Manatees are large aquatic herbivorous marine mammals sometimes known as sea cows. There are three accepted living species of Trichechidae, representing three of the four living species in the order Sirenia: the Amazonian manatee, the West Indian manatee, the West African manatee, they measure up to 4.0 metres long, weigh as much as 590 kilograms, have paddle-like flippers. The etymology of the name is dubious, with connections having been made to Latin "manus", to a word sometimes cited as "manati" used by the Taíno, a pre-Columbian people of the Caribbean, meaning "breast". Manatees are called sea cows, as they are slow plant-eaters and similar to cows on land, they graze on water plants in tropical seas. Manatees are three of the four living species in the order Sirenia; the fourth is the Eastern Hemisphere's dugong. The Sirenia are thought to have evolved from four-legged land mammals more than 60 million years ago, with the closest living relatives being the Proboscidea and Hyracoidea; the Amazonian's hair color is brownish gray, it has thick wrinkled skin with coarse hair, or "whiskers".
Photos are rare. Manatees weigh 400 to 550 kilograms, average 2.8 to 3.0 metres in length, sometimes growing to 4.6 metres and 1,775 kilograms. At birth, baby manatees weigh about 30 kilograms each; the manatee has a large, prehensile upper lip, used to gather food and eat and for social interaction and communication. Manatees have shorter snouts than the dugongs; the lids of manatees' small spaced eyes close in a circular manner. The adults have no incisor or canine teeth, just a set of cheek teeth, which are not differentiated into molars and premolars; these teeth are replaced throughout life, with new teeth growing at the rear as older teeth fall out from farther forward in the mouth, somewhat as elephants' teeth do. At any time, a manatee has no more than six teeth in each jaw of its mouth, its tail is paddle-shaped, is the clearest visible difference between manatees and dugongs. The female manatee has two teats, one under each flipper, a characteristic, used to make early links between the manatee and elephants.
The manatee is unusual among mammals in having just six cervical vertebrae, a number that may be due to mutations in the homeotic genes. All other mammals have other than the two-toed and three-toed sloths. Like the horse, the manatee has a simple stomach, but a large cecum, in which it can digest tough plant matter; the intestines are about 45 meters, unusually long for an animal of the manatee's size. Apart from mothers with their young, or males following a receptive female, manatees are solitary animals. Manatees spend 50% of the day sleeping submerged, surfacing for air at intervals of less than 20 minutes; the remainder of the time is spent grazing in shallow waters at depths of 1–2 metres. The Florida subspecies has been known to live up to 60 years. Manatees swim at about 5 to 8 kilometres per hour. However, they have been known to swim at up to 30 kilometres per hour in short bursts. Manatees are capable of understanding discrimination tasks and show signs of complex associative learning.
They have good long-term memory. They demonstrate discrimination and task-learning abilities similar to dolphins and pinnipeds in acoustic and visual studies. Manatees breed once every two years. Gestation lasts about 12 months and to wean the calf takes a further 12 to 18 months. Manatees emit a wide range of sounds used in communication between cows and their calves, their ears are large internally but the external openings are small, they are located four inches behind each eye. Adults communicate to maintain contact and during sexual and play behaviors. Taste and smell, in addition to sight and touch, may be forms of communication. Manatees eat over 60 different freshwater and saltwater plants. Using their divided upper lip, an adult manatee will eat up to 10%–15% of their body weight per day. Consuming such an amount requires the manatee to graze for up to seven hours a day. To be able to cope with the high levels of cellulose in their plant based diet, manatees utilize hindgut fermentation to help with the digestion process.
Manatees have been known to eat small numbers of fish from nets. Manatees use their flippers to "walk" along the bottom whilst they dig for plants and roots in the substrate; when plants are detected, the flippers are used to scoop the vegetation toward the manatee's lips. The manatee has prehensile lips; the lips use seven muscles to tear at plants. Manatees use front flippers to move the plants into the mouth; the manatee does not have front teeth, behind the lips, on the roof of the mouth, there are dense, ridged pads. These horny ridges, the manatee's lower jaw, tear
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring by sexual reproduction. Other ways of defining species include their karyotype, DNA sequence, behaviour or ecological niche. In addition, paleontologists use the concept of the chronospecies since fossil reproduction cannot be examined. While these definitions may seem adequate, when looked at more they represent problematic species concepts. For example, the boundaries between related species become unclear with hybridisation, in a species complex of hundreds of similar microspecies, in a ring species. Among organisms that reproduce only asexually, the concept of a reproductive species breaks down, each clone is a microspecies. All species are given a two-part name, a "binomial"; the first part of a binomial is the genus.
The second part is called the specific epithet. For example, Boa constrictor is one of four species of the genus Boa. None of these is satisfactory definitions, but scientists and conservationists need a species definition which allows them to work, regardless of the theoretical difficulties. If species were fixed and distinct from one another, there would be no problem, but evolutionary processes cause species to change continually, to grade into one another. Species were seen from the time of Aristotle until the 18th century as fixed kinds that could be arranged in a hierarchy, the great chain of being. In the 19th century, biologists grasped. Charles Darwin's 1859 book The Origin of Species explained how species could arise by natural selection; that understanding was extended in the 20th century through genetics and population ecology. Genetic variability arises from mutations and recombination, while organisms themselves are mobile, leading to geographical isolation and genetic drift with varying selection pressures.
Genes can sometimes be exchanged between species by horizontal gene transfer. Viruses are a special case, driven by a balance of mutation and selection, can be treated as quasispecies. Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics. Early taxonomists such as Linnaeus had no option but to describe what they saw: this was formalised as the typological or morphological species concept. Ernst Mayr emphasised reproductive isolation, but this, like other species concepts, is hard or impossible to test. Biologists have tried to refine Mayr's definition with the recognition and cohesion concepts, among others. Many of the concepts are quite similar or overlap, so they are not easy to count: the biologist R. L. Mayden recorded about 24 concepts, the philosopher of science John Wilkins counted 26. Wilkins further grouped the species concepts into seven basic kinds of concepts: agamospecies for asexual organisms biospecies for reproductively isolated sexual organisms ecospecies based on ecological niches evolutionary species based on lineage genetic species based on gene pool morphospecies based on form or phenotype and taxonomic species, a species as determined by a taxonomist.
A typological species is a group of organisms in which individuals conform to certain fixed properties, so that pre-literate people recognise the same taxon as do modern taxonomists. The clusters of variations or phenotypes within specimens would differentiate the species; this method was used as a "classical" method of determining species, such as with Linnaeus early in evolutionary theory. However, different phenotypes are not different species. Species named in this manner are called morphospecies. In the 1970s, Robert R. Sokal, Theodore J. Crovello and Peter Sneath proposed a variation on this, a phenetic species, defined as a set of organisms with a similar phenotype to each other, but a different phenotype from other sets of organisms, it differs from the morphological species concept in including a numerical measure of distance or similarity to cluster entities based on multivariate comparisons of a reasonably large number of phenotypic traits. A mate-recognition species is a group of sexually reproducing organisms that recognize one another as potential mates.
Expanding on this to allow for post-mating isolation, a cohesion species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. A further development of the recognition concept is provided by the biosemiotic concept of species. In microbiology, genes can move even between distantly related bacteria extending to the whole bacterial domain; as a rule of thumb, microbiologists have assumed that kinds of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97% to each other need to be checked by DNA-DNA hybridisation to decide if they belong to the same species or not. This concept was narrowed in 2006 to a similarity of 98.7%. DNA-DNA hybri
Crabs are decapod crustaceans of the infraorder Brachyura, which have a short projecting "tail" entirely hidden under the thorax. They live in all the world's oceans, in fresh water, on land, are covered with a thick exoskeleton and have a single pair of pincers. Many other animals with similar names – such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs, crab lice – are not true crabs. Crabs are covered with a thick exoskeleton, composed of mineralized chitin, armed with a single pair of chelae. Crabs are found in all of the world's oceans, while many crabs live in fresh water and on land in tropical regions. Crabs vary in size from the pea crab, a few millimetres wide, to the Japanese spider crab, with a leg span of up to 4 metres. About 850 species of crab are terrestrial or semi-terrestrial species, they were thought to be a monophyletic group, but are now believed to represent at least two distinct lineages, one in the Old World and one in the New World. The earliest unambiguous crab fossils date from the Jurassic, although Carboniferous Imocaris, known only from its carapace, may be a primitive crab.
The radiation of crabs in the Cretaceous and afterward may be linked either to the break-up of Gondwana or to the concurrent radiation of bony fish, crabs' main predators. Crabs show marked sexual dimorphism. Males have larger claws, a tendency, pronounced in the fiddler crabs of the genus Uca. In fiddler crabs, males have one claw, enlarged and, used for communication for attracting a mate. Another conspicuous difference is the form of the pleon; this is. Crabs attract a mate through chemical, acoustic, or vibratory means. Pheromones are used by most aquatic crabs, while terrestrial and semiterrestrial crabs use visual signals, such as fiddler crab males waving their large claws to attract females; the vast number of brachyuran crabs have mate belly-to-belly. For many aquatic species, mating takes place just after the female is still soft. Females can store the sperm for a long time before using it to fertilise their eggs; when fertilisation has taken place, the eggs are released onto the female's abdomen, below the tail flap, secured with a sticky material.
In this location, they are protected during embryonic development. Females carrying eggs are called "berried"; when development is complete, the female releases the newly hatched larvae into the water, where they are part of the plankton. The release is timed with the tides; the free-swimming tiny zoea larvae can take advantage of water currents. They have a spine, which reduces the rate of predation by larger animals; the zoea of most species must find food, but some crabs provide enough yolk in the eggs that the larval stages can continue to live off the yolk. Each species has a particular number of zoeal stages, separated by moults, before they change into a megalopa stage, which resembles an adult crab, except for having the abdomen sticking out behind. After one more moult, the crab is a juvenile, living on the bottom rather than floating in the water; this last moult, from megalopa to juvenile, is critical, it must take place in a habitat, suitable for the juvenile to survive. Most species of terrestrial crabs must migrate down to the ocean to release their larvae.
After living for a short time as larvae in the ocean, the juveniles must do this migration in reverse. In many tropical areas with land crabs, these migrations result in considerable roadkill of migrating crabs. Once crabs have become juveniles, they will still have to keep moulting many more times to become adults, they are covered with a hard shell. The moult cycle is coordinated by hormones; when preparing for moult, the old shell is softened and eroded away, while the rudimentary beginnings of a new shell form under it. At the time of moulting, the crab takes in a lot of water to expand and crack open the old shell at a line of weakness along the back edge of the carapace; the crab must extract all of itself – including its legs, mouthparts and the lining of the front and back of the digestive tract – from the old shell. This is a difficult process that takes many hours, if a crab gets stuck, it will die. After freeing itself from the old shell, the crab is soft and hides until its new shell has hardened.
While the new shell is still soft, the crab can expand it to make room for future growth. Crabs walk sideways, because of the articulation of the legs which makes a sidelong gait more efficient. However, some crabs walk forwards or backwards, including raninids, Libinia emarginata and Mictyris platycheles; some crabs, notably the Portunidae and Matutidae, are capable of swimming, the Portunidae so as their last pair of walking legs is flattened into swimming paddles. Crabs are active animals with complex behaviour patterns, they can communicate by waving their pincers. Crabs tend to be aggressive towards one another, males fight to gain access to females. On rocky seashores, where nearly all caves and crevices
A mangrove is a shrub or small tree that grows in coastal saline or brackish water. The term is used for tropical coastal vegetation consisting of such species. Mangroves occur worldwide in the tropics and subtropics between latitudes 25° N and 25° S; the total mangrove forest area of the world in 2000 was 137,800 square kilometres, spanning 118 countries and territories. Mangroves are salt-tolerant trees called halophytes, are adapted to life in harsh coastal conditions, they contain a complex salt filtration system and complex root system to cope with salt water immersion and wave action. They are adapted to the low oxygen conditions of waterlogged mud; the word is used in at least three senses: most broadly to refer to the habitat and entire plant assemblage or mangal, for which the terms mangrove forest biome, mangrove swamp are used, to refer to all trees and large shrubs in the mangrove swamp, narrowly to refer to the mangrove family of plants, the Rhizophoraceae, or more just to mangrove trees of the genus Rhizophora.
The mangrove biome, or mangal, is a distinct saline woodland or shrubland habitat characterized by depositional coastal environments, where fine sediments collect in areas protected from high-energy wave action. The saline conditions tolerated by various mangrove species range from brackish water, through pure seawater, to water concentrated by evaporation to over twice the salinity of ocean seawater; the term "mangrove" comes to English from Spanish, is to originate from Guarani. It was earlier "mangrow", but this word was corrupted via folk etymology influence of the word "grove". Mangrove swamps are found in subtropical tidal areas. Areas where mangals occur include marine shorelines; the intertidal existence to which these trees are adapted represents the major limitation to the number of species able to thrive in their habitat. High tide brings in salt water, when the tide recedes, solar evaporation of the seawater in the soil leads to further increases in salinity; the return of tide can flush out these soils, bringing them back to salinity levels comparable to that of seawater.
At low tide, organisms are exposed to increases in temperature and desiccation, are cooled and flooded by the tide. Thus, for a plant to survive in this environment, it must tolerate broad ranges of salinity and moisture, as well as a number of other key environmental factors—thus only a select few species make up the mangrove tree community. About 110 species are considered "mangroves", in the sense of being a tree that grows in such a saline swamp, though only a few are from the mangrove plant genus, Rhizophora. However, a given mangrove swamp features only a small number of tree species, it is not uncommon for a mangrove forest in the Caribbean to feature only three or four tree species. For comparison, the tropical rainforest biome contains thousands of tree species, but this is not to say mangrove forests lack diversity. Though the trees themselves are few in species, the ecosystem that these trees create provides a home for a great variety of other species. Mangrove plants require a number of physiological adaptations to overcome the problems of anoxia, high salinity and frequent tidal inundation.
Each species has its own solutions to these problems. Small environmental variations within a mangal may lead to differing methods for coping with the environment. Therefore, the mix of species is determined by the tolerances of individual species to physical conditions, such as tidal inundation and salinity, but may be influenced by other factors, such as predation of plant seedlings by crabs. Once established, mangrove roots provide an oyster habitat and slow water flow, thereby enhancing sediment deposition in areas where it is occurring; the fine, anoxic sediments under mangroves act as sinks for a variety of heavy metals which colloidal particles in the sediments have scavenged from the water. Mangrove removal disturbs these underlying sediments creating problems of trace metal contamination of seawater and biota. Mangrove swamps protect coastal areas from erosion, storm surge, tsunamis; the mangroves' massive root systems are efficient at dissipating wave energy. They slow down tidal water enough so its sediment is deposited as the tide comes in, leaving all except fine particles when the tide ebbs.
In this way, mangroves build their own environments. Because of the uniqueness of mangrove ecosystems and the protection against erosion they provide, they are the object of conservation programs, including national biodiversity action plans. Mangrove swamps' effectiveness in terms of erosion control can sometimes be overstated. Wave energy is low in areas where mangroves grow, so their effect on erosion is measured over long periods, their capacity to limit high-energy wave erosion is in relation to events such as storm surges and tsunamis. The unique ecosystem found in the intricate mesh of mangrove roots offers a quiet marine region for young organisms. In areas where roots are permanently submerged, the organisms they host include algae, oysters and bryozoans, which all require a hard surface for anchoring while they filter feed. Shrimps and mud lobsters use the muddy bottoms as their home. Mangrove crabs munch on the mangrove leaves, adding nutrients to the mangal muds for other bottom feeders.
In at least some cases, export of carbon fixed in mangroves is imp
Fish are gill-bearing aquatic craniate animals that lack limbs with digits. They form a sister group to the tunicates. Included in this definition are the living hagfish and cartilaginous and bony fish as well as various extinct related groups. Tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered paraphyletic by excluding the tetrapods; because in this manner the term "fish" is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology, unless it is used in the cladistic sense, including tetrapods. The traditional term pisces is considered a typological, but not a phylogenetic classification; the earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts. Fish would continue to evolve through the Paleozoic era.
Many fish of the Paleozoic developed external armor. The first fish with jaws appeared in the Silurian period, after which many became formidable marine predators rather than just the prey of arthropods. Most fish are ectothermic, allowing their body temperatures to vary as ambient temperatures change, though some of the large active swimmers like white shark and tuna can hold a higher core temperature. Fish can communicate in their underwater environments through the use of acoustic communication. Acoustic communication in fish involves the transmission of acoustic signals from one individual of a species to another; the production of sounds as a means of communication among fish is most used in the context of feeding, aggression or courtship behaviour. The sounds emitted by fish can vary depending on the stimulus involved, they can produce either stridulatory sounds by moving components of the skeletal system, or can produce non-stridulatory sounds by manipulating specialized organs such as the swimbladder.
Fish are abundant in most bodies of water. They can be found in nearly all aquatic environments, from high mountain streams to the abyssal and hadal depths of the deepest oceans, although no species has yet been documented in the deepest 25% of the ocean. With 33,600 described species, fish exhibit greater species diversity than any other group of vertebrates. Fish are an important resource for humans worldwide as food. Commercial and subsistence fishers hunt fish in wild fisheries or farm them in ponds or in cages in the ocean, they are caught by recreational fishers, kept as pets, raised by fishkeepers, exhibited in public aquaria. Fish have had a role in culture through the ages, serving as deities, religious symbols, as the subjects of art and movies. Fish do not represent a monophyletic group, therefore the "evolution of fish" is not studied as a single event. Early fish from the fossil record are represented by a group of small, armored fish known as ostracoderms. Jawless fish lineages are extinct.
An extant clade, the lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils; the diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, or a combination of factors. Fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways; the first ancestors of fish may have kept the larval form into adulthood, although the reverse is the case. Fish are a paraphyletic group: that is, any clade containing all fish contains the tetrapods, which are not fish. For this reason, groups such as the "Class Pisces" seen in older reference works are no longer used in formal classifications. Traditional classification divides fish into three extant classes, with extinct forms sometimes classified within the tree, sometimes as their own classes: Class Agnatha Subclass Cyclostomata Subclass Ostracodermi † Class Chondrichthyes Subclass Elasmobranchii Subclass Holocephali Class Placodermi † Class Acanthodii † Class Osteichthyes Subclass Actinopterygii Subclass Sarcopterygii The above scheme is the one most encountered in non-specialist and general works.
Many of the above groups are paraphyletic, in that they have given rise to successive groups: Agnathans are ancestral to Chondrichthyes, who again have given rise to Acanthodiians, the ancestors of Osteichthyes. With the arrival of phylogenetic nomenclature, the fishes has been split up into a more detailed scheme, with the following major groups: Class Myxini Class Pteraspidomorphi † Class Thelodonti † Class Anaspida † Class Petromyzontida or Hyperoartia Petromyzontidae Class Conodonta † Class Cephalaspidomorphi † Galeaspida † Pituriaspida † Osteostraci † Infraphylum Gnathostomata Class Placodermi † Class Chondrichthyes Class Acanthodii † Superclass Osteichthy
A meadow is a open habitat, or field, vegetated by grass and other non-woody plants. They attract a multitude of wildlife and support flora and fauna that could not thrive in other conditions, they provide areas for courtship displays, food gathering, pollinating insects, sometimes sheltering, if the vegetation is high enough, making them ecologically important. There are multiple types of meadows, such as agricultural and perpetual, each important to the ecosystem. Meadows may be occurring or artificially created from cleared shrub or woodland. In agriculture, a meadow is grassland, not grazed by domestic livestock, but rather allowed to grow unchecked in order to produce hay. In the United Kingdom and Ireland, the term meadow is used in its original sense to mean a hay meadow, signifying grassland mown annually in the summer for making hay. Agricultural meadows are lowland or upland fields upon which hay or pasture grasses grow from self-sown or hand-sown seed. Traditional hay meadows are now in decline.
Ecologist Professor John Rodwell states that over the past century and Wales have lost about 97% of their hay meadows. Fewer than 15.000 hectares of lowland meadows remain in the UK and most sites are small and fragmented. 25% of the UK's meadows are found in Worcestershire, with Foster's Green Meadow managed by the Worcestershire Wildlife Trust being a major site. A similar concept to the hay meadow is the pasture, which differs from the meadow in that it is grazed through the summer, rather than being allowed to grow out and periodically be cut for hay. A pasture can refer to any land used for grazing, in this wider sense the term refers not only to grass pasture, but to non-grassland habitats such as heathland and wood pasture; the term, grassland, is used to describe both hay meadows and grass pastures. The specific agricultural practices in relation to the meadow can take on various expressions; as mentioned, this could be hay production or providing food for grazing cattle and livestock but to give room for orchards or honey production.
A transitional meadow occurs when a field, farmland, or other cleared land is no longer cut or grazed and starts to display luxuriant growth, extending to the flowering and self-seeding of its grass and wild flower species. The condition is however only temporary, because the grasses become shaded out when scrub and woody plants become well-established, being the forerunners of the return to a wooded state. A transitional state can be artificially-maintained through a double-field system, in which cultivated soil and meadows are alternated for a period of 10 to 12 years each. In North America prior to European colonization, Algonquians and other Native Americans peoples cleared areas of forest to create transitional meadows where deer and game could find food and be hunted. For example, some of today's meadows originated thousands of years ago, due to regular burnings by Native Americans. A perpetual meadow called a natural meadow, is one in which environmental factors, such as climatic and soil conditions, are favorable to perennial grasses and restrict the growth of woody plants indefinitely.
Types of perpetual meadows may include: Alpine meadows occur at high elevations above the tree line and maintained by harsh climatic conditions. Coastal meadows maintained by salt sprays. Desert meadows restricted by low lack of nutrients and humus. Prairies maintained by periods of severe subject to wildfires. Wet meadows saturated with water throughout much of the year. Artificially or culturally conceived meadows emerge from and continually require human intervention to persist and flourish. In many places, the natural, pristine populations of free roaming large grazers are either extinct or limited due to human activities; this reduces or removes their natural influence on the surrounding ecology and results in meadows only being created or maintained by human intervention. Existing meadows could and decline, if unmaintained by agricultural practices, but a reintroduction of large grazers could influence meadows to reappear as natural habitats in the landscape. Mankind has influenced the ecology and the landscape for millennia in many parts of the world, so it can sometimes be difficult to discern what is natural and what is cultural.
Meadows are one example. As extensive farming like grazing is diminishing in some parts of the world, the meadow is endangered as a habitat; some scientific projects are therefore experimenting with reintroduction of natural grazers. This includes deer, goat, wild horse, etc. depending on the location. A more exotic example with a wider scope, is the European Tauros Programme; some environmental organization recommend to convert Lawns to meadows by stopping or reducing mowing. They claim that meadows can better preserve biodiversity, reduce the use of fertilizers. For example, in 2018 environmental organizations with the support of the Department for Environment Food and Rural Affairs of England, concerned by the decline in the numbre of Bees worldwide, in the first day of Bees' Needs Week 2018 give some recommendation how preserve bees; the recommendations include 1) growing flowers and trees, 2) letting the garden grow wild, 3) cutting grass less 4) leaving insect nest and hibernation spots alone, 5) using careful consideration with pesticides.
Foundation for Restoring European Ecosystems UK Wild Meadows Website Irish Wild Meadows Website Meadow Planting A Year in a Meadow Grow a Back Yard Meadow Adrian Higgins, "Today, 32,000 Seedlings.
The coast known as the coastline or seashore, is the area where land meets the sea or ocean, or a line that forms the boundary between the land and the ocean or a lake. A precise line that can be called a coastline cannot be determined due to the Coastline paradox; the term coastal zone is a region where interaction of the land processes occurs. Both the terms coast and coastal are used to describe a geographic location or region. Edinburgh for example is a city on the coast of Great Britain. A pelagic coast refers to a coast which fronts the open ocean, as opposed to a more sheltered coast in a gulf or bay. A shore, on the other hand, can refer to parts of land adjoining any large body of water, including oceans and lakes; the somewhat related term "" refers to the land alongside or sloping down to a river or body of water smaller than a lake. "Bank" is used in some parts of the world to refer to an artificial ridge of earth intended to retain the water of a river or pond. While many scientific experts might agree on a common definition of the term "coast", the delineation of the extents of a coast differ according to jurisdiction, with many scientific and government authorities in various countries differing for economic and social policy reasons.
According to the UN atlas, 44% of people live within 150 kilometres of the sea. Tides determine the range over which sediment is deposited or eroded. Areas with high tidal ranges allow waves to reach farther up the shore, areas with lower tidal ranges produce deposition at a smaller elevation interval; the tidal range is influenced by the shape of the coastline. Tides do not cause erosion by themselves. Waves erode coastline. Coastlines with longer shores have more room for the waves to disperse their energy, while coasts with cliffs and short shore faces give little room for the wave energy to be dispersed. In these areas the wave energy breaking against the cliffs is higher, air and water are compressed into cracks in the rock, forcing the rock apart, breaking it down. Sediment deposited by waves comes from eroded cliff faces and is moved along the coastline by the waves; this forms an cliffed coast. Sediment deposited by rivers is the dominant influence on the amount of sediment located on a coastline.
Today riverine deposition at the coast is blocked by dams and other human regulatory devices, which remove the sediment from the stream by causing it to be deposited inland. Like the ocean which shapes them, coasts are a dynamic environment with constant change; the Earth's natural processes sea level rises and various weather phenomena, have resulted in the erosion and reshaping of coasts as well as flooding and creation of continental shelves and drowned river valleys. The coast and its adjacent areas on and off shore are an important part of a local ecosystem: the mixture of fresh water and salt water in estuaries provides many nutrients for marine life. Salt marshes and beaches support a diversity of plants and insects crucial to the food chain; the high level of biodiversity creates a high level of biological activity, which has attracted human activity for thousands of years. More and more of the world's people live in coastal regions. Many major cities have port facilities; some landlocked places have achieved port status by building canals.
The coast is a frontier that nations have defended against military invaders and illegal migrants. Fixed coastal defenses have long been erected in many nations and coastal countries have a navy and some form of coast guard. Coasts those with beaches and warm water, attract tourists. In many island nations such as those of the Mediterranean, South Pacific and Caribbean, tourism is central to the economy. Coasts offer recreational activities such as swimming, surfing and sunbathing. Growth management can be a challenge for coastal local authorities who struggle to provide the infrastructure required by new residents. Coasts face many human-induced environmental impacts; the human influence on climate change is thought to contribute to an accelerated trend in sea level rise which threatens coastal habitats. Pollution can occur from a number of sources: industrial debris. Fishing has declined due to habitat degradation, trawling and climate change. Since the growth of global fishing enterprises after the 1950s, intensive fishing has spread from a few concentrated areas to encompass nearly all fisheries.
The scraping of the ocean floor in bottom dragging is devastating to coral and other long-lived species that do not recover quickly. This destruction alters the functioning of the ecosystem and can permanently alter species composition and biodiversity. Bycatch, the capture of unintended species in the course of fishing, is returned to the ocean only to die from injuries or exposure. Bycatch represents about a quarter of all marine catch. In the case of shrimp capture, the bycatch is five times larger, it is believed that melting Arctic ice will cause sea levels to rise and flood coas