Adaptations for aquatic life

Life in water

In the beginning, all life on Earth was aquatic. Although water covers over two-thirds of our planet, precisely how life in the oceans came to be is one of our unanswered questions. Many of these animals have been around for millions of years. Over time, they have adapted in such a way that allows them to live and reproduce in water. One unusual example of long-term ocean survival is that of the coelacanth. Fossils of this armored fish dating back more than 75 million years have been discovered, and it was thought to have been extinct. In 1938, however, one was caught off the coast of South Africa. Since then, more than 100 of these prehistoric, deep-dwelling fish have been examined. They have no scales or eyelids, as do "modern" fish, and have quietly kept to themselves in the deepest areas of the ocean.

For the most part, aquatic creatures spend their entire lives submerged. However, a few aquatic animals—those that are descended from land animals—come all or part of the way out of the water for one reason or another: sea turtles, pinnipeds, and penguins come ashore to breed, for example. Mammals, such as whales and dolphins, have also acquired some handy adaptive techniques for life in the water, coming to the surface only to breathe.

The smallest of the marine mammals is the sea otter (Enhydra lutris), at 5 ft (1.5 m) long, including the tail, and up to 70 lb (32 kg). The largest is the blue whale (Balaenoptera musculus)—the largest animal alive—which can be 110 ft (33.5 m) long and weigh 300,000 lb (136,000 kg). To varying degrees, these mammals that have returned to the water have retained vestiges of their terrestrial forms, including hair, which only mammals have. Sea otters, seals, and sea lions are thickly furred; manatees and dugongs have a sparse pelage, but they have many whiskers around their mouths. Dolphins and whales are hairless, but in some species hairs are present at birth (they are soon lost). Sea otters have hand-like paws on their front legs, but their hind feet have become webbed, so that they're almost flippers. The four legs of pinnipeds have become flippers, and the sirenians have front flippers (some of them have fingernails), but no hind legs, and a flattened tail for propulsion. Whales and dolphins have no hind legs, flippers instead of forelegs, and a horizontal tail (fluke) for propulsion.

Evolution of aquatic animals

Marine fossils paint an idyllic scene of aquatic animal life in its infancy some 670 million years ago (mya): soft coral fronds arch from the ocean floor, jellyfishes undulate in the currents, and marine worms plow through the ooze. But a geologically brief 100 million years later, at the dawn of the Cambrian period, the picture suddenly changes. Animals abruptly appear cloaked in scales and spines, tubes and shells. Seemingly out of nowhere, and in bewildering abundance and variety, the animal skeleton emerges.

For more than a century, paleontologists have tried to explain why life turned hard. Hypotheses abound, some linking the skeletal genesis to changing chemistries of the seas and skies. Yet a recent analysis of old fossil quarries in Canada and new ones in Greenland is providing evidence supporting the notion that the skeletal revolution was more than a chemical reaction—it was an arms race.

High in Canadian Rockies of British Columbia, in an extraordinary 540-million-year-old fossil deposit called the

Burgess shale, a mid-Cambrian marine community comes to life. Like many less exceptional deposits, the Burgess harbors mollusks, trilobites (the ubiquitous, armored "cockroaches" of the Cambrian seas), and clam-like brachiopods. But other imprints in the smooth black shale dispel any image of a peaceful prehistoric aquarium. In these waters lurked a lethal cast of predators, eyeing little shells with bad intent: Sidneyia, a flattened, ram-headed arthropod with a penchant for munching on trilobites, brachiopods, and cone-shelled hyolithids; Ottoia, a chunky burrowing worm that preferred its hyolithids whole, reaching out and swallowing them with a muscular, toothed proboscis; and even some trilobites with predatory tastes. These findings have helped resurrect the arms race hypothesis: the 80-year-old idea that skeletons evolved primarily as fortresses against an incoming wave of predators.

Take Wiwaxia, a small, slug-like beast sheathed in a chain-mail-like armor. With two rows of spikes running along its back, Wiwaxia was the mid-Cambrian analogue to a marine porcupine. The chinks in its armor are telling. Some of Wiwaxia's spines appear to have broken and healed. The healed wounds of trilobite and Wiwaxia specimens suggest that predators strongly influenced the elaborate new skeletal designs of the mid-Cambrian.

What sort of creature could gouge such wounds in a tough trilobite? One likely culprit is Anomalocaris, the largest of Cambrian predators. This half-meter-long creature glided through the seas with ray-like fins and chomped with a ring of spiked plates that dispatched trilobite shells like a nutcracker.

From the treacherous maw of Anomalocaris to the healed wounds of Wiwaxia, much of the support for the arms race argument hinges on the Burgess shale collection. But what about the small shelly fauna that emerged 30 million years earlier? For an arms race hypothesis to be complete, predators must have roamed then, too.

New finds strengthen the case for an early Cambrian arms race. From an extraordinary fossil bed discovered in 1984 in north Greenland, predating the Burgess shale by perhaps as much as 15 million years, comes a jigsaw puzzle already assembled: a suspiciously familiar, slug-like beast sheathed in chain-mail armor, proposed to be the long-sought ancestor of the armored slug Wiwaxia.

The creature sports a disproportionately large, saucer-like shell at each end of its elongated body. From another fossil discovery at a quarry in south China, which appears even older than the Greenland site, emerges the bizarre Microdictyon. Unveiled in 1989 by Chinese paleontologists, Microdictyon is a wormish creature with a row of pointed appendages and a body studded with oval phosphate plates. About 30 quarries

worldwide are beginning to yield Burgess-quality fossils, with perhaps many more waiting to be discovered.

Since that explosion of new forms some 530 mya, however, few new marine animals have evolved. Analysis of the evolution of marine animals suggests that a sufficient variety of life forms in an environment suppresses further innovation. About 530 mya, during the Cambrian period, after a long period in which animals were essentially jellyfishes or worms, marine animal life exploded into a variety of fundamentally new body types. Arthropods turned up inside external skeletons, mollusks put on their calcareous shells, and seven other new and different body plans appeared; an additional one showed up shortly thereafter. But since then, there's been nothing new in terms of basic body types, which form the basis of the top-level classification of the animal kingdom called phyla.

Research presented at a 1994 meeting of the Geological Society of America lends support to the idea that once evolution fills the world with sufficient variety, further innovation may be for naught. There are only so many ways marine animals can feed themselves—preying on others or scavenging debris, for example. And there are only so many places to

do it: on the sea floor, beneath it, or some distance above it. When all the nooks and crannies of this "ecospace" are filled, latecomers never get a foot in the door.

Challenges

Because water is so dense (up to 800 times denser than air), it can easily support an animal's body, eliminating the need for weight-bearing skeletons like terrestrial animals. Water is also more viscous than air, and this coupled with the high density has resulted in aquatic animals adapting a very streamlined shape, particularly the carnivores. This makes them very fast and powerful swimmers, enabling them to catch their prey.

Many of the adaptations of aquatic organisms have to do with maintaining suitable conditions inside their bodies. The living "machinery" inside most organisms is rather sensitive and can only operate within a narrow range of conditions. Therefore, aquatic organisms have devised ways to keep their internal environments within this range no matter what external conditions are like.

Thermoregulation

Most aquatic animals are ectotherms, or poikilotherms, or what is often referred to as "cold-blooded." As the temperature of the surrounding water rises and falls, so does their body temperature and, consequently, their metabolic rate. Many become quite sluggish in unusually cold water. This "slowing down" caused by cold water is a disadvantage for active swimmers. Some large fish, such as certain tunas and sharks, can maintain body temperatures that are considerably

warmer than the surrounding water. They do this by retaining the heat produced in their large and active muscles. This allows them to remain active even in cold water.

Aquatic mammals are able to keep their body temperatures more or less constant regardless of water temperature. Marine mammals deposit most of their body fat into a thick layer of blubber that lies just underneath the skin. This blubber layer not only insulates them but also streamlines the body and functions as an energy reserve. The fusiform body shape and reduced limb size of many marine mammals and organisms decreases the amount of surface area exposed to the external environment. This helps conserve body heat. An interesting example of this body form adaptation can be seen in dolphins: those adapted to cooler, deeper water generally have larger bodies and smaller flippers than coastal dolphins, further reducing the surface area of their skin.

Arteries in the flippers, flukes, and dorsal fins of marine mammals are surrounded by veins. Thus, some heat from the blood traveling through the arteries is transferred to the venous blood rather than the outside environment. This countercurrent heat exchange also helps to conserve body heat.

The air-breathers

Some water birds, such as cormorants and pelicans, simply hold their breath until completely out of the water. However, it is not appropriate for all air breathers to leave the water to breathe, especially if only a small portion of them can do it. This also has two evolutionary advantages: it reduces the amount of time at the surface of the water so they can spend more time feeding, and it reduces the amount of wave drag they encounter. The external nares of aquatic mammals, such as beavers, hippopotamuses, and dolphins, are always dorsal in position, and the owner seems always to know when they are barely out of water. A ridge deflects water from the blowhole of many whales. When underwater, the nares are automatically tightly closed. Sphincter muscles usually accomplish this, but baleen whales use a large valve-like plug, and toothed whales add an intricate system of pneumatic sacs so that great pressure can be resisted in each direction.

To avoid inhaling water, aquatic mammals take very quick breaths. Fin whales can empty and refill their lungs in less than two seconds, half the time humans take, even though the whale breathes in 3,000 times more air. Exhaling and inhaling takes about 0.3 seconds in bottlenosed dolphins (Tursiops truncatus). When swimming quickly, many pinnipeds and dolphins jump clear out of the water to take a breath. Cetaceans have the advantage of having a blowhole on top of the head. This allows them to breathe even though most of the body is underwater. It also means that cetaceans can eat and swallow without drowning.

The long, deep dives of aquatic mammals require several crucial adaptations. For one thing, they must be able to go a long time without breathing. This involves more than just holding their breath, for they must keep their vital organs supplied with oxygen. To get as much oxygen as possible before dives, pinnipeds and cetaceans hold their breath for 15 to 30 seconds, then rapidly exhale and take a new breath. As much as 90% of the oxygen contained in the lungs is exchanged during each breath, in contrast to 20% in humans. Not only do diving mammals breathe more air faster than other mammals, they are also better at absorbing and storing the oxygen in the air. They have relatively more blood than nondiving mammals. Their blood also contains a higher concentration of red blood cells, and these cells carry more hemoglobin. Furthermore, their muscles are extra rich in myoglobin, which means the muscles themselves can store a lot of oxygen. To aid in diving, marine mammals also increase buoyancy through bone reduction and the presence of a layer of lipids (fats or oils).

Aquatic mammals have adaptations that reduce oxygen consumption in addition to increasing supply. When they dive, their heart rate slows dramatically. In the northern elephant seal, for example, the heart rate decreases from about 85 beats per minute to about 12. A bottlenose dolphin's average respiratory rate is about two to three breaths per minute.

Blood flow to nonessential parts of the body, like the extremities and the gut, is reduced, but it is maintained to vital organs like the brain and heart. In other words, oxygen is made available where it is needed most.

Another potential problem faced by divers results from the presence of large amounts of nitrogen in the air. Nitrogen dissolves much better at high pressures, such as those experienced at great depths. When nitrogen bubbles form in the blood after diving, they can lodge in the joints or block the flow of blood to the brain and other organs. Aquatic mammals have adaptations that prevent nitrogen from dissolving in the blood, whereas human lungs basically work the same underwater as on land. When aquatic mammals dive, their lungs actually collapse. They have a flexible rib cage that is pushed in by the pressure of the water. This squeezes the air in the lungs out of the places where it can dissolve into the blood. Air is moved instead into central places, where little nitrogen is absorbed. Some pinnipeds actually exhale before they dive, further reducing the amount of air, and therefore nitrogen, in the lungs.

Buoyancy

Unlike fishes, secondary swimmers (terrestrial animals that returned to an aquatic environment) have no such specific adaptations to the buoyancy problem. They all rely on simple density adaptations to help them. For example, the bones of diving birds are less pneumatic, and their air sacs are reduced (loons, penguins). Mammals that dive deep may hyperventilate before submerging, but they do not fill their lungs. Indeed, they may exhale before diving. Deep-diving whales have relatively small lungs. Sirenians, which may feed while resting on the bottom or standing on their tails, have unusually heavy skeletons; their ribs are swollen and solid. Likewise, the skeleton of the hippopotamus is also unusually heavy. The presence of blubber in marine mammals also contributes to their overall density, and walruses (Odobenidae) have two large air pouches extending from the pharynx, which can be inflated to act like a life preserver to keep the animals' head above water while sleeping.

Convergence

The largest group of marine mammals, the cetaceans, is also the group that has made the most complete transition to aquatic life. While most other marine mammals return to land at least part of the time, cetaceans spend their entire lives in the water. Their bodies are streamlined and look remarkably fish-like. Interestingly, even though all marine mammals have evolved from very different evolutionary groups, there are certain similarities in lifestyle and morphology, and they are considered good examples of the principle of convergence. Convergent evolution is the process by which creatures unrelated by evolution develop similar or even identical solutions to a particular problem; in this case, life in water.

Resources

Books

Ellis, Richard. Aquagenesis. New York: Viking, 2001.

Periodicals

Alexander, R. McNeill. "Size, Speed, and Buoyancy Adaptations in Aquatic Animals." American Zoologist 30 (Spring 1990): 189–196.

Butler, Patrick J., and David R. Jones. "Physiology of Diving Birds and Mammals." Physiological Reviews 77 (1997): 837–894.

Fish, Frank E. "Transitions from Drag-based to Life-based Propulsion in Mammalian Swimming." American Zoologist 36 (December 1996): 628–641.

Graham, Jeffrey B. "Ecological, Evolutionary, and Physical Factors Influencing Aquatic Animal Respiration." American Zoologist 30 (Spring 1990): 137–146.

Thompson, David, and Michael Fedak. "Cardiac Responses of Grey Seals During Diving at Sea." Journal of Experimental Biology 174 (1993): 139–164.

Webb, P. M., D. E. Crocker, S. B. Blackwell, D. P. Costa, et al. "Effects of Buoyancy on the Diving Behavior of Northern Elephant Seals." Journal of Experimental Biology 201 (August 1998): 2349–2358.

Gretel H. Schueller