Marine animals

Approximately 71 percent of earth's surface is covered by salt water, and the marine environments contained therein constitute the largest and most diverse array of life on the planet. Life originated in the oceans, and the salt water that comprises the largest constituent of the tissues of all living organisms is a vestigial reminder of the aquatic origins of life.

Marine Zones
The marine environment can be divided broadly into different zones, each of which supports numerous habitats. The coastal area between the high and low tide boundaries is known as the intertidal zone; beyond this is the neritic zone, relatively shallow water that extends over the continental shelves. The much deeper water that extends past the boundaries of the continental shelves is known as the oceanic zone. Open water of any depth away from the coastline is also known as the pelagic zone. The benthic zone is composed of the sediments occurring at the sea floor. Areas in which freshwater rivers empty into the saltwater oceans produce a continually mixed brackish water region known as an estuary. Estuarine zones often also include extensive wetland areas such as mud flats or salt marshes. Zones in the marine environment are distributed vertically as well as horizontally. Life in the ocean, as on land, is ultimately supported by sunlight in most cases, used by photosynthetic plants as an energy source. Sunlight can only penetrate water to a limited depth, generally between one hundred and two hundred meters; this region is known as the photic or epipelagic zone. Below two hundred meters, there may be sufficient sunlight penetrating to permit vision, but not enough to support photosynthesis; this transitional region may extend to depths of one thousand meters and is known as the disphotic or mesopelagic zone. Below this depth, in the aphotic zone, sunlight cannot penetrate and the environment is perpetually dark, with the exception of small amounts of light produced by photoluminescent invertebrate and vertebrate animals. This aphotic zone is typically divided into the bathypelagic zone, between seven hundred and one thousand meters as the upper range and two thousand to four thousand meters as the lower range, where the water temperature is between 4 and 10 degrees Celsius. Beneath the bathypelagic zone, overlying the great plains of the ocean basins, is the abyssalpelagic zone, with a lower boundary of approximately six thousand meters. Finally, the deepest waters of the oceanic trenches, which extend to depths of ten thousand meters, constitute the hadalpelagic zone. In each of these zones, the nature and variety of marine life present is dictated by the physical characteristics of the zone. However, these zones are not absolute, but rather merge gradually into each other, and organisms may move back and forth between zones.

Plankton
Marine life can be divided broadly into three major categories. Those small organisms that are either free-floating or weakly swimming and which thus drift with oceanic currents are referred to as nekton. Plankton can be further divided into phytonekton, which are plantlike and capable of photosynthesis; zoonekton, which are animallike; and bacterionekton, which are bacteria and blue-green algae suspended in the water column. Larger organisms that can swim more powerfully and which can thus move independently of water movements are known collectively as the nekton. Finally, organisms that are restricted to living on or in the sediments of the seafloor bottom are referred to as the benthos. The phytonekton, which are necessarily restricted to the photic zone, are by far the largest contributors to photosynthesis in the oceans. The phytonekton are therefore responsible for trapping most of the solar energy obtained by the ocean (the primary productivity), which can then be transferred to other organismswhenthe phytonekton are themselves ingested. The phytonekton are composed of numerous different types of photosynthetic organisms, including diatoms, which are each encased in a unique "pillbox" shell of transparent silica, and dinoflagellates. The very rapid growth of some species of dinoflagellates in some areas results in massive concentrations or blooms that are sometimes referred to as red tides. Chemicals that are produced by red tide dinoflagellates often prove toxic to other marine organisms and can result in massive die-offs of marine life. Smaller photosynthetic nekton forms comprise the nanonekton and also play an important role in the photosynthetic harnessing of energy in the oceans. The zoonekton are an extremely diverse group of small animal organisms. Unlike the phytonekton, which can make their own complex organic compounds via photosynthesis, the phytonekton must ingest or absorb organic compounds produced by other organisms. This is accomplished by either preying upon other nektonic organisms or by feeding on the decaying remains of dead organisms. Anumber of zoonekton species also exist as parasites during some portion of their life cycles, living in or upon the bodies of nekton species. The largest group of zoonekton are members of the subphylum Crustacea, especially the copepods. These organisms typically possess a jointed exoskeleton, or shell, made of chitin, large antennae, and a number of jointed appendages. Space precludes a definitive listing of all of the zoonektonic organisms; however, virtually all of the other groups of aquatic invertebrates are represented in the bewildering variety of the zoonekton, either in larval or adult forms. Even fishes, normally a part of the nekton, contribute to the zoonekton, both as eggs and as larval forms. The bacterionekton are found in all of the world's oceans. Some of these, the blue-green algae (cyanobacteria), play an important role in the photosynthetic productivity of the ocean. Bacterionekton are usually found in greatest concentrations in surface waters, often in association with organic fragments known as particulate organic carbon, or marine snow. Bacterionekton play an important role in renewing nutrients in the photic zones of the ocean; such renewal is important in maintaining the photosynthetic activity of the phytonekton, upon which the rest of marine life is in turn dependent. One of the principal problems facing nekton is maintaining their position in the water column. Since these organisms are slightly denser than the surrounding seawater, they tend to sink. Clearly this is a disadvantage, particularly since nekton typically have very limited mobility. This is especially true for the photosynthetic phytonekton, which must remain within the photic zone in order to carry on photosynthesis.Anumber of strategies have evolved among nektonic species to oppose this tendency to sink. Long, spindly extensions of the body provide resistance to the flow of water. Inclusions of oils or fats (which are less dense than water) within the body provide positive buoyancy by decreasing the overall density of the nekton. Finally, some species, such as the Portuguese man-o'-war, generate balloonlike gas bladders, which provide enough buoyancy to keep them at the very surface of the epipelagic zone.

Nekton
The nekton is composed of those larger animals that have developed locomotion to a sufficient degree that they can move independently of the ocean's water movements. Whereas the nekton are principally invertebrates, most of the nekton are vertebrates. The majority of the nekton are fishes, although reptile, bird, and mammalian species are also constituent parts. The oceanic nekton are those species which are found in the epipelagic zone of the open ocean. These include a wide variety of sharks, rays, bony fishes, sea birds, marine mammals, and a few species of reptiles. Some members of the oceanic nekton, such as blue sharks, oceanic whitetip sharks, tuna, flying fish, and swordfish, spend their entire lives in the pelagic environment; these are said to be holoepipelagic. Others, the meroepipelagic nekton, only spend a portion of their lives in the epipelagic zone, returning to coastal areas to mate, as with herring and dolphins, or returning to freshwater, as with salmon and sturgeon. Sea birds are a special case: Although they spend much of their time flying over the epipelagic zone and nest on land, they feed in the epipelagic zone and some species may dive as deep as one hundred meters in search of prey. Some members of the nekton enter the epipelagic only at certain times in their life cycle. Eels of the family Anguillidae spend most of their lives in freshwater but return to the epipelagic zone to spawn. Additionally, at night many species of deep-water fishes migrate up into the epipelagic to feed before returning to deeper waters during the daylight hours. The pelagic environment, unlike the terrestrial one, is profoundly three-dimensional. Nektonic animals can move both horizontally and vertically within the water column. Furthermore, since most of the pelagic environment is essentially bottomless, since there is no apparent or visible ground or substrate, the environment is basically uniform and featureless. These characteristics play an important role in the evolution of the behavior of nektonic animals. Fishes suspended in an essentially transparent and featureless medium have no shelter in which to hide from predators, nor are there any apparent landmarks to serve as directional cues for animals moving horizontally from place to place. Life in the open ocean has therefore favored adaptations for great mobility and speed with which to move across large distances and escape from predators, as well as camouflage and cryptic coloration designed to deceive potential predators or prey. As is the case for nekton, most nektonic animals are denser than the surrounding seawater, and maintaining position in the water column is of the first importance. Most fishes possess a swim bladder, a gas-filled membranous sac within their body that opposes the tendency to sink and provides the fish with neutral buoyancy. Sharks and rays lack a swim bladder, but accumulate large concentrations of fats and oils in their liver, which also help counter the tendency to sink. Large, fastswimming species of shark, tuna, and many billfish also rely on the generation of hydrodynamic lift to maintain vertical position in the water column. The tail and body of these fishes generate forward thrust, moving the animal through the water, and the fins, notably the pectoral fins, generate lift fromthe water flowing over them in a manner similar to that of an airplane's wing. Thus these animals fly through the water, but are in turn required to move continuously in order to generate lift. All members of the nekton are carnivores, feeding on other nektonic species or upon nekton, particularly the larger zoonekton. In general, the size of the prey consumed by nekton is directly related to the size of the predator, with larger species consuming larger prey. However, the organisms that feed upon nekton, the planktivores, include a wide variety of fish species such as herring, salmon, and the whale shark, the largest extant fish species. They also include the largest marine animals of all, the baleen whales. The case of large animals feeding upon very small nekton directly addresses the need of all animals to meet their energy requirements. For all animals, the amount of energy obtained from food consumed must necessarily exceed the energy expended in acquiring the prey. Very large animals, such as whales and whale sharks, require a great deal of energy to move their bodies through the aquatic environment, but because of their great size they are necessarily less agile than smaller forms. The amount of energy required to chase and catch these smaller animals would generally exceed the energy derived from ingesting them. Plankton, however, are relatively easy to obtain due to their very limited mobility. However, because of their small size, vast quantities of nekton must be ingested in order to meet the metabolic requirements of large marine animals. Some very large species that are not planktivores solve the energy problem by evolving behaviors for acquiring specialized diets that yield higher energy. White sharks, for example, feed on fish when young, but as they age and increase in size, marine mammals, notably seals and sea lions (pinnipeds), become a major part of their diet. Marine mammals all possess blubber, an energy-rich substance that yields much more energy than fish. Similarly, sperm whales, the largest hunting carnivores on the planet, have a diet that consists in large part of giant squid, which are hunted in the ocean depths largely using the whale's acoustic echolocation sense. Orcas (killer whales) effectively use pack hunting techniques to hunt larger whales and other marine mammals. The deeper regions of the ocean are dominated by different types of nekton. However, we know even less about their ecology due to their relative inaccessibility. The disphotic or mesopelagic zone contains many animal species that migrate vertically into surface waters at night to feed upon the nekton there. Many of these organisms possess large, well-developed eyes and also possess light organs containing symbiotic luminescent bacteria. The majority of the fish species in this group are colored black and the invertebrates are largely red (red light penetrates water less effectively than do longer wavelengths, and these animals appear dark-colored at depth). Beneath this zone, in the bathypelagic and abyssalpelagic zones, there are many fewer organisms and much less diversity than in the shallower levels. Animals in this region are typically colorless and possess small eyes and luminescent organs. Because organisms in these deep regions are few and far between, many species have become specialized in order to maximize their advantages. Thus, deep-sea fish are characterized by large teeth and remarkably hinged jaws that allow them to consume prey much larger than might be expected from their size. Similarly, since encounters with potential mates are presumably scarce, a number of unique reproductive strategies have evolved. In the anglerfish (Ceratius), all of the large individuals are female and the comparatively tiny males are parasitic, permanently attaching themselves to the female. Much, however, still remains to be learned of the ecology of these deep-sea organisms.

The benthos of the world's oceans consists of animals that live on the solid substrate of the water column, the ocean floor. Scientists typically divide benthic organisms into two categories, the epifauna, which live on the surface of the bottom at the sediment-water interface, and the infauna, those organisms living within the sediments. In shallow water benthic communities, members of virtually every major animal group are represented. Ecologists generally differentiate between soft bottom benthic communities (sand, silt, and mud, which comprise the majority of the benthic zone) and rocky bottom communities, which are less common proportionately. Soft bottom communities have an extensive diversity of burrowing infauna, such as polychaete worms, and mollusks, such as clams. Rocky bottom communities possess a larger proportion of epifauna, such as crustaceans and echinoderms (starfish, sea urchins, and brittle stars), living on the surface of what is essentially a two-dimensional environment. Vertical faces of the hard bottom environment, such as canyon walls or coral reefs, are often home to a wide variety of animals occupying various crannies and caves. In some parts of the world, kelp plants that are anchored to the substrate and which extend to the water surface dominate the rocky bottom substrate. In these kelp forests, large kelp plants (actually a species of brown algae) form a forestlike canopy that plays host to a wide and complex array of animals extending throughout the water column. On the deep ocean floor, the benthos is composed of representatives of virtually every major animal group: crustaceans such as amphipods, segmented polychaete worms, sea cucumbers, and brittle stars. Less common are starfish, sea lilies, anemones, and sea fans. The fishes of the deep benthos include rat tails and a number of eel species. Estuaries, where freshwater rivers empty into marine environments, are typified by large, cyclic changes in temperature and salinity. Although estuaries have played an important role in human history as the sites of major ports, the variety and number of estuarine species tend to show less diversity of animal species due to the difficulty in adapting to the large swings in environmental conditions. Animal life in the sea, like that on land, shows an astonishing variety of forms and behaviors, the result of natural selection. The inaccessibility and hostility of much of the world's oceans to human exploration and observation leaves much yet to be learned about the biology of marine life. Much remains to be achieved in order to obtain a useful body of knowledge concerning life in the sea.