Species deep dive · Testudines

Sea turtle size and weight: the ancient voyagers of the Mediterranean

Sea turtles were cruising warm oceans a hundred million years before the Mediterranean existed. Three species still visit its waters — and when they are injured off the Riviera, many end up in the care of the Oceanographic Museum of Monaco.

The three Mediterranean species at a glance

Loggerhead — length ≈ 90 cm The Mediterranean’s most common turtle; a heavy, broad head built for crushing shellfish

Loggerhead — weight 70 – 170 kg Large adults are formidable animals well over a metre long

Green turtle — length ≈ 1.1 m An occasional visitor from warmer eastern waters

Green turtle — weight 110 – 190 kg Adults are herbivores, grazing seagrass and algae

Leatherback — length 1.8 – 2.2 m The giant of the family, following jellyfish into the western Mediterranean in summer

Leatherback — weight 300 – 600 kg The largest verified leatherbacks approached 900 kg — the heaviest reptile on Earth

Lifespan 50 – 80 + years Turtles mature slowly; a nesting female may be older than her beach’s regulars

Nest temperature decides sex ≈ 29 °C Warmer sand produces females, cooler sand males — a climate-sensitive design

Built for a hundred million years of ocean

A sea turtle’s shell is not a borrowed house but a transformed skeleton: the ribs and spine have fused into a single armoured case, so a turtle can never leave it. What it loses in flexibility it gains in endurance. Leatherbacks take the design furthest — replacing the hard shell with a leathery, oil-slick carapace over a lattice of bone, they can dive past 1,200 metres and cross entire ocean basins, guided by a magnetic map written into their nervous system before they ever see the sea.

Size and slowness are the turtle’s bargain with time. A loggerhead needs twenty to thirty years to reach breeding age; a green turtle may need forty. Females then navigate back to the beach where they themselves hatched, sometimes after decades and thousands of kilometres, to lay a hundred eggs at a time in the sand. Of every thousand hatchlings that scramble to the surf, perhaps one survives to breed — which is why the loss of adult turtles to fishing gear, boat strikes and plastic is so devastating, and why protecting nesting beaches changes the fate of whole populations.

The Mediterranean is a hard neighbourhood for turtles: busy shipping lanes, dense coastal development, and fishing fleets working waters that turtles have used since the sea was young. Yet loggerheads still nest on beaches in Greece, Turkey, Cyprus and occasionally even the western basin, and every summer leatherbacks cruise past the Riviera feeding on jellyfish blooms — a reminder that the Mediterranean remains, against the odds, a living sea.

Sea turtle swimming upward through deep water

Monaco’s turtle hospital

The Oceanographic Museum of Monaco has turned turtle care into one of its signature missions. Injured loggerheads — tangled in longlines, struck by propellers, or weakened after swallowing plastic — are brought to the museum’s care centre, treated by veterinarians, and released back into the Mediterranean once they recover, some fitted with satellite tags that let researchers follow their journeys. Visitors can meet resident turtles in the lagoon and on the rooftop terrace, where the museum’s famous tortoise enclosure looks out over the very sea its patients come from. The work is practical conservation with a visible result: a turtle swimming away from the release boat is the best exhibit a museum can offer.

Sea turtles: quick answers

How much does a sea turtle weigh?

Mediterranean loggerheads typically weigh 70–170 kg, green turtles 110–190 kg, and leatherbacks — the giants — 300–600 kg, with exceptional individuals near 900 kg.

What is the biggest sea turtle?

The leatherback, reaching over 2 metres in length. It is the largest living reptile after the big crocodiles.

Are there sea turtles in Monaco?

Yes. Loggerheads and leatherbacks pass through Monaco’s waters, and the Oceanographic Museum runs a rescue and rehabilitation centre for injured turtles.

How long do sea turtles live?

Estimates range from 50 to 80 years or more. They grow slowly and do not breed until they are decades old.