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The Structures & Adaptations to Marine Living

Marine Life / NEXT: The Grazers & Predators »

Over the last 2,000 million years, plant and animal life on earth has continuously evolved from its simple beginnings in the oceans to the complex existence lived today. It's no accident that protoplasm, a substance found in every living cell, strongly resembles seawater. Although some animals emerged from the sea millions of years ago to fill all available niches on land, some remained in the ocean and evolved and adapted to life beneath the surface.

The ocean covers the majority of the planet, yet it remains a little understood realm as scientists are limited in the study of habitats that lack physical boundaries and can span thousands of miles.

Each form of marine life has become adapted to a specific niche with a relatively narrow variation in salinity, temperature, and light. The high salt content found in the ocean can support the large bodies of giant squids and whales, which has allowed them to evolve without the use of strong limbs for support. Nevertheless, salt water exerts enormous pressure on the air spaces of marine animals at depth (fluids like blood are practically incompressible). For every 33 feet of water, pressure increases by 14.7 pounds per square inch (equal to one atmosphere every 10 meters) which limits our depths significantly unless we use diving craft specifically designed to maintain one atmosphere.

And yet all sorts of other organisms thrive at high pressure. Some of them are even air-breathing surface dwellers like us. Weddell seals and elephant seals can dive up to a mile (sperm whales go much deeper than that). All these animals seem to share the same secret: Instead of fighting the pressure, they let it collapse their lungs completely. Some oxygen remains in their lungs, but they mostly store it in their muscles, where it's needed; their muscle tissue contains much higher concentrations of oxygen-binding myoglobin than ours does.

Moreover, collapsed lungs give deep-diving mammals another big advantage, as a team led by Terrie Williams of the University of California at Santa Cruz reported last year. Once a seal's lungs have collapsed, it becomes heavier than water, and so it sinks. Thus it doesn't have to flap flukes or flippers all the way down; it reaches great depths mostly by gliding effortlessly, saving its oxygen stores for the strenuous climb back to the surface.

The deep seafloor itself, well beyond the range of diving mammals, is inhabited by an incredible diversity of animals. Some of the fish even have lunglike swim bladders to control their buoyancy: They move up in the water column by secreting gas into the bladder and inflating it, and down by reabsorbing gas into their blood. With Jason, the researchers aboard the Knorr have observed such fish hang motionless a few feet above the seafloor. But they've made no effort to bring the fish up to the ship, because they know the results would not be pretty. A swim bladder doesn't collapse at depth because the gas inside is at the same pressure as the water outside—which means if that external pressure suddenly decreases, the bladder will swell catastrophically. "When we bring a fish up from depth, its swim bladder is often sticking out of its mouth," says Shana Goffredi of the Monterey Bay Aquarium Research Institute. "So those animals don't fare so well." - Discover Magazine

Clown anemonefish, Amphiprion ocellarisMarine animals must also regulate the interaction of freshwater and saltwater in their bodies. Specially developed kidneys, gills, and body functions help prevent the water from equalizing salt concentrations across membranes through osmosis. Marine animals must also be able to absorb dissolved gases like oxygen from the water needed to release the energy from food. Simple animals, such as anemones or worms, absorb the gases through their skin. Mobile animals use gills, or even lungs to absorb oxygen from the water and air. All animals in the ocean release carbon dioxide into the water as waste, which is then used by plants to produce energy.

Temperatures vary dramatically between the surface and the ocean floor. Marine life has developed many adaptations to the variations in temperature. Many marine mammals have blubber for insulation from the cold, and some fish have an antifreeze-like substance in their blood to keep it flowing. It is interesting to study the dramatically different adaptations in marine life on a vertical scale in the water. Animals and plants living in surface waters have access to high nutrient levels, increased temperatures, reduced pressure, and more light and therefore lack the adaptations of deep sea creatures that must live in highly pressurized, cold, dark waters with scarce nutrients.

Marine life has adapted to an incredible variety of conditions and habitats. Barnacles and mussels have developed mechanisms that allow them to cling to rocks in environments where they might otherwise be easily washed out by strong waves. Brightly-colored clownfish have adapted symbiotic relationships with anemones to protect both the clownfish and the anemone from predation. Sperm whales and herring gulls have adapted the ability to travel long distances and the ability to survive in a variety of environments.

Although the focus here is primarily on the adaptations of marine body structures, marine adaptations also include symbiosis, camouflage, defensive behavior, reproductive strategies, contact and communication, and adaptations to environmental conditions like temperature, light and salinity.

Chordate Origins

Animals in the Phylum Chordata include the vertebrates and some of the more primitive nonvertebrates like the protochordates, lancelets, acorn worms, tunicates, and the pterobranchs. The first vertebrates appearing in the fossil record during the Cambrian age were animals that resembled fishes and had respiratory gills formed by pharyngeal gill slits located in