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 plankton 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 nek

Nekton Fis
The nekton is made up of animals with independent locomotion, such as fish. (Digital Stock)

Hydrothermal Vents

One of the most exciting discoveries made by deep-sea biologists occurred as recently as 1977. Scientists aboard the deep submersible Alvin happened upon a previously unknown hydrothermal vent community while studying the Galápagos Rift Zone, 2,700 meters below the sea surface. These communities were composed of high densities of organisms never before seen by humans, in an otherwise barren landscape of sparse benthic life. Since their discovery, over four hundred species have been recovered from hydrothermal vents, including one new class, three new orders, and twenty-two new families of animals. Among the more notable animals are giant clams, mussels, and vestimentiferan tubeworms. Vestimentiferan tube worms are about two meters long and lack a digestive tract.

These dark ecosystems obviously cannot be supported by photo-synthetic organisms, but instead, chemoautotrophic bacteria that derive energy from sulfide are the primary producers that feed this unique food web. Vestimentiferan tube worms have these chemo-autotrophic bacteria living symbiotically within their tissues. These bacteria directly provide the tube worms with the fixed carbon they need to survive, which is why the worms do not have digestive tracts. They do not need to eat. Since the discovery of hydrothermal vent communities in the Galápagos rift, similar communities have been discovered along rifts in the Atlantic, Indian, and western Pacific Oceans.

ton, 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 Anguil-lidae 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 verti cally 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 plankton, 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, fast-swimming species of shark, tuna, and many bill-fish 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 from the 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 plankton, particularly the larger zooplankton. 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 plankton, 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 plankton 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 plankton 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 plankton 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.

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  • matilda
    What roles do nekton have in the ocean?
    6 years ago
  • dillon
    What is an nektonic species the is not carnivorous?
    6 years ago

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