Introduction to Sea Turtles: Families and Species
A sea turtle is one of seven species of marine turtles that belong to the families Dermochelyidae (leatherbacks) and Cheloniidae (green turtles, flatbacks, loggerheads, hawksbills, and ridleys). Both families are primarily aquatic, with most species only appearing on coastal beaches to lay eggs; however, the green turtle (Chelonia mydas) occasionally sunbathes in terrestrial habitats. Adult sea turtles live primarily in tropical and subtropical oceans, but juveniles from both families can be found in more temperate regions.
Divergence and Evolution of Sea Turtles
Dermochelyids and cheloniids are distantly related, diverging between 100 and 150 million years ago. Yet, both species have streamlined shells, forelimbs designed as flippers to drive their bodies through the water, figure-eight swimming strokes, and big, completely webbed rear feet that serve as rudders. Cheloniids are hard-shelled sea turtles with a bony carapace (top shell) and plastron (bottom shell) covered in scutes (scales). Dermochelyids’ leatherback shell, on the other hand, has much reduced bony architecture and bones that are less solidly articulated; scutes emerge in hatchlings but are quickly shed, so a thick, leathery skin covers the bony shell.
Size, Diet, and Habitat of Sea Turtles
The size of the seven species varies widely; yet, diet and habitat have common characteristics. With few exceptions, most sea turtles are carnivores who favor warm, coastal marine settings. The leatherback sea turtle (Dermochelys coriacea) lives in pelagic (open ocean) settings. It appears to roam widely over the oceans, following the blooms of its jellyfish prey. Few individuals have shell lengths that exceed 1.6 meters (5 feet), while some have been observed to reach 2.4 meters (8 feet).
Adult and juvenile olive ridleys (Lepidochelys olivacea) are primarily aquatic, but they are known to visit coastal areas such as bays and estuaries. The olive ridley and its relative, the Kemp’s ridley sea turtle (L. kempii), have small, wide-rounded shells. As adults, both species have shells that measure 58-78 cm (23-31 inches) long. Leatherbacks and ridleys are primarily carnivores, eating a varied range of crustaceans and mollusks.
Adult shell lengths for loggerhead (Caretta caretta) and green sea turtles (Chelonia mydas) range from 0.9 to 1.2 meters (3 to 4 feet). The loggerhead is a carnivore that favors coastal maritime habitats. It has the largest head of any sea turtle; this could be an adaptation that boosts jaw strength to crush the shells of huge mollusks like whelks. The green turtle lives in warm coastal waters all over the world; but, unlike other sea turtles, it is largely herbivorous, feeding on algae or marine grasses.
The hawksbill sea turtle (Eretmochelys imbricata) is mainly tropical and lives in coral reef settings, where it feeds on sponges and other invertebrates. The flatback sea turtle (Natator depressa) lives in the oceans between Australia and New Guinea and feeds on a variety of crustaceans. Adult shells of both species measure 90 to 100 cm (35 to 39 inches).
Reproductive Habits and Nesting Cycles of Sea Turtles
While reproductive habits and timing differ between populations and species, all sea turtles follow a common pattern. All are egg layers, and the females must come ashore to bury their eggs in sandy areas. Apart from Lepidochelys, which has a nesting cycle of 1-3 years, females only nest once every third or fourth year; however, they sometimes nest many times throughout a nesting season. Although most species lay two to four eggs every nesting season, the loggerhead has up to seven.
The female emerges from the surf at night, crawling slowly to sandy regions above the high tide line, and digging a nest. The nest or egg chamber is dug entirely with the hind limbs, the tail is placed over the center of the nest entrance, and eggs are laid. The nest is subsequently filled with sand before the female returns to the sea. Each nest is built 12 to 14 days apart, and clutch size varies by species and population; approximately 100 eggs in each nesting session are typical.
Eggs incubate for around 50 to 60 days. Temperature affects development, therefore a warmer nest results in an earlier hatching. Within a given nest, hatching is almost simultaneous. Because of the nest’s depth, numerous hatchlings must dig upward simultaneously to emerge. Hatchlings normally emerge from the ground at night and can recognize the horizon over the open ocean, but they can be fooled by artificial illumination behind or along the shore.
Hatchling Journey and Magnetic Field Imprinting
There is evidence that female loggerhead turtles imprint on the magnetic field signature of the beach as hatchlings and use Earth’s magnetic field to return to the same beach as adults to deposit their eggs. Once in the surf, hatchlings swim out into the open ocean, while most species enter the massive marine gyres. Young sea turtles are typically pelagic for 5 to 10 years before returning to warm nearshore waters to continue their development.
Conservation Challenges and the Path to Maturity
A wide number of these species are unprotected or endangered. They are often slow to mature, long-lived, and migratory; ahead of attaining sexual maturity, many are caught and killed in coastal fisheries, either purposefully or by accident. The minimum time from hatching to first reproduction appears to be 10 to 15 years, which is shared by both the largest species, D. coriacea, and the smallest species, L. kempii and L. olivacea.