Since the Triassic, sea turtles have not changed a great deal. About 200 million years ago, the common ancestor to today’s Chelonians developed an “armored tank” body form, enclosing its body inside a tough shell as a mode of defense from large toothed predators. This was one of the early steps that led to the evolution of the sea turtle. The development of this hard encasing was achieved by many other anatomical changes. Many vertebrae from the trunk region were done away with and the ribs were no longer attached to the sternum. Because of the fusion of the axial skeleton to the shell, almost all of the dorsal trunk muscles were lost or became modified to aid in respiration. These many changes of body form also led to the development of a new swimming mode, since undulatory swimming was no longer an option. This new swimming mode came from modifications of the forelimbs, which became stiffened and elongated. Now the animal could move through the water by propulsion from up and down strokes of the modified front limbs, while using its back flippers as a rudder.
These modifications of body form allowed the Chelonians to move into an unoccupied niche space in the marine environment. However, because of the restrictions of hardened shell, there was not sufficient room to expand the cloacal area, and so the Chelonians were still dependent on the terrestrial environment for the laying of eggs and development of their offspring. But with the new modifications to the limbs sea turtles were no longer able to move very well on land. This makes them extremely vulnerable to predators when it comes time for them to lay their eggs. Therefore sea turtles developed the behavior of choosing remote beaches as their nesting sites and do not care for their nests. This in turn, leaves the eggs very vulnerable to predation during incubation, as well as the hatchlings as they make their way to the ocean. Thus, there is a high mortality rate in the early life stages of the Chelonian. This is somewhat compensated for by the laying of larger numbers of eggs.
The most commonly recognized species of sea turtles are all very similar, but each has minor differences in morphology and each occupies its own ecological niche. Some of the species are migratory between feeding and nesting sites, some species are neritic and one species is strictly pelagic. However, all species are dependent on land for their nesting sites. Most Chelonians are herbivores, with a few species being carnivorous. The pelagic species feed on planktonic jelly fish, while the neritic species feed either on mollusks or crustaceans. The migratory species consist mostly of the herbivores, as well as one species that is omnivorous and another species that is carnivorous.
According to Hendrickson, the leathery sea turtle, Dermochelys coriacea, is more adapted to its pelagic lifestyle because of more powerfully developed front flippers, which are proportioned and structured differently from those of any other sea turtle. He also believes the carapace ridges found on their shells to be adaptations for dealing with the laminar flow associated with a sustained swimming speed. Because of their life in the open ocean, these turtles are obligate jelly fish feeders.
The green sea turtle, Chelonia mydas, is one of the species that migrates between feeding and nesting sites. Some populations of this species migrate longer distances than others, while some species practice a coastal-oriented migration and never cross open ocean waters to get to their nesting sites. The populations that live in Hawaii and the Galapagos, however, seem to be permanent residents of these islands. Thus, Hendrickson states that this species seems to be divided in to distinct variant populations, more so than the other species of sea turtles.
The flatback sea turtle, Chelonia depressa, is an Australian species that is characterized by its large head and flattened shell. Hendrickson believes that this Chelonian evolved to species status from a population of C. mydas fairly recently. This species is one of the non-migratory species of sea turtles and is thought to subsist on a diet of sea cucumbers. Interestingly, C. depressa has lost a majority of its hard shell, so much so that even a light scratch can draw blood. Hendrickson attributes this to the fact that this Chelonian evolved in the absence of placental carnivores.
Eretmochelys imbricata is better known as the hawksbill turtle. It is thought that this species separated from C. mydas earlier than C. depressa and intergenic breeding of E. imbricata and C. mydas produces viable offspring. These turtles are omnivorous, feeding on coral reefs. This species is also a solitary nester and does not do long migrations. They nest mainly on islands near the coral reefs in which they feed.
The neritic sea turtle, Caretta caretta, is a carnivore feeding on mollusks. This turtle is distributed mainly along the eastern coasts of continents over productive sea bottoms and characteristically nest on beaches of the mainland. Hendrickson states that their large head and jaw structure is a direct adaptation to their diet of mollusks, however they have been known to eat jellyfish and mangrove leaves. This species is the only other species besides D. coriacea that can tolerate lower water temperatures.
The olive ridley turtle, or Lepidochelys olivacea, shares a similar distribution and diet as the loggerhead turtle, but during the non-breeding portion of its life it is thought to reside farther offshore than the loggerhead. Hendrickson believes that reported sightings of green sea turtles far out at sea in the eastern Pacific are actually olive ridley turtles. He also suspects that these turtles, in addition to long dives to feed on the benthic crustaceans of the neritic waters, will float above abyssal waters and feed on the crustaceans that surface at night.
Hendrickson believes that the Kemp ridley turtle, Lepidochelys kempi, is a recent offshoot of the L. olivacea line after the Panamanian land barrier isolated these turtles from the rest of the population of L. olivacea. This turtle is only found in the Gulf of Mexico and the entire population nests in a small section of coast in Mexico. This species will come ashore to nest in enormous arribadas during the day, unlike the olive ridley turtle which does this at night. A problem these turtles encounter is being swept up the eastern coast of the U.S. and as far away as Europe in the strong Gulf currents. It is not known whether these turtles are ever able to make their way back to their home to participate in the reproduction of the species.
Hendrickson, J.R. (1980). The Ecological Strategies of Sea Turtles. American Zoology, 20, 597-608.