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 
Marine 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 a set of pouches. The first purpose of the skeleton and scales were to protect the animal, to add support to the notochord, and to keep the brain protected. Later, a true backbone (rather than a notochord) evolved in marine animals. In all vertebrates, a heart developed to pump blood throughout the capillaries for the exchange of gases and oxygen. The blood in most fish goes from the heart to the gills and from there it is moved to the brain and other important body structures.
The Agnatha, or jawless fish, lived from the Late Cambrian until the end of the Devonian period. These fish were covered in bony armor, an adaptation that helped protect them from other animals. Parasitic lampreys and deep-sea hagfish are descended from the weak swimming, bottom dwelling jawless fish. Later in the Middle Silurian, a fish with jaws and teeth, known as the Gnathostomata vertebrate, evolved. Most fish are descended from this vertebrate, including all of the tetrapods. The jaws were actually adapted from the front elements of the gills and the teeth came from very bony scales near the skin of the mouth of the fish. Once jaws had developed in fish, many new strategies of surviving in the ecosystem became available. During this time, swimming capabilities were enhanced with the development of paired fins.
This was a time of great diversification in the oceans. Four groups of fishes branched out: the Placodermi (extinct now), the Acanthodii (extinct), the Chondrichthyes (sharks, rays and chimaeras) and the Osteichthyes (more highly evolved bony fishes). The Placodermi had extreme amounts of armor and were highly prevalent carnivores in the Silurian and Devonian periods. The Acanthodii were small filter-feeders. The Chondrichthyes and Osteichthyes classes survived, adapting to many different ocean conditions and branching out further into a vast array of species. Some of the many adaptations are as follows.
Most sharks in the Class Chondrichthyes have to keep swimming, otherwise they will sink to the bottom of the ocean. This characteristic has led to two distinct forms of sharks: the pelagic and benthic forms. The pelagic sharks move constantly through the water and rely on this movement to pass water across the gills for respiration. The benthic forms lie on the bottom and take in water through a pair of holes at the top of their head called spiracles. Rays also can lie on the ocean floor and respire through a spiracle at the top of their head. Rays have a flattened body type that allows them to hide under the mud and dig up crabs and shelled animals. The intestines and livers of sharks and rays are also shorter and larger than bony fish. Rays have developed stingers at the ends of their tails as a form as protection and some even have developed a type of battery that can deliver a strong electric shock. Another important development aiding in the survival of species in the Class Chondrichthyes was the appearance of the lateral line. The lateral line is a sensory organ in pelagic sharks and some fish. This line runs all the way from the head to the tail and functions to triangulate distances so the shark or fish can locate prey with great precision even in total darkness.
The Class Osteichthyes consists of all the bony fish. It is important to note that bony fish are also referred to as Teleost Fishes. Bony fish include many familiar fish like the bass, perch, cod, tuna, halibut—basically any fish with a bony skeleton. The general characteristics of a fish in this class include a longer intestine than sharks and rays, a single gill slit on each side, a mouth at the front of the body, a tail fin that is equal in size on the top and the bottom and external fertilization of eggs. Bony fish produce thousands of eggs, so there is plenty of genetic variation for natural selection to occur and adaptations in bony fishes abound. The flat fish is a good example of some of the stranger adaptations. The young flat fish appears to be a normal fish but as it develops, one eye actually migrates over to the other side of the body so that both eyes are on the same side. After the eye moves, the fish flips over so it looks like both eyes are on the same side but actually the top is just one side of the body. Another example is the male seahorse, which has adapted a pouch and, unlike most male animals, takes care of the young while the female swims away. The remora has developed a plate on its head to latch on to other fish and feed on food the larger fish leaves behind. The mola mola, or ocean sunfish, cannot swim very well, weighs over 2,000 lbs and has been said to be the largest type of zooplankton. This fish reaches a top speed of 3 miles per hour and floats around eating jellyfish. Some freshwater fish have developed the ability to climb trees, squirt water at insects, breathe air and stay out of water for long periods of time.
Reptiles
The reptiles came about as a novel group of terrestrial animals from the amphibians. Reptiles were extremely successful on land and quickly became the dominant animal for the next 150 million years. When mammals evolved, they took over the dominant position leaving the reptiles to crawl back into the ocean. The reptiles that survived include the snakes, turtles and lizards many of which have changed a little so they can live more successfully in salt-water environments. Although crocodiles have also adapted to saltier conditions, they never made a full change and still prefer brackish waters. Reptiles that abandoned the land for the sea include the sea turtles in the Family Cheloniidae, the marine iguana in the Family Iguanidae, and the sea snakes in the Family Hydrophiidae.
The turtles have not changed too much over the last 100 million years. The hard shell characteristic of turtles has been a great help in protection and the prevention of drying out. Land turtles have a problem with their shell being too heavy but when turtles are in the water—the buoyancy of the water lifts the weight of the shell and allows the turtle to move gracefully through the medium. Sea turtles developed longer feet that were more paddle-like allowing the turtle to fly through the water with great speed and agility. Another adaptation of sea turtles to the sea is a hinge in the lower portion of the turtle that allows them to take in much more air and come up for air less often.
Mammals
Marine mammals include the Order Cetacea (porpoises and whales), the Order Carnivora (animals like seals), and the Order Sirenia (dugongs, manatees and sea cows). Marine mammals are still warm-blooded and have to keep the temperature of their bodies above that of the ocean. Adaptations that have helped solve this problem include the reduction of surface area and the increase in internal volume, a fatty layer of blubber under very thick skin, and a reduction in the amount of blood going to areas in contact with the cold water. Unlike land animals, marine mammals are also able to dive very deep into the water without getting the bends because as they dive down deeper they exhale instead of inhale like we do. They expel air from their lungs, and therefore do not absorb excess nitrogen. Other adaptations to marine living include: a slower heartbeat during dives, reduced blood flow to non-vital organs, unusually high hemoglobin count in blood, and an unusually high myoglobin count in muscles.
One fundamental difference between cetaceans and fish is the tail. The tails of mammals are horizontal enabling to swim both vertically and horizontally. The tails of most fish are vertical, so the swimming motion is side to side. The streamlined shape observed in both marine fish and marine mammals is an example of biological convergence. The rounded head and tapering body shape allows marine fish and mammals to glide smoothly through the water, wasting little energy due to resistance. Animals that are not streamlined, like the stingray or the globefish, have sacrificed efficient swimming for benefits of camouflage or body armor.
Most of the power generated for swimming in marine animals comes from the tail at the back. Most fish will move their tail from side to side so that water is pushed backwards and around the side and the fish moves forward. Fins at the side of the fish help counteract the tendency of the head to swing from side to side as the tail moves. Fish also have fins on their back, their sides and underneath their bodies. Fish, whales, turtles and even seals have specialized limbs for swimming.
Animals with Shells
About 500 million years ago, animals with hard-shells became prominent in the fossil record in the Phylum Molluska. The evolution of an impenetrable shell was obviously a very helpful trait for an animal to possess because now mollusks are found in almost every known environment. Animals with hard shells are protected from predation and drying out and some can even use their shell to float if necessary among other things. The seven Classes of mollusks are the Polyplacophora (the chitons), Gastropoda (the snails), Bivalvia (the clams), Cephalopoda (octopus and squid), Scaphopoda (the tusk shells) and Aplacophora (small worm-like organisms). There are at least 30,000 species of gastropods and it is the largest taxonomic class.
The chitons are the most primitive animals in the Phylum Molluska. Every chiton shell is made so that it will fit together and bend. Chitons live only in marine environments and are also recognizable by the eight plates that overlap on their back. The gills are located safely under the shell on either side of their foot. The adaptations seen in chitons allow these organisms to survive heavy surf, so they are often found in tide pools.
Organisms in the Class Gastropoda are most commonly known as snails, limpets, abalones, conchs, and whelks. Other gastropods perhaps less familiar include the nudibranchs, slugs, and some pteropods and heteropods. Gastropods can usually be identified by a shell that spirals to the right although some like the nudibranchs do not have a shell and in others the shell twists to the left. In order to fit into this shell, many gastropods have organs that are reduced in size. Although some gastropods have lost their shell throughout evolution, most still have a shell and benefit from the protection. Many gastropods like limpets and abalone will retreat into their shell when disturbed and close off the opening with a special plate called the operculum. There are many different types of shells and most of the variety is a direct result of adaptation to the environment. For example, in rough waters most animals have flat shells to reduce water resistance. Animals that need to crawl into rocks to hide also have flat shells to fit into smaller cracks. Most gastropods move forward with the help of a foot that is very similar to that of a terrestrial snail.
Cephalopods, like octopuses and squid are feared by many, however they are actually quite gentle, delicate and "intelligent" creatures. Squid and octopuses are the most advanced mollusks. They have highly developed eyesight, the ability to swim quickly and the amazing ability to rapidly change color using their chromatophores. The female octopus has excellent parenting skills and keeps her eggs safe and clean until they hatch. Most cephalopods have soft bodies with no shell and can walk on ocean floor or swim using a siphon that squirts water in a powerful jet. Some segments of giant squids have been recovered indicating that the whole animal may weigh up to 900 kgs and be 18 meters long. Some scientists believe there are may be squid with lengths over 30 meters. Another interesting adaptation in the cephalopods is the development of an inky substance used to block the senses of sight and smell in predators.
