Everybody loves sea turtles. With their cool shells, patient demeanors and adorable hatchlings, who wouldn’t? The broad carapaces covering their backs – punctuated by patterns of scutes, or hard scales – practically tell stories and present beautiful images.
SEA TURTLES MAY BE LUMBERING ON SHORE but they’re perfectly suited for the life aquatic. They’re streamlined, with large, powerful front flippers that give them the wherewithal for great speed when they need it. And, anyway, they don’t go on shore much.
They’re found in oceans worldwide, even though there are only seven sea turtle species, identifiable by their scute and prefrontal scale patterns: Leatherbacks, Greens, Hawksbills, Loggerheads, Flatbacks, Olive Ridleys and Kemp’s Ridleys. Four of them – Leatherbacks, Greens, Hawksbills and Loggerheads – are found throughout the Atlantic/Caribbean and Indo/Pacific basins. Flatbacks and the two Ridley species are more restricted, at least in nesting.
ANCIENT LINEAGES
Sea Turtles have been in Planet Earth’s oceans since Leatherbacks emerged some 150 million years ago, when their fellow reptilian dinosaurs roamed. The others came later, but it’s said they haven’t changed much for 110 million years.
Obviously, they’re related to their terrestrial turtle and tortoise cousins but adapted to life in the sea with tapered shells that reduce drag and facilitate swimming. And, those strong front flippers. Tapering also deprives them of a land turtle defensive measure: They can’t withdraw their heads and legs into their shells.
BY THEIR SCUTES, YE SHALL KNOW THEM
The shells in all sea turtles except Leatherbacks are hard, broad structures composed of bone and cartilage adapted from ribs, pelvis and other skeletal elements. They’re fixed parts of their owners’ bodies, not structures they can leave behind or shed. (Leatherbacks differ in that their shells instead are, well, leathery. More on that below).
The carapaces of hard-shelled turtles are covered by hard keratinous segments called scutes.
The number of scutes, the shape of the head and the number of prefrontal scales between the eyes are key species identifiers. A series of thin nerves are embedded in the shells, so they can tell when they’ve been touched.
SEA TURTLE LIFECYCLES
Cable nature programs are full of footage of female sea turtles coming ashore on moonlit nights to lay eggs in the sands and, subsequently, of adorable little hatchlings making their way into the sea. In fact, sea turtles eat, sleep and mate in the sea, never setting foot on land once they’re in the water, except for the females’ fleeting nesting visits.
The nesting part: For most species, turtle feeding grounds are far distant from their mating and nesting phases, requiring migration of hundreds to thousands of miles.
Apparently guided by magnetic fields and chemical cues (a phenomenon called “natal homing”), most females return to beaches within 5 to 35 miles of their birthplaces. The males return to their feeding areas.
Once ashore, the females use their front flippers to dig “body pits” to bring themselves level with the sand and their rear flippers to create egg chambers 2 to 5 feet deep. Numbers vary by species but they typically lay between 50 and 200 eggs, carefully covering them with sand to hide the nest before returning to the ocean. Depending on the species, they may nest as many as a half-dozen times at 7 to 10 day intervals throughout the nesting season.
Hatchlings emerge from their shells and dig their way out after some 50 to 70 days, then dash for the water, heading for patches of sargassum to hide out while they mature.
Plenty hatch but the perils of predators and other factors are majestic. It’s estimated that fewer than 10 percent make it to the one -year mark. Only one in a thousand survive the decades-long journey to sexual maturity. Depending on species, lifespans of surviving turtles can reach to 50 or more years.
LIFESTYLE SPECIFICS
Sea turtles may spend their lives in the water – the large majority of their time underwater – but they’re still air-breathing animals who must return to the surface regularly to take in air. Large lungs help. And the amount of time they can spend underwater depends on what they’re doing.
A dive for active foraging underwater can last a quarter hour or so. A sleeping turtle can snooze down below for up to 7 hours. But they have an advantage we lack: When they’re underwater (and not under stress), they can slow their heart rate down – to as much as nine minutes between heartbeats.
As with other marine reptiles, ingesting salty water in their ocean environment presents the risk of excessive salt build up, a potentially lethal issue. To deal with it, they expel salt through special glands near their eyes. The problem is particularly significant for Leatherbacks,due to diets of mostly-watery jellyfish. Their glands are oversized – to more than twice as large as their brains.
SEA TURTLE SPECES SNAPSHOTS
Leatherback Sea Turtles (Dermochelys coriacea)
Having emerged 150 million years ago, Leatherbacks are the oldest of the seven extant sea turtle species. They’re also largest, reaching as much as 8 feet in length and weighing as much a 2,000 pounds (More typically, 6 feet and 700 pounds).
Most notably, of course, they lack the hard shells of Hawksbills and the rest. Instead, as their name suggests, they sport thick, leathery carapaces covering a flexible arrangement of underlying bone. They don’t have scutes but they do have “keels” along their backs.
Adaptations that allow them to maintain their core body temperatures allow them to tolerate cold water much more easily than their hard-shelled cousins, so that they are found ranging wide in tropical and temperate oceans worldwide, as far north as Alaska and the Arctic Circle. They spend much of their lives in the open oceans but return to tropical and subtropical areas to mate and nest.
On the other hand, their jaws are relatively weak and their diet is restricted to soft prey like jellyfish and salps. Still, they can consume nearly their own body weight in an average day. Like nearly all sea turtle species, they’re considered to be at risk: Endangered in the U.S. and Vulnerable around the world.
Green Sea Turtles (Chelonia mydas)
Green Sea Turtles don’t get their names from going around looking like The Hulk. Instead, they’re named for the greenish color of their body fat and cartilage, a consequence of their dietary preference for green algae and seagrasses.
Since an herbivorous diet depends on plants found in shallow waters, they’re found primarily in coastal tropical and subtropical areas worldwide.
In the Caribbean and in the parts of the Pacific basin I’ve dived, Greens and Hawksbills have been the species I’ve encountered most often. They’re considered Threatened or Endangered in all oceans, although it appears that their survival rates are improved in protected marine areas.
Hawksbill Sea Turtles (Eretmohelys imbricata)
The hawk-like beaks that give. E. imbricata their common name come in handy for their dietary lifestyle of chunking off and devouring sponges. Found mostly in tropical and subtropical coastal waters worldwide, they’ve been targeted for richly patterned shells, used in tortoiseshell jewelry and other products.
I’ve encountered plenty of Hawksbills in the Caribbean basin and Philippines, but they’re considered Critically Endangered species in the ICUN Red List.
Loggerhead SeaTurtles (Caretta caretta)
Loggerheads are carnivores almost at the other end of the dining spectrum from Leatherbacks. They’re characterized by large, blockish heads with powerful jaws for crushing hard-shelled prey like conchs, barnacles and crabs, including horseshoe crabs.
Found in oceans worldwide, they’re second only to Leatherbacks in their distribution, although it appears that they’re more likely to be encountered in temperate waters than in the tropics.
A Mediterranean Loggerhead population regularly nests on beaches in Greece, Turkey and Israel. Another is notable for journeying 8,000 miles between feeding areas in Baja California and South America and nesting grounds in Japan and Australia. Few nest in the Caribbean.
Monitoring agencies list nine distinct populations of Loggerheads, counting two in the Pacific. Important nesting areas include Oman on the Arabian Sea and the Atlantic coast from North Carolina to Florida and in the Gulf of Mexico. They’re listed as Threatened or Endangered in all of them.
Kemp’s Ridley Sea Turtles (Lepidochelys kempii)
To get the burning question out of the way: the Kemp part is named after a Key West fisherman who discovered and helped identify the species in 1906. As for the Ridley part, nobody where it comes from, even though Kemp’s shares the nomenclature with Olive Ridleys.
They’re probably known better for their nesting behaviors and their perilous population status than for themselves. There are only some 2,500 nesting females, centered in the Gulf of Mexico, although juvenile Kemp’s have been spotted along the Atlantic coast as far north as New England. They don’t nest in the Caribbean at all, so visitors to that region, like me, are unlikely to have encountered them.
They’re listed as Critically Endangered on the ICUN’s Red List, and turtle conservation organizations put enormous effort into helping the species survive. Their major nesting site is on Rancho Nuevo Beach on the Mexican Gulf Coast, although a smaller population nests on beaches in South Texas. The proximity to American news media brings it significant attention.
Their nesting behaviors differ from the non-Ridley species. They come ashore on their nesting beaches in groups of hundreds (or even thousands) called arribadas, the Spanish term for “arrivals.” And, they’re the only sea turtle species that nests in the daytime.
They’re primarily carnivores, favoring a diet of crustaceans, mollusks, jellyfish, small fishes and sea urchins. But they’ll eat algae, as well. The Kemp’s lifespan is unclear but believed to be in the 30-year range.
Olive Ridley Sea Turtles (Lepidochelys olivacea)
Olive Ridleys sport shells that actually are olive green, hence their name (no word on their body fat colors). Actually, they’re primarily carnivores.
In contrast to Kemp’s Ridleys’ status as the smallest population of sea turtles, Olive Ridleys are the largest. The number estimates are mixed, from 80,000 nesting females to hundreds of thousands. .
They’re found in tropical waters worldwide, with major nesting sites on the west coast of Mexico and the west coast of India, but also the east coast of Brazil and Gabon on the west coast of Africa. They, too, are famed for arriving on nesting beaches in arribada formations, sometimes quite massive.
Despite all the coastal nesting, Olive Ridleys are primarily denizens of the open oceans, sometimes migrating in great numbers. They’re know to dive to depths of more than 600 feet and feed on jellyfish, salps, fish and fish eggs, sea urchins, bivalves, crustaceans and worms.
Despite their abundance, their population has declined and they’re listed on the ICUN Red List as vulnerable to extinction.
Australian Flatback Sea Turtles (Natator depressus)
In turtle nomenclature, “flatback” is a relative term. Flatback Turtle carapaces are domed, as with other sea turtles, just not as much. They grow up to some 3 feet and 200 pounds, with 1 pair of prefrontal scales and 4 pairs of costal scutes, with upturned marginal scutes.
They spend so much time at the surface that birds sometimes perch on them. They primarily feed on sponges, jellyfish sea cucumbers, crustaceans and other soft prey.
Flatbacks probably have the least public awareness of all the 7 species. Their distribution is confined to the coasts of northern Australia and southern Papua New Guinea/Indonesia. And even the Papua New Guinea/Indonesia populations return to Australian beaches to nest. Possibly the least-studied of the sea turtles, its population numbers and ICUN Red List status are uncertain.
PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Reef Creature Identification Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Les Wilk; Reef Creature Identification, Tropical Pacific, Paul Humann, Ned DeLoach; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Galapagos – A Natural History, Michael H. Jackson; SEA TURTLES, Cheloniidae and Dermatochelyidae, Smithsonian Ocean Life; Sea Turtle Species, WWF (World Wildlife.org); Identifying sea turtle species, “Turtles and Tides, WordPress.com; Sea Turtle Conservation Program, Nova Southeastern University; Sea Turtles of the Caribbean, SWOT – The State of the World’s Sea Turtles; Can a Turtle Outgrow Its Shell? How Stuff Works; Types of Sea Turtles, Outforia; Sea Turtle Species, The LIfe Cycle of Sea Turtles, seeturtles.org; 7 Sea Turtle Facts for the Ocean Lover, National Oceanic and Atmospheric Administration; Green, Hawksbill, Leatherback, Loggerhead, Kemp’s Ridley, Olive Ridley Sea Turtles, NOAA; Turtle Species, Hawkbill, Green, Loggerhead Sea Turtles, et. al, Wikipedia.org.