The Lost Years in the Life of Sea Turtles
When they first emerge from the nest, the sea turtle hatchlings may not see the water. Yet, unless confused by artificial stimuli, they usually move directly toward the ocean.
The tiny turtles figure out the direction of the ocean in one of three ways. On a natural beach, vegetation and dunes create black silhouettes on the landward horizon, while the reflection of starlight and moonlight creates a brighter horizon on the seaward side. Hatchling turtles ignore the sky overhead and concentrate on the horizon, moving away from any black silhouettes and toward the brightest portion of the horizon. When artificial lighting and/or the clearing of dunes and vegetation reverse the natural lighting ratio on the beach, the results can be disastrous. In the absence of visual cues, hatchlings will move down a slope. If there is neither a slope nor visual cues, hatchlings will move toward the sound of the surf.
When the little turtles reach the ocean, their first contact is usually with a breaking wave that pushes them back up the beach. Initially they must swim directly against this powerful surge as the backwash pulls them out, they must swim with it before diving beneath the next approaching wave. Buffeted by violent surge forces in the dark of night, the hatchlings must quickly escape the roiling surf, or risk being thrown back onto the beach. No longer able to see the horizon, they orient themselves using built-in biological accelerometers. As a wave approaches the beach, it moves objects in the water in a circular path. An object is lifted upward, pushed toward the beach, carried downward, and then pulled away from the beach by the undertow. By monitoring the acceleration of its body in different directions, a hatchling can orient itself to face the waves and swim away from the beach.
This strategy works in very shallow water because waves are refracted by the sea bottom as they approach the shore, arriving at the shoreline head on. Only a few yards out to sea, however, the waves may move in almost any direction. Yet the hatchling must continue to swim away from the shoreline as fast as possible if it is to survive. Many fish, birds, and other animals close to shore will devour baby sea turtles.
A turtle hatchling is about the size of a small mouse. It can only lift its head a few centimeters (an inch) above the surface of the water and waves block its view. Its own buoyancy prevents a hatchling from diving more than a few yards; therefore, it cannot see the seafloor. So how do the hatchlings find their way? Laboratory experiments have shown that sea turtles can detect the magnetic field of the earth. After establishing a course of travel from cues such as brightness of the horizon or wave action at the shoreline, the hatchlings switch to magnetic guidance to maintain the same course as they continue out to sea.
In some parts of the world, hatchlings enter the sea from beaches where there are no waves, and from beaches that do not face out toward open sea. Yet somehow when they reach the water they can swim away from the beach and then establish a course that will carry them out to sea. They can even navigate around reefs and islands blocking their path. How sea turtle hatchlings accomplish this remains unknown.
The “frenzy” (the frantic nonstop crawling/swimming action that characterizes newly hatched sea turtles) lasts about twenty-four hours in green, loggerhead, and leatherback turtles. The duration of the frenzy in other species is not yet known, but there are indications that it may be much shorter in Flatback, Kemp’s Ridley, and hawksbill turtles - perhaps as short as two hours in the Hawksbill. During its first couple of days in the ocean, the hatchling does not feed. The rest of the egg yolk, which is carried in the turtle’s abdominal cavity, fuels the near-constant activity. This rapid migration takes the hatchling offshore to the pelagic environment, where it begins the period of its life known as the pelagic phase or developmental migration.
The Flatback turtle and perhaps some Hawksbill turtles appear not to have a pelagic phase. Evidence shows that Flatback turtles stay in near-shore waters throughout their lives, rarely entering waters deeper than about 45m (150 feet) as juveniles or adults. Leatherback turtles spend their entire lives (apart from the reproductive phase) in pelagic habitat. After entering the ocean, the hatchlings swim offshore for at least six days and are not seen again for four years or longer. We know little about where they go. Some (but not all) Olive Ridleys, may also spend their entire lives in open ocean, while some (but not most) Hawksbills may never leave coastal waters.
For other sea turtles, the pelagic phase encompasses a distinct period of their lives - a time when feeding, sleeping, and migratory habits are markedly different from those they will adopt once they return to coastal waters. During this period, the young turtles are rarely seen by humans. Until recently, where baby turtles go between the time they leave the beach with shells roughly the size of the bowl of a spoon, and when they are again seen close to shore with shells that are dinner plate size or larger, was a significant mystery. Biologists called this period the “lost years”. The duration of the “lost years” varies dramatically among species of sea turtles, different populations of a species, and unique individuals within a population. We estimate the pelagic phase to last between one and ten years for most sea turtles. Sea turtles that live in the Pacific Ocean have a longer pelagic phase than sea turtles that live in the Atlantic.
During the first part of the pelagic phase, Loggerhead and Hawksbill hatchlings often seek floating patches of sargassum and other types of seaweed, where they can hide from predators. They feed on small shrimps, crabs, snails, worms, insects, fish, and other organisms that live in the weed, and on sea jellies and other creatures that float by. Kemp’s Ridley and Olive Ridley hatchlings probably do the same. Green and Leatherback turtles, however, are believed to prefer open water, where they likely consume larger amounts of small sea jellies and other organisms that float in open surface waters. Green Sea Turtle hatchlings have been found in sargassum at least twice but may have been pushed into it by converging currents.
In laboratory experiments, Loggerhead and Hawksbill hatchlings were attracted to floating artificial weed beds, while green turtle hatchlings avoided them. When threatened, the baby Green Sea Turtles dove and swam away, while the Loggerheads and Hawksbills remained motionless - an excellent strategy if hidden in seaweed. These ecological differences are reflected in the coloring of the baby turtles. Green Sea Turtles, Leatherback, and Flatback hatchlings sport a two-tone pattern that biologists call countershading. A dark carapace helps the turtle blend into the dark waters of the open ocean when viewed from above, while a white plastron renders the hatchling less visible when seen from below against a background of sky and clouds. Ridley, Loggerhead, and Hawksbill hatchlings are a uniform brown to gray color, which may help them blend into a patch of floating weed. Predation levels are extremely high on these small turtles, so a camouflage pattern that matches their habitat is crucial to their survival. Floating seaweed, however, is common in the Atlantic, but too rare in the Pacific to provide dependable habitat.
From captures by sea eagles and shrimp trawls, we know that juvenile Flatbacks are usually found within 150km (100 miles) of a nesting beach. In other species of sea turtles, the juveniles are likely to be found hundreds to thousands of kilometers (miles) from the beach where they hatched.
Although the pelagic periods of sea turtles are not well known, scientists have been able to work out the basics of the developmental migrations of Loggerheads. From these findings, they have been able to generalize what the pelagic stage might be like for other sea turtles. Loggerheads make one of the most phenomenal migrations in the animal kingdom. Loggerheads that hatch in Japan traverse the Pacific, spending time off the coast of Mexico, where they feed on aggregations of pelagic red crabs, before they return to Japan. One turtle that was tagged and released off the coast of Mexico was caught a little more than a year later in a net in Japan, 10,600km (6,500 miles) away. During the developmental migration, these turtles travel a minimum of 20,000km (12,400 miles) and they probably often travel much farther!
Loggerhead turtles hatched on the southeast coast of North America enter the North Atlantic Gyre, a huge current system that spins clockwise around the entire North Atlantic. The great Gulf Stream current sweeps north along the east coast of North America to Newfoundland, where it heads east and crosses the Atlantic as the North Atlantic Current. About two-thirds of the way across the Atlantic, near the Azores Islands, the current splits into the North Atlantic Drift, which flows northeast toward the British Isles, and the Canary Current, which flows south, past the Madeira Islands. Somewhere near the Canary Islands, which are located off the coast of North Africa, the current turns westward to cross the Atlantic again as the North Equatorial Current.
Turtles that stay within this gyre can successfully complete their developmental migration and return to North America to begin the bottom-dwelling phase of their lives. Those that, for example, are caught in the North Atlantic Drift and swept up to Britain usually die from the cold. The turtles can avoid being transported out of the gyre by swimming, but to choose the right direction, they must know where they are.
Scientists have determined that baby Loggerheads can detect both the strength and angle of magnetic force-field lines. Since the combination of these characteristics varies throughout the earth’s magnetic field, it is theoretically possible for turtles to carry an internal map and compass that could guide them to any place on the earth. A map sense has not been proven, but it has been shown that baby Florida Loggerheads, when exposed in the lab to magnetic fields replicating the fields at specific locations in the North Atlantic, swim in directions that would keep them within the gyre. The ability to sense magnetic fields, however, does not by itself explain the amazing navigational abilities of sea turtles.
We believe the pelagic phase of North Atlantic Loggerheads to last three to fifteen years. Some turtles wander into the Mediterranean for a while. Many of the turtles spend several of these years floating around the Azores and Madeira Islands. Although there appears to be extraordinarily little life in these clear, blue oceanic surface waters, scientists believe that the small turtles can locate areas where currents converge, and sea jellies and other animals accumulate. As the turtles grow, they are better able to dive for food below the surface.
Almost nothing is known about the return migration to the western Atlantic, but when they reach a carapace length of 25-84cm (10 to 33 inches), Loggerheads show up as bottom-dwellers along the coast of North America from Cape Cod, Massachusetts, to southern Texas. Some of them may spin around the North Atlantic more than once before settling down. Some apparently move back and forth between the coastal and pelagic habitats for a while, and even mature loggerheads may make trips offshore to feed at oceanic “fronts” where water masses converge. Most Loggerheads, however, stick mostly to bottom feeding from the time they reach a shell length of about half a yard in the Atlantic, and a little less than a yard in the Pacific.