary according to age and underwater light. The green color is that of the animal's subcutaneous fat, which takes on this characteristic hue after exposure to air, particularly during nesting or dissection. This fat long made the species a prime target for turtle soup, before international protections ended this trade.

Olive-Brown Carapace and Prefrontal Scales

In the field, the carapace alone is not enough to identify the species with certainty. What immediately distinguishes the green sea turtle from other species is the presence of two large prefrontal scales located between the eyes, clearly visible even while snorkeling at a reasonable distance. Most other sea turtles have four or more, arranged differently.

Blunt Beak vs. Pointed Beak

The beak is the second quick criterion to check. The adult green sea turtle has a short, blunt beak adapted for grazing seagrass. The hawksbill turtle (Eretmochelys imbricata), often confused with it, has a narrow, pointed beak that is clearly hooked. This contrast is visible from a few meters away, even without underwater optical equipment. These two combined criteria, prefrontal scales and beak shape, allow reliable identification in the vast majority of cases.

Size and Silhouette

An adult measures between 80 and 140 cm in carapace length and can weigh from 70 to 230 kg. The head is large and rounded, proportionally wider than in the loggerhead turtle (Caretta caretta). In clear water, the flattened silhouette and long, tapered front flippers are recognizable from the surface.

The Two Large Prefrontal Scales

This is the most reliable and quickest criterion to check. These two prefrontal scales form a symmetrical pair clearly visible between the eyes, on top of the head. The hawksbill turtle has four prefrontal scales. The loggerhead turtle also has two, but the head is more massive and the beak more robust.

Surface Behavior

The green sea turtle regularly surfaces to breathe. During grazing activity, intervals between breaths range from a few minutes to about twenty minutes depending on the effort intensity. A resting turtle can remain motionless on the bottom for over an hour. Observing these surfacings from the surface is often the first sign of the species' presence.

Distinguishing Male and Female

Outside the breeding season, distinguishing juveniles is difficult. In adults, the male's tail extends well beyond the carapace, sometimes by 30 cm or more, unambiguously distinguishing it from the female. This criterion is visible while free-diving if the animal is seen in profile or from below.

Juvenile vs. Adult

Juveniles have a darker coloration, often dark brown to black on top, with bluish reflections. Their behavior also differs radically: they inhabit pelagic zones and feed on invertebrates, unlike the strictly herbivorous adults. A small individual observed far from seagrass beds is likely a juvenile in the pelagic phase.

Carnivorous Juvenile, Herbivorous Adult

The dietary shift in the green sea turtle is one of the most remarkable aspects of its biology. Juveniles are omnivorous with a carnivorous tendency: they consume jellyfish, crustaceans, sponges, and benthic invertebrates. Upon reaching sexual maturity, the diet shifts to a strictly herbivorous regime, centered on seagrass beds (Thalassia, Posidonia, Cymodocea) and algae. This change explains why adults concentrate in coastal seagrass areas, while juveniles often remain in pelagic environments for several years.

Sexual Maturity and Longevity

Sexual maturity is reached between 20 and 25 years, sometimes later depending on trophic conditions. Estimated longevity exceeds 80 years in some cases. This late maturity means each adult represents two decades of biological investment, making adult mortality particularly damaging to population dynamics.

Nesting: Night, Warm Sand, Sex Determined by Temperature

Females return to nest on the beach where they were born, sometimes after migrations of thousands of kilometers. Nesting occurs at night, in a nest dug in the sand above the high tide line. Incubation temperature determines embryo sex: above about 29.5 °C, hatchlings are predominantly female. Climate warming directly disrupts this balance.

Navigation by Magnetic Field

Green sea turtles use Earth's magnetic field as both compass and map to navigate thousands of kilometers with remarkable precision (Lohmann et al., 2008). Each nesting beach has a unique magnetic signature that females memorize at hatching and rediscover decades later.

Mayotte

Mayotte's lagoon seagrass beds are one of the most accessible observation sites in the Indian Ocean for the green sea turtle. The nesting population is estimated at several thousand individuals, with active nesting beaches particularly on Mayotte island and surrounding islets. The Mayotte Marine Natural Park coordinates population and nesting site monitoring. Local operators offer guided snorkeling trips over lagoon seagrass beds, with near-guaranteed sightings in season.

French Polynesia

The atolls of Moorea, Rangiroa, and Tetiaroa are among the Pacific's best-documented sites for the species. Field observers report high densities on coral heads and inner lagoon seagrass beds. Tetiaroa benefits from an active scientific monitoring program, with photo-identification data available via local networks.

Réunion and Scattered Islands

Europa Island, in the French-administered Scattered Islands, is one of the major Indian Ocean nesting sites, with thousands of nesting females each year (TAAF report). Réunion hosts feeding individuals on its coastal seagrass beds, with regular sightings reported by local dive clubs.

French Antilles

In Martinique and Guadeloupe, resident populations frequent coastal seagrass beds year-round. The French Office for Biodiversity (OFB) coordinates a restoration plan for nesting populations in the Antilles, with nesting monitoring on several protected beaches.

Outside French Territories

In the Maldives, field observers report some of South Asia's highest densities, particularly around Baa Atoll's reefs. The Great Barrier Reef (Australia) hosts one of the world's largest populations, followed by the Queensland Turtle Conservation Project. In Costa Rica, Tortuguero beaches are an iconic nesting site, with structured ecovolunteer programs.

Recommended Minimum Distances

Rules vary by territory. In Mayotte, the Marine Natural Park recommends maintaining a minimum distance of 2 meters and avoiding physical contact. In French Polynesia, local decrees protect turtles in managed marine areas, with similar distances. In Réunion, national regulations apply, reinforced by marine managers' recommendations. Always check local rules before each outing, as they evolve.

Behaviors to Avoid

Touching a turtle is prohibited and counterproductive: it disrupts behavior, can transmit skin pathogens, and desensitizes it to predators. Blocking a turtle's path as it surfaces to breathe is the most serious mistake: a turtle prevented from breathing can drown. Underwater flash disorients the animal and must be avoided. Never chase a turtle that swims away.

French Regulations

In France, the green sea turtle enjoys full protection under the October 14, 2005 decree. This protection covers individuals, eggs, nests, and habitats. Any intentional disturbance, capture, injury, or destruction is punishable by criminal penalties.

Washington Convention (CITES) and IUCN Status

The species is listed in CITES Appendix I, prohibiting international trade. It is classified as "Endangered" on the IUCN Red List (IUCN, 2022). These two legal instruments form the basis of its international protection.

Reporting a Turtle in Distress

If observing an injured, stranded, or net-entangled individual, contact local authorities or the OFB without handling the animal. In Mayotte, the Oulanga Na Nyamba network centralizes reports. In other territories, marine turtle care centers and local OFB delegations are the appropriate contacts.

Bycatch in Fishing Nets

Bycatch remains the leading cause of unnatural mortality for adult sea turtles worldwide. Longlines, gillnets, and trawls capture individuals that drown unable to surface. Turtle exclusion devices (TEDs) exist and are mandatory in some fisheries, but their deployment remains insufficient.

Plastic Ingestion

Transparent plastic bags are mistaken for jellyfish by still-omnivorous juveniles. Plastic ingestion causes intestinal blockages and buoyancy changes that prevent diving. Recent studies show over 50% of necropsied sea turtles have plastics in their digestive tract (Schuyler et al., 2014).

Poaching and Egg Collection

In Mayotte and several Indian Ocean territories, poaching of adults and egg collection remain documented issues despite legal protection. Pressure is particularly high on isolated nesting sites, hard to monitor continuously.

Climate Warming and Female-Biased Hatchlings

Rising sand temperatures cause progressive feminization of clutches. On some Great Barrier Reef sites, researchers measured ratios exceeding 99% females in recent hatchlings (Jensen et al., 2018). Long-term sex imbalance threatens population reproductive viability.

Positive Signals

The outlook is not uniformly negative. Some populations protected for decades show clear signs of stabilization or increase, particularly in Florida, the Caymans, and certain Indian Ocean sites (IUCN, 2022). These successes prove that protection measures work when consistently applied.

I have used photo-identification for years for cetaceans, via platforms like Happywhale and local databases. The same logic applies to sea turtles, with some adaptations.

Photo-Identification by Facial Scales

Each green sea turtle has a unique facial scale pattern, especially the scales around the eyes and on top of the head. A sharp profile or frontal photo, with good resolution on the head area, allows identifying an individual and matching it in databases. For a useful shot: frame the head frontally or at three-quarters, avoid backlighting, do not use flash that distorts colors. Date, time, and GPS coordinates of the observation are essential.

Platforms to Know

iNaturalist accepts all geolocated sea turtle observations and submits them automatically for community validation. Happywhale now includes sea turtles in some regions. In Mayotte, the Oulanga Na Nyamba network has its own local photo-identification database, directly used by field researchers. In French Polynesia, local monitoring programs collect data from divers and operators.

What a Well-Documented Observation Brings to Researchers

A properly documented observation, with photo, coordinates, and noted behavior, allows researchers to reconstruct individual movements, estimate population sizes via photo-recapture, and detect distribution changes linked to warming. For a species whose adults live decades, each photographic recapture data point has long-term value. The amateur observer becomes a full-fledged producer of scientific data, provided protocols are followed.