Pick from the Past
Natural History, March 1992
Bound for Deep Water
Leatherback turtles can pursue their
prey half a mile straight down.
By Scott A. Eckert
OF THE DEEP-DIVING ANIMALS, the champions have always been assumed to be the marine mammals: the great whales and earless seals. Weddell seals, for instance, have been recorded diving to nearly 2,000 feet, elephant seals to nearly 4,000 feet, and sperm whales to more than 7,000 feet. Our recent investigations, however, suggest that a reptile may also be ranked among the ocean’s greatest air-breathing divers. While measuring the dives of leatherback sea turtles near Saint Croix, in the U. S. Virgin Islands, my wife, Karen, and I recorded a 650-pound female that sounded to more than 3,330 feet and remained submerged for some thirty-seven minutes. Unfortunately, this unexpected behavior continued beyond the range of our recording instruments; we estimate that she reached a maximum depth of about 4,265 feet. While such behavior demonstrates that the leatherback is indeed a magnificent diver, it only hints at this living fossil’s unique adaptations to life in the open ocean.
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The leatherback is the only surviving member of the family Dermochelyidae; all other species of marine turtles belong to another group, the Cheloniidae. Genetics research conducted by Brian Bowen, of the University of Georgia, suggests that leatherbacks may be the most ancient species of living sea turtles. His preliminary mitochondrial DNA analysis, comparing leatherbacks with other sea turtles, indicates that leatherbacks diverged some time between 60 and 100 million years ago. Bowen’s chronology is corroborated by evidence in the fossil record that the leatherback lineage emerged contemporaneously with the dinosaurs.
Not surprisingly, given the large size of many Mesozoic reptiles, the leatherback is, on average, the most massive reptile alive. While mature females typically exceed 660 pounds, the largest-known individual was an adult male that drowned after becoming entangled in a fishing net off Wales. It weighed 2,015 pounds and had a flipper span of eight feet. This unfortunate giant is the centerpiece of an exhibit on leatherback conservation at the National Museum of Wales.
A leatherback has an unusual life style for a turtle, since it lives almost exclusively in pelagic, or open ocean, habitats. While other sea turtle species commonly graze in shallows near the shoreline, leatherbacks seldom approach land for any reason other than to nest. After entering the surf as small hatchlings, leatherbacks are seldom seen again by humans until they reach subadult size. Only a handful of juveniles have ever been encountered, and we know virtually nothing about the adult males. Females in the Atlantic reach sexual maturity when their top shell, or carapace, reaches a length of fifty-four to fifty-seven inches, while Pacific females are a bit smaller. Their age at maturity is unknown, but other, better-known sea turtles require some twenty to fifty years before they breed for the first time. When it is her time to reproduce (the time and place of mating remain a mystery), a female journeys to a nesting beach. Females arrive at these breeding grounds asynchronously over a four-month period.
A gravid female approaches the chosen tropical beach—some believe it is her natal beach—and under the cloak of darkness leaves the shelter of the sea. Using her large hind flippers, she excavates a hole in the sand, then lays from seventy to ninety yolked eggs in the nest. Each cueball-sized egg is moist, soft, and leathery; it will not break when it falls into the deep cavity. Near the end of egg laying, a variable number of small, sometimes misshapen eggs, containing neither embryo nor yolk,
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When the full clutch has been laid, the turtle fills the nest cavity with sand, again using her hind flippers to pack it firmly. Next, she sweeps sand over the area, presumably to camouflage the nest’s precise location. Finally, she returns to the sea. Ten days later, she will come back to this beach, dig another nest, and lay another clutch of eggs—a cycle that will be repeated as many as eleven times over the course of the nesting season. The large number of offspring helps insure that some survive; each one measures only three and a half inches from head to tail and must enter an ocean full of hungry predators. After she has laid her last clutch of the season, the female will not return to her nesting beach again for two or three years.
While leatherbacks must nest on warm tropical beaches, they do not generally live in tropical waters. Adults are usually found in temperate or cold-temperate latitudes, as far north as Newfoundland and the Barents Sea, and also along the cold southern coast of Chile. Indeed, leatherbacks are encountered throughout the world’s oceans, although nowhere in large numbers; their geographic range is the most extensive of any living reptile. Some of the longest migrations recorded for any animal have been made by this marine turtle. In 1970, a female leatherback was tagged while she nested in Suriname on the northeast coast of South America; less than one year later, she was captured off the coast of Ghana, West Africa, having traveled a minimum of 3,700 miles across the Atlantic Ocean. Similar, although shorter, post-nesting migrations have been recorded for females dispersing from eastern Caribbean breeding grounds and traveling north along the Atlantic seaboard of the United States.
One reason the leatherback ranges so far may be its specialized diet. This giant of the sea feeds mainly on jellyfishes. Although a variety of jellyfishes and related prey are abundant, they are eaten by few other vertebrates. As one might suspect, a life spent eating mostly jellyfish requires some special adaptations to make the job easier.
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Once the turtle has snapped up a jellyfish, the slippery prey is kept moving toward the stomach by long, flexible, downward-pointing spines that line the inner surface of the reptile’s mouth and esophagus. How the turtle digests the jellyfish has been little studied, but the process is probably rapid, as a jellyfish consists largely of water. In most animals the stomach is an area for food storage and preliminary absorption, but in leatherbacks it is poorly defined. Food travels through the turtle’s long, muscular esophagus and small stomach and into the small intestine, where most nutrient absorption occurs.
Leatherbacks are particularly well designed for long-distance travel. Broad shoulders and a wide anterior shell taper, teardrop fashion, to a blunt point at the rear, allowing nearly ideal hydrodynamic efficiency. Seven ridges along the length of the carapace improve laminar flow, much as a keel improves the efficiency of a boat moving through the water. The entire body, including the shell, is sheathed in the leathery but very smooth skin from which the animal derives its scientific name, Dermochelys coriacea, “the turtle covered in leathery skin.”
Are there demonstrable advantages to the leatherback’s proportions and streamlined shape? Jeanette Wyneken, of Florida Atlantic University, has shown that swimming leatherback hatchlings have a lower drag coefficient than hatchlings of other species, indicating more efficient propulsion. To swim the same distance as other sea turtles of comparable size and weight, leatherback hatchlings expend, on average, 20 percent less energy. Steering with their rear flippers, leatherbacks propel themselves forward (they seem incapable of going backward) during both the upstroke and the downstroke of the front flippers. The front flippers are more than half the length of the turtle’s body and derive power from huge pectoral muscles, which can account for 30 percent of the animal’s total body weight.
Leatherbacks are such consummate swimmers that they rarely stop moving, a behavior that has made it impossible to keep them in captivity. They adjust poorly to the confines of a tank and usually swim persistently against the walls until the tank breaks or they seriously harm themselves. Since leatherbacks cannot easily be studied in captivity,
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During the 1984 and 1985 nesting seasons, with the assistance of Kooyman and Paul Ponganis, also of the Scripps Institution, we attached recorders to ten female leatherbacks that had just laid eggs. As a result, we were able to monitor their behavior when they returned to the sea. To our astonishment, we found that the turtles were diving almost continuously, day and night, averaging ten minutes per dive and five dives per hour. A typical dive was far from gradual; both descent and ascent were almost vertical. Upon returning to the surface, the turtles gulped a quick breath and immediately headed straight down again. Little or no time was given to sleep or rest. On closer examination, we noticed differences in the dives depending on the time of day. Night dives were shallower, with less variation in depth than day dives, although the overall time spent underwater at night was greater. The turtles spent relatively long periods at the surface during midday, perhaps basking to raise their body temperature.
Why the incessant diving? At first we were baffled, but we soon realized that the turtles were probably following their food source. In tropical waters, jellyfish are most common at great depths, in a biological zone called the deep scattering layer. Discovered shortly after the development of sonar, this zone consists of a horizontal layer (or layers) of zooplankton that hovers below 1,800 feet during the day and migrates to the surface at night. It is so dense with living creatures that early sonar equipment interpreted it as a false ocean bottom. Despite years of research, scientists cannot agree on why organisms aggregate in these deep, dense layers, which represent a large portion of the zooplankton biomass in tropical oceans. However, we do know that they migrate to the surface at night to feed on phytoplankton, then gradually retreat from the light of day, maintaining themselves within a gloomy zone that receives less than one percent of the surface illumination.
 Illustration by Joe LeMonnier |
A leatherback’s dives seem to follow the crepuscular movements of the deep scattering layer. As dusk approaches, the turtle executes shallower and shallower dives at more consistent depths than during the day. As dawn approaches, dives become increasingly deep, probably reflecting the pursuit of jellyfish as the invertebrates retreat from the light of dawn. We believe that the turtles feed in the upper layers of the water at night; during the day they either bask or resort to a more random dive pattern, because the aggregation of prey sinks beyond typical dive depths. Many species of jellyfish are luminescent, which enables the turtles to find them at night or even in deep water during the day.
These are inferences, of course, since we have not been able to directly observe leatherbacks feeding, but we have collected some additional circumstantial evidence. During the 1985 nesting season at Saint Croix, we weighed turtles each time they came ashore to nest. The results tend to corroborate the hypothesis that gravid females feed between bouts of nesting.
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In addition to learning when and why turtles dive we would very much like to know how they dive. Pressure at great depths can be extreme. For every 32.8 feet of descent, a turtle is subjected to an additional pressure of 14.7 pounds per square inch. At 3,280 feet, the turtle is pressed over each inch of its surface by 1,470 pounds of force; the impact is both physical and physiological. Pressure acts on air trapped in the body, the lungs, the trachea, and even within vascular tissue. Tissue around these gas pockets can deform and rupture.
Under great pressure, the nervous system, including the brain, can become “oversensitized”—a phenomenon known as high-pressure nervous syndrome, which can result in convulsions and even death. Also, there is the more familiar decompression sickness, commonly known as the bends, caused by nitrogen bubbles forming in the blood during steep, rapid ascents from deep dives.
How do the deep-diving vertebrates—whales, seals, penguins, and leatherback turtles—avoid decompression sickness and other hazards when they dive to great depths, stay down for long periods of time, and ascend quickly? First, only small amounts of nitrogen are available to the tissues. Many of the deepest-diving marine mammals have small lungs and forcefully exhale prior to diving. In addition to reducing buoyancy so that they can dive more easily, this severely limits the amount of nitrogen (a major component of air) available to the bloodstream.
We don’t know if leatherbacks exhale prior to diving, but they do have relatively small lungs for their body size. Small lungs, however, also mean reduced carrying capacity for oxygen. Some deep-diving mammals have compensated for this lack by evolving an increased blood volume, a greater density of red blood cells to carry the oxygen, and elevated concentrations of an oxygen-storing pigment (myoglobin) in the muscles. The leatherback is the only reptile to employ this same overall strategy. The concentration of myoglobin is almost three times higher in leatherbacks than in any other reptile, and they have very dense concentrations of red blood cells in the bloodstream.
High pressure also poses a challenge to the deep diver in terms of potentially crushing forces that could compress the chest cavity, causing broken bones or tissue damage. Having small lungs reduces the risk of damage by external squeezing. Furthermore, the leatherback’s frame is extremely flexible. The entire skeleton exhibits a high cartilage-to-bone ratio, which allows any internal air stores to be compressed without damaging tissues or breaking bones. Unlike its hard-shelled relatives, with their armorlike shields of fused ribs, the leatherback has a shell made up of widely separated ribs embedded in a thick, oily, cartilaginous tissue. A matrix of thin, bony plates, three-fourths of an inch in diameter, gives the tissue shape, and the whole structure is overlaid with leathery skin. The plastron (belly) has almost no bone, only a flexible cartilaginous ring of tissue for structural support. Even the long bones in the flippers are very pliant.
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The extensive geographic range over which the leatherback roams also poses special physiological challenges for a reptile. The turtle must be able to tolerate enormous fluctuations in environmental temperature. During the four-month nesting season, it lives in waters that may be as warm as 86 degrees F. However, during the intervening twenty months or more, the turtle is likely to reside in waters that may be 41 degrees F or even less. Typically, reptiles and fishes have body temperatures roughly equivalent to that of their surroundings. In a cool environment, metabolic enzyme efficiency is greatly reduced and the animals cannot remain active. Some scientists hypothesize that climatic cooling was partly responsible for the extinction of some former contemporaries of the leatherbacks, such as the dinosaurs. But adult leatherbacks have been observed swimming around icebergs!
Research by zoologists Wayne Friar, Robert Ackerman, and Nicholas Mrosovsky has shown that the leatherback can maintain a body temperature of 64 degrees F in 41-degree F water. Several adaptations conserve heat generated by muscular activity or gained during basking. A heavy layer of fat, oil, and cartilage acts as insulation, much as the blubber in marine mammals does. And like some marine mammals, the leatherback has bundies of vascular tissue in its front flippers. Each outgoing artery is paired with an incoming vein, an arrangement that acts as a countercurrent heat exchanger. This unusual mechanism prewarms the blood flowing in from the extremities, preventing the venous blood from cooling the internal organs and muscles. At the same time, it cools the outgoing blood, precluding the loss of core heat to the extremities.
Shape and color may also be advantageous for keeping warm. The leatherback’s nearly cylindrical body minimizes the surface area exposed to cooling water. If the turtles do indeed bask at midday, then the overall black color is likely to be advantageous for heat absorption. Finally, preliminary evidence indicates that leatherbacks may have a layer of a special thermogenetic tissue called brown fat, a substance known to generate heat metabolically in some other species.
All these recent discoveries about the leatherback’s physiology and behavior show it to be an oceanic specialist whose adaptations are finely tuned to a life of deep diving in the open ocean. Studying them, I have become much less "mammalocentric"; many features that I had previously assumed to be the exclusive invention of marine mammals may have actually evolved long before, in grandfather turtle.
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