At the mention of the leatherback turtle[1], most people generally knowledgeable about animals may visualise merely one of the half dozen-odd types of marine turtles collectively associated with tropical seas. However, this particular species deserves a special shelf in the mind of the nature-lover because it differs from all other turtles. In a sense it is categorically more successful than any other living reptile.Here we offer some original interpretations. We also point out an intriguing convergence with the widely coexisting ocean sunfish[2], which shares with the leatherback turtle a wobbly and watery diet shunned by other reptiles and fishes.
In a class of its own?
The leatherback turtle is distinct in four related ways besides the reduction of the shell found in other marine turtles. Firstly, it’s the largest of all living reptiles by a certain yardstick. Secondly, it’s the fastest-travelling and deepest-diving of all living reptiles. Thirdly, in a surprisingly large portion of the world, it’s the only reptile that occurs at all. And fourthly, it is – like the world’s largest mammals and fishes – a consumer of prey so insubstantial that most people don’t think of them as prey in the first place.
Both females and males of the leatherback turtle can weigh over 800 kilograms. While the males of several species of crocodiles are known to exceed 1000 kilograms, the females of all crocodiles are relatively small: less than 800 kilograms. In organisms in general, it’s the female that is energetically the more important sex, because she converts food energy into offspring. So if we take the mature females of turtles and crocodiles as a standard – putting the fully-grown males of crocodiles in their place as sexually dimorphic show-offs – the leatherback turtle emerges as arguably the largest living species of reptile on Earth. It grows remarkably rapidly as a hatchling, reaches sexual maturity at a size approaching the maximum size for other species of marine turtles, and continues to grow after this.
It may surprise some readers that the speediest[3] of living reptiles is a turtle. After all, the related tortoises symbolise slowness. And the leatherback turtle is a ‘living fossil’ originating at the time of the dinosaurs, 100 million years ago. However, the maximum sustained speed at which the leatherback turtle can swim exceeds that of any other reptile, including those that run or slither. This is partly because of exceptional size and an internal temperature which can be up to 18o Celsius higher than that of the sea water. But it’s also the result of an extreme development of the forelimbs: the leatherback turtle has proportionately larger flippers than any other turtle, and a unique form of streamlining owing to the reduction of the massive shell, typical of turtles, to several low ridges that perhaps function like the fins on a torpedo.
In a world of its own?
It’s partly because of its speed and its ability to remain submerged for more than 40 minutes that the leatherback turtle is the deepest-diving of all reptiles. It can dive down to 1.2 kilometres below the sea surface. This capacity is also related to the exceptional body size of the leatherback turtle; deep waters are far too cold for most reptiles but a large body can resist chilling for a relatively long time.
Much of the sea, in both area and depth, contains no reptile other than the leatherback turtle. The distributions of all other marine reptiles – turtles, sea snakes, and one species of lizard[4] –collectively cover most of the tropical and subtropical seas. However, other marine turtles need water of at least 20o Celsius, and there are no sea snakes whatsoever in the Atlantic Ocean. By contrast, the leatherback turtle fares better in the Atlantic than in the Indian or Pacific oceans, and forages (at least as adults) only in relatively cold water. It extends thousands of kilometres farther north and south than any other marine turtle and hundreds of kilometres beyond even the most latitudinally widespread of sea snakes[5]. The result is that the leatherback turtle has exclusive access, among living reptiles, to a vast, irregularly shaped portion of its habitat.
Males of the leatherback turtle may never need to leave relatively cold seas, with only the females making the long journeys[6] to beaches warm enough for egg-laying. The tropical and subtropical climes of the beaches chosen for incubation are not representative of the cold waters that are the true home of the leatherback turtle. This species may actually avoid the tropics for foraging, because warm waters tend to be too nutrient-poor for its prey of jellyfishes. We suspect that the water in which the adult leatherback turtle prefers to forage is so cold as to be avoided by all other species of marine turtles. And if it does forage in subtropical parts of its range which are shared with one or more species of turtle or sea snake, the leatherback turtle may monopolise considerable volumes of water too deep to be penetrated by these other reptiles.
In its diet of pelagic animals, the leatherback turtle resembles the largest of mammals and fishes. Baleen whales and the whale shark, basking shark, and megamouth shark all depend on mere invertebrates for food. The greatest body sizes among marine animals are sustained by the marine equivalent of insects, which have rapid turnover and can continually be strained out of the water in large numbers. What may be overlooked in a general knowledge of the leatherback turtle is that it is, likewise, a ‘filter-forager’ of sorts. While the jellyfishes and salps of its staple diet would not be called ‘plankton’, and are not strained out by baleen plates, as in whales, or gill rakers, as in plankton-eating sharks, nevertheless the prey items of this reptile are insubstantial. Jellyfishes contain more than 95 percent water, which means that a mouthful may yield only a few grams of real food. In the case of bluebottles[7], most of the body is air.
Furthermore, the leatherback turtle has a broad throat containing ‘horny spines’ 5-7.5 centimetres long, the function of which is – at least partly – the gripping of slippery prey during swallowing. As in baleen whales and large filter-foraging sharks, there’s an advantage in eating pelagic invertebrates because these animals reproduce more rapidly than larger prey animals would; but there’s also a disadvantage in that much water must be discarded in order to concentrate the flesh for digestion. We suspect that an additional function of the throat spines and buccal papillae of the leatherback turtle is to strain stomach contents by regurgitation, i.e. to recycle water and salts back into the sea from the capacious stomach while retaining the fleshy fragments of the disintegrating bodies of jellyfishes. It’s unlikely that most of the waste fluid released during the digestion of prey is absorbed into the blood and then excreted in urine. This is because the reptilian kidneys cannot excrete salt rapidly enough.
One possible reason why a consumer of jelly-like animals might process its prey differently from the familiar plankton-eaters is that the stinging tentacles would tend to adhere to any straining device in the mouth. The nematocysts can fire even when the jellyfish is dead, and are neutralised only once broken down by enzymes. Therefore, it might pay the leatherback turtle to bite large jellyfishes to pieces and to swallow these as rapidly as possible with minimum use of the spines in the throat. These spines could engage afterwards, retaining material not so much on the way in but more as the water and salt released from the dead prey come back out during digestion.
A fish emulating a turtle?
Although the largest of all fishes are sharks, various lineages of fishes have achieved large body sizes. Bony fishes[8] are quite different from cartilaginous fishes[9] such as sharks and manta rays[10]. But what’s not generally realised is that even the largest bony fish – the ocean sunfish – is somewhat analogous to the leatherback turtle.
Firstly, the ocean sunfish, like the leatherback turtle, has a staple diet of jellyfishes, and a mouth with stubby, beak-like jaws able to bite chunks – quite different from the gaping mouths of baleen whales, the largest sharks, and manta rays. Secondly, the body form of the ocean sunfish is odd for a fish but, in certain ways, convergent with the leatherback turtle. The ocean sunfish has converted the tail normally found in fishes to what is effectively a pair of flippers – located at the rear of the body instead of the fore position seen in turtles, and used vertically instead of horizontally as in turtles. Like the leatherback turtle, the ocean sunfish can dive deeply into dark, cold water, but also spends time basking at the surface, emulating the body configuration of the turtle as it lolls on its side to rest.
Both the ocean sunfish and the leatherback turtle have lost the scales typical of fishes and reptiles, instead adopting a thick skin with a leathery or rubbery texture. In both cases, this skin is embedded with fragments of bone of various sizes, either small dermal bones in the turtle or denticles[11] in the ocean sunfish. Another convergence can be found in the fact that the ocean sunfish, although evolved from bony ancestors, has lost its internal bones, and has a skeleton as cartilaginous as that of sharks. Although the leatherback turtle retains bones, it’s remarkable that this extremely large species is less bony than other reptiles. Many of its skeletal components have reverted to cartilage. Even the skull of the leatherback turtle, which is bonier than that of the ocean sunfish, retains unfused sutures throughout life, so that the mature skull falls apart when the corpse decomposes. We wonder whether this loss of bone in both species reflects the chemical composition of boneless prey such as jellyfishes.
A final possible similarity, unexplored by biologists, is in fecundity. The ocean sunfish is exceptional among fishes in the number of eggs it lays. And because the leatherback turtle can lay up to 12 clutches per nesting season, with 65 to 85 eggs per clutch, it may perhaps emerge as the most fecund reptile of all in the sense of number of eggs laid per lifetime. If so, it would be almost as if both of these plankton-eaters aspire to planktonic proliferation in their own juvenile stages.
What we’re left with is a reptile exceptional in size and cold-tolerance, at the same time converging with the largest marine non-reptilian vertebrates in ways related to diet. The fact that a reptile and a bony fish, rather than a marine mammal or marine bird, have specialised on jellyfishes is perhaps partly explained by the peculiar adversity of this food. We refer to defence by adhesive tentacles and extreme dilution. Jellyfishes, although abundant in places, can seem like little more than mouthfuls of sting. The rapid metabolism of mammals and birds requires richer fare than this, perhaps explaining why no whale, seal or penguin specialises on a diet of jellyfishes and salps. Leatherback turtle and ocean sunfish share a skeleton rubbery enough and a metabolism slow enough to make ends meet on a dilute diet – even if these ends meet at odds because of the back-to-front and topsy-turvy relationship of their fin-like flippers versus flipper-like fins.
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1 Order Testudines: Family Dermochelyidae: Dermochelys coriacea
2 Tetraodontiformes: Molidae: Mola mola
3 Capable of commuting right across the Pacific Ocean, from its foraging habitat in the cold seas of Oregon to its nesting beaches in New Guinea. Tagged individuals have been tracked from Nova Scotia to the Caribbean in the Great Turtle Race and at speeds of over 35 km/h.
4 marine iguana (Iguanidae: Amblyrhynchus cristatus)
5 The most widespread of which is the yellow-bellied sea snake (Pelamis platurus).
6 It is noteworthy that females foraging off California travel not to the nesting beaches in nearby western Mexico, but westwards right across the Pacific, to lay their eggs.
7 Siphonophora: Physaliidae: Physalia
8 Osteichthyes
9 Chondrichthyes
10 Manta
11 similar to those of sharks