Unless one visits the ancestral nesting beaches at the exact night when females come ashore to lay eggs, observing sea-turtles is limited to shipboard sightings at sea, or underwater encounters while SCUBA diving. I've done the latter two, but have yet to experience the nesting beach phenomena.
The typical sea-turtles
The Green Sea-turtle Chelonia mydas is found in tropical oceans around the globe, except the eastern tropical Pacific where Black Sea-turtle is the tropical species. It is often found close to shore in bays and lagoons, seen while diving in the Caribbean and on the Great Barrier Reef, Australia [the underwater photo of Green Sea-Turtle, right, is from Heron I. at the southern end of the Great Barrier Reef, east of Townsville, Queensland, Australia. It is a grazer, feeding on beds of eel-grass or seaweed, or algae around mangroves. Migrational movements take some of these beasts far offshore. It has perhaps the most prized turtle meat, and now there are commercial turtle farms in lagoons around the southern Caribbean. The largest natural nesting beach is at Tortugera, Costa Rica.
The name "Green Sea-Turtle" comes from the color of its fat, but the carapace
(shell) is often greenish to olive. It has a long prehensile tail.
Identification within its range is made by counting four costal shields (the
large plates on the sides of the shell, not to be confused with a row of
vertebral shields down the center of the carapace) and the two very large
prefrontal scales between the eye-lids.
The Black Sea-turtle Chelonia agassizi is the "Green Turtle" representative in the eastern tropical Pacific. Like the Green, it is a grazer. It seems most common around the Galapagos Island. Our research boat disturbed this pair of Black Sea-turtles, left, just east of Espaņola I., Galapagos. They were engaged in mating before our ship just about ran them over. These mating sessions can last for hours if undisturbed ....
My brief experience with these turtles is that they do look dark -- essentially blackish -- compared to the more widespread Green Sea-Turtle. Photos in the National Geographic article cited above tend to support this idea. Their range extends to western Mexico and occasional one will reach southern California (there are even a couple records from Monterey Bay. Like the Green Sea-Turtle, they have two very large and very long prefrontal scales between the eyes, helping to separate them from Loggerheads (which has two pairs of two scales on the forehead; see the photo below).
The Loggerhead Sea-turtle Caretta caretta has a huge head and heavy jaws used for crushing mollusks and crustaceans. This is the most likely sea-turtle to be seen around southern Florida, and indeed some 16,000 females lay eggs on Florida beaches (each female generally comes ashore about four times between October-April). One can see them from the sea-plane flight to the Dry Tortugas, or sometimes around Caribbean islands. I photographed this Loggerhead Sea-turtle, above, from a small boat off the southern tip of Dominica in the Lesser Antillies.
The Loggerhead is a big turtle (it can weigh 650 pounds). The carapace (shell) is often reddish-brown and the head is large and usually washed reddish-brown [young turtles can be yellowish buff, brown, or grayish]. Note the two prefrontal scales over the eyes; there are two pairs of these (one pair above each eye) and the heavy jaws. Loggerheads also have five costal plates on each side (not four as in Green and Black sea-turtles), but with the growth of algae and barnacles, it can be hard to distinguish between carapace plates.
The Olive Ridley Lepidochelys olivacea is apparently the most common sea-turtle in the world -- at least based on the numbers of females coming ashore in Central America -- but it is threatened by being slaughtered when they do come ashore (a decade ago, some 75,000 were killed annually in Mexico alone). Both species of ridley come ashore to lay eggs in huge arribadas -- spectacular arrivals of thousands of turtles on the same beach on the same night. The Olive Ridley is also caught in long lines and gill nets in the open sea. During my four month cruise way offshore in the eastern tropical Pacific, it was certainly the most common sea turtle. One of my cruise duties was to attempt to identify and document each turtle we discovered; this Olive Ridley, below, was thousands of miles offshore at 10°39'N, 139°47'W. The primary distribution of this turtle is tropical, but there are a couple records as far north as Monterey Bay.
The Olive Ridley is decidedly smaller than other Pacific sea-turtles (Kemp's
Ridley in the Atlantic is also small; both have an average shell length of 30
inches) and does tend to be olive in color. Some (like this one) have a high
anterior dome on the carapace. They feed on crustaceans. I've seen Brown Boobies
riding on live ridleys, and Red-foots sitting on dead ones at sea. Their small
size, shape, and color, plus far offshore habitat, makes them fairly easy to
identify; they also have more costal scutes than other Pacific turtles,
typically 6-8 on each side.
The Leatherback Sea-Turtle Dermochelys coriacea is a very distinctive and different sea-turtle. First, it is by far the world's largest sea-turtle, typically weighing 600-1000 pounds and some have tipped the scales at 2000 pounds. Second, it lacks scutes and scales and instead has a leathery carapace with prominent longitudinal ridges. Third, it has the longest migrations of any pelagic turtle and wanders widely the tropical and subtropical oceans of the world. Yet there are only a very few limited nesting beaches: one large and critical beach in western Mexico, another in Indonesia, and a few others scattered elsewhere (including Costa Rica). Fourth, it feeds only jellyfish and relatives, and can dive up to 3200 feet deep in search of giant jellyfish. Yay!
It is hard to think of anything more unique than this. It is the only sea-turtle that appears annually in Monterey Bay. It generally is associated with warmer fingers of the east Pacific gyre that reach closer to shore during our "pelagic period" in the northern summer.
Egg collecting on the few nesting beaches is pushing this remarkable species toward extinction. Indeed, as I understand it, computer modeling indicates that if the current rate of losses in breeding females continues, the Pacific Ocean population of Leatherback will go extinct in our lifetime. Additional hazards are long line and gill-netting, and the presence of rubber balloons or plastic bags as garbage at sea. Leatherbacks can mistake this trash for jellyfish and die of internal blockage. For that reason alone, the release of balloons at weddings and other events should stop. Leatherbacks have special backward pointing nubs in their throat which keeps a jellyfish going down the gullet, but also precludes regurgitation of balloons.
In July 2000 there were unusual multiple sightings of Leatherbacks on Monterey
Bay coincident with a bloom of jellyfish. This individual Leatherback
(below) was actively munching on a Pelagia jelly. The beast was lazying
at the surface with its head in the large transparent jelly. Mostly the head
was underwater eating the jelly, but as an air-breather it lifts its head briefly
every few moments.