Five Facts About Turtles for World Turtle Day 2018
It's World Turtle (and tortoise) Day 2018, and as divers, turtles are one of our favourite creatures to spot when we're underwater. They can, by turns, be rather inquisitve and yet often completely ignore the divers who surround them looking for the perfect turtle picture. But, much as we love them, turtles are under threat due to damage to our coral reefs, poor fishing practice, and an underground market for their eggs and shells. Here's five facts about some of our best-loved dive buddies.
Sea turtles are air-breathing reptiles of the order Testudines, which aslo includes tortoises (also known as turtles in some countries) and terrapins. Turtles are divided into two families: Cheloniidae, turtles with a hard carapace (shell) and the soft-shelled Dermochelyidae. There are 6 known species of Cheloniidae, the green turtle (Chelonia mydas), hawksbill (Eretmochelys imbricata), loggerhead (Caretta caretta), olive ridley (Lepidochelys olivacea), Kemp's ridley (Lepidochelys kempii) and the flatback (Natator depressus). The only member of the Dermochelyidae family is the leatherback turtle (Dermochelys coriacea), the largest in the world, growing up to 2.2m long and weighing in at three-quarters of a tonne. The six species of Cheloniidae are poikilothermic, meaning their body temperature changes with their surroundings. Leatherbacks are endothermics, or 'warm-blooded', meaning they maintain a high internal body heat through their metabolism, just like mammals. This makes leatherbacks unique in the reptilian world.
Hard-shelled reptiles are recorded to have existed from around 220 million years ago, making them older than crocodiles and snakes. The ancestors of modern sea-going turtles first appear in the fossil record around 120 million years ago, with the extinct Desmatochelys padillai only being described in 2015, even though the bones and shell were discovered in the 1940s in Columbia. Turtles are long-lived creatures, along with their land-based cousins. It is thought that turtles do not reach sexual maturity until they are approximately 30 years old. Nobody knows how long they really live, although it is estimated that some reach more than 100 years of age.
Turtles are typically encountered in tropical and sub-tropical waters, although they often range outside these areas. As poikilotherms, the six species of Cheloniidae are usually found in warmer water, but the warm-blooded leatherback has been sighted in the open oceans as far north as Alaska and as far south as New Zealand. Five of the species are distributed around the globe, but the Kemp's ridely turtle is only found in the western Atlantic along the US east coast and the Gulf of Mexico. The flatback is only found around the northern coastline of Australia. Some turtles have a very wide range - the leatherback has been known to swim more than 20,000km in a year.
Turtles mate in the water but they nest on land, almost exclusively in tropical and sub-tropical waters. Females will return to the area where they themselves hatched - often to the very same beach where they were born - to nest and lay eggs, which require the warmth to incubate successfully. The nests are not attended, and as the baby turtles hatch, they run a gauntlet of predators as they instinctively head out into the open ocean. Most of them sadly never make it into adulthood, however, what happens between birth and maturity, nobody really knew until recently, when the BBC Blue Planet II documentary filmed a hatchling sheltering under a piece of driftwood in the open ocean.
The conservation status of sea turtles, according to the IUCN, is not good. Loggerheads and olive ridley turtles are officially listed as 'vulnerable', although some of their individual populations are listed as endangered. Leatherbacks and green turtles are listed as 'endangered' and the hawksbill and Kemp's ridley listed as 'critically endangered'. So little is known about the flatback that it is simply listed as 'data deficient'. Threats to turtles include climate change, the destruction of the reefs (and seagrass fields, in the case of green turtles) through both natural and man-made activities, poaching of their eggs for food and their shells to make ornaments. Some species love to feed on jellyfish, making them prone to suffocation through the ingestion of the very similar looking plastic bags. Turtles are also victims, by the hundreds and thousands, of destructive fishing practices, often the result of accidental bycatch or through abandoned ghost gear where entangled and unable to breathe, they drown. World Turtle Day might occur on 23 May every year but - like all special days - every day is turtle day, and they need our help.